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1957 The lecE tron Spectra of Cesium-134 and Barium-131. Leon Stanley August Louisiana State University and Agricultural & Mechanical College

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Recommended Citation August, Leon Stanley, "The Electron Spectra of Cesium-134 and Barium-131." (1957). LSU Historical Dissertations and Theses. 189. https://digitalcommons.lsu.edu/gradschool_disstheses/189

This Dissertation is brought to you for free and open access by the Graduate School at LSU Digital Commons. It has been accepted for inclusion in LSU Historical Dissertations and Theses by an authorized administrator of LSU Digital Commons. For more information, please contact [email protected]. I T T ' 134 131 - * i . - o i . ' OP J3 AIID Ba

A Dissertation submitted to the graduate Pacully of the Louisiana Dtate University and -w;ricultural and mechanical College in partial fulfillment of the requirement a for the decree of Doctor of Philosophy

in

The Department of Physics

by Leon Dtenley August . Louisiana vteto University, 1950 Tulono University, 195D June, 1957 The author wishes to expreo3 hi a si ace re r.pprooi >> tion

to i rofos.-or 'OOurioh f :ir his constant interest ’~ncl help

i 11 thi., work, rad to 1 rc To l : o i ior:' C • *.;ulph for his nssivt-

?.uco i’i 0 osia'ii.to the maauet. rivJiu. oi jo -5T i' o

rage

AOXnO t i

L I n T o*' T ^ c x c c ...... V

LToI 01 lIOUnEJ...... vl

’ ’ V i l i

C. I. J'liv

1 I X JiikUJ I. J l'I Oii...... 1 ^ -r h .• ' ■ ’i * II. TEE IlL TLU.-EOT -4 .1 l A / U. -i. • J. 'J j | t • t « < • • I • t ■ * * • * O

1. h is to r y ami 'Jhoiue o i In stru m en...... t 5 s. The Components of the Instrument...... 10 h , One La-met mid Vacuum Chamber ...... 10 b• The Electronic lower supply lor the La.- n e t ...... 16 c. source Preparation...... 19 d. Detactor...... 0:4 e. Lagnetic nipidity Leasuremerts ...... £8 0. The col3V Test spectrum...... 00

III. X iiill -tJjLa± iUx'ti** I j.t.-T 1 k j -< Oil joj-i /-j-u.I jii . j Onli H n . t T 09

1. Importanoe of Conversion Coei1ieionts j nid L/X -'laties...... 09 0. Let hod of beterrr;ininf- Conversion Coefficients...... 41 / . Vest o f Let ho 6 for Ooiormi ni np Conversion Coefficients...... 45

IV, THE Cs134 BETA-OPLCill...... 58

1. Previous -iork...... 58 £. Present -.ork...... 59

i i i Pn^o

V. THE COHVEHEIQII ELEGT^OH Sl’GGTHUi: OF Cs131...... 63

1. Introduction...... 63 Lt Previous tJork...... 64 P re sent rtork...... 70

J CIHJ0TEI) BIBLIOJH.J.-HY...... 87

VITA...... 83

i v LI LIT 03 .’A ! i l l

T ab le P age

I . Jumrnary o l D ata f o r D e te r m in in g a lc 's • 56

II . ^-Oouvorsi.)ii Jo of f 1 cl eat a of Ba^"*^...... 56 1 '7j4 Ill . maximum ^nerrfies o f Js i3eta--ipeot ra . » * 50 IV A. Reported Unergries of Transitions in Cs1^ - Iviagrnotio Spectrometers...... 65

IV B. Reported Energies of Transitions in Scintillation opectrometers...... 66

V. Reported lonversion Coofiioionts and A/L Ratios for Cs*31 ...... 67

VI . uummar^ of Cs^*^ Bata...... 84

v LIST OF FIGUHE3

Figure Page

1. Grose Section of Magnet and Vacuum Chamber...... 11

2. iiadial Field Plot in Median Plane ...... 13

3. Top View of Vacuum Chamber...... 14

4. Regulated Current Supply...... 17

5. Top View of Source...... 21

0. Cross Section of Detector...... 25

7. Momentum Calibration Curve for Spectrometer...... 31 137 6. Cs Electron Spectrum...... 32

CJ. Decay scheme for Cb^37...... 34

10. Fermi Plot for Cs^37...... 36 134 11. 1 artial Cs Electron Speotrum...... 48

12 . Part.U -1 Gamma-E«y npectrum fl/B" oouroo ). . . • 50

13. Partial Ba^37 Gamma-Say apectrum fl/l6" Source)... 51

14. Partial Ba Gamma-Eay spectrum fl/4" oouroe ) .... 52

15. K-Convorsion Peak of Ba^7 fl/4" Souroe...... ) 53

16. k-Oonversion Peak of Ba^37 fl/lo" Souroe)...... 54

17. Fermi Plot lor Csx"4...... 61

18. Cs'1'31 Conversion Electron Spectrum (Source I) 72

v i ■Figure Page

19. Partial O b 1 3 1 Conversion Kleatron Spectrum (Source I)... 76

20. Partial Ob131 Co nve rsion .Clect ron Spectrum (souroeV) . . . 77 ■ „131 21. Part ial m 8 Coaversion Electron C p e o t r u m f Sou roe h i ). 79 * ' 08131 £* ^ « Partial Conversion PJleat ron Spectrum (source III ) . 80 * * 131 2 3 . P a rt i a 1 ’■j B Co nverslon Electron Bp eat rum (SouroeIV). . 81

24. K and L + U Conversion Peaks of Ba^*^ (Souroe IV).. 3 2

vi i ABBTHAOT

-v double-foousi ng t iron oo re magnetic spectrometer employing a scintillation neteotor was constructed, tested, 1 rxA I'3)! and used to study the electron speotra of Gs and Ba •

The performance of the spectrometer was shown to be satisfactory by studies that were made on the Gs^-'^ electron spectrum. This spectrum has seen thoroughly investigated by a number of other workers. The spectrum obtained for the priucioal beta component of showed * unique first- forbidden shape and had a maximum energy of 511 ±20 kev.

The K/(L + Ll) ratio of the 661 kev transition in BqI37 was found to be 4.5 ±0,5. These results are in good agreement with the generally accepted data on this radioisotope.

The method employed in determining conversion coeffi­ cients involved using the 661 kev, I >14 transition in as a standard of comparison. The theoretical volue of the h- conversion coefficient for this transition was used In deter­ mining other conversion coefficients. Other workers have shown that the theoretical and experimental v- lues for this

^-conversion coefficient agree to within the experimental error. The required gamma-ray intensities were determined with a scintillation spectrometer. The method was shown to be satisfactory by determining the 5-conversion coefficients of the 605 kev and 797 kev transitions in

v 1 i i A—conversion onelliaient for the 605 kev transition was found to he (5.Q± 0.6) x 10”^ while that of the 797 kev *" 3 transition wuu (£.8±0.3) x 10 . Those values are In reasonable agreement with those of other investigators who used different methods that seem reliable.

In the course of working with a beta-speut rum wae observed which had not been previously reported, A i’ermi plot of the data gives a maximum energy of 0.95 ± o. 15 kev for this spectrum. 131 The work on Ba was performed using a specially prepared source of relatively high specific activity. The conversion electron spectrum confirmed the existence of a

133 kev transition in This transition had been reported in only one previous investigation. All of the principal transitions in 0s were observed as well as the three highest energy ones whose energies are 630, 915 and lo4G kev. Previously these highest energy transit.ions had only been observed with a scintillation spectrometer. There was _Iso evidence of other weaker transitions whose energies were found to be 94, 158 and 4 05 kev. The two lowest energy transitions reported by several investigators could not be confirmed because of aetector out-off at these low energies.

The A-conversion coef1icie;ts of the principal transitions and the three weak, high energy ones were determined. iron these data probable multipolarities are given. These conclu sions were verified when possible by the a / l or A/fX+M) ratios for these transitions. The values for the conversion

i x oooflioionts and the IC/l or h/fX+Il) ratios indicate that n oaoh transition studied in Cs *■' is one of the following typos: 131, 132 , lul or BC + ill. ’Phe Informsti on on multipolar­ ities has been used in proposing an improved and more com­ plete ueoey scheme by other workers in this laboratory who have done other experiments on Ba^^.

X CHAPTER I

InPAODUdTION

The principal problems of nuclear physios are the

oolleatlon and Interpretation of data obtained is a result o f various finds of nuoleur experiments and their explana­ t i o n In terms o f a nuclear theory. A number of nuclear theories and models hove been developed as e result of making certain simplifying assumptions regarding the nature of nuclear forces. unilied and completely satisfactory theory of the nucleus does not exist because the exact n a tu r e o f the forces between nucleons is not known. The ultimate objective o f both theorists and experimentalists

Is to develop an adequate explanation ol nuclear phenomena as a result of their Joint efforts.

One area of experimental investigation which has yielded much useful information ©bout nuclei is that of the speotrosoopy of radiations omitted by radioactive substance i^ost studies in this area ere made on Isotopes th: t undergo bota-decay. The research reported in this paper in ' n ex­ am; le of such an Investigation.

Prom an experiment© 1 point of v 1 ov, , the object ■; vc 1 studying a radioisotope that undergoes beta-decay is to obtain an energy level diagram for the product, nucleus. auch a diagram showa all of ■the oner-ry levels that oan be reached by beta-decay and the gamma-ray and conversion elec­ tron transitions that occur between these levels. A com­ plete energy level diagram must also include spin and parity assignments lor each level since these quantities are of considerable importance for purposes of comparison with theory.

In obtaining the required data for a complete decay scheme for radioisotopes that emit both electrons and gamma— rays, it is convenient and usually necessary to study these two types of radiation separately. Of the different kinds of instruments used for these studies, the scint i Hat i on spectrometer and the magnetic spectrometer are the ones most generally used. llxperi ence has shown that gamma-ray inten­ sity measurements are more reliably made with a scintilla-

1 ion spectrometer, while electron intensity measurements ore more reliably made with a magnetic spectremeter.

While the magnetic spectrometer can be employed to make gamna-rey Intensity measurements by studying the photo - electrons ejected from a radiator, there is an aggular corre­ lation between the ejected electrons and the photons which produce them. This correlation results in suppressing the lower energy part of the spectrum relative to the higher energy part. The necessary correction of the data is quite difficult end is usually not made. Considerable error can be introduced into the intensity values when the effect is not corrected for and when the measurements extend over a 3 wide range of energy. The effect of this correlation is most important for photon energies below 0.6Mev.

Conversion electron and beta-spectra are more reli­ ably studied with a magnetic spectrometer because the exper­ imental data ore usually cleaner than that obtained with a scintillation spectrometer. i'or conversion electron studies the greater resolution of the magnetic spectrometer is of considerable advantage.

In comparing these two instruneuts, mention should also be made of the greater efficiency of detection of the scintillation spectrometer.

if'rom a comparison o± these instruments it is clear that beta and gamma-ray studies are best made if the instru­ ments are used together so as to complement each other.

The specific research reported in this paper is con­ cerned with the construction and testing of a magnetic spec­ trometer, testing a somewhat different method of determining conversion coefficients, and the application of the spectro­ meter and the method to the study of the decays of Cs-^4 end

Ba1B1.

The magnetic spectrometer built was used to study the electron spectra of Os-^’4, and Cesium 137 was studied to test the performance of the instrument.

Gesium 134 was studied to test the method used in determin­ ing conversion coefficients. In the wor’: on Cs^?4 a beta- spectrum was found which had not been previously reported.

As a result of finding this beta-spectrum further work is planned on at this laboratory, and the work chscusGed in this paper is perhaps best regarded as an initial stop in this proposed program.

Tho convoreion electron spectrum resulting from the decay of was studied with the magnetic spectrometer primarily in order to determine the relative intensities of the conversion lines. From these intensities, from the cor­ responding gamma-ray intensities that were determined with a scintillation spectrometer, and u si tig the 661 kev transition in Ba^-37 as a standard of comparison, the 1-oonvcrsion coef­ ficients were determined as accurately as possible for a number of transitions in The K/L and k/(L + L.) ratios of these transitions were clso determined to serve as a check. 011 the conclusions reached from the conversion coeffi­ cient measurements• from these quantities multipolarity assignments were made for a number of the transitions in

i'/hen used in conjunction with other information on

Bal*Jlt these multipolarity assignments can yield v lues for the sp i 11 b and parities of the various levels in Is-1-*'.

The working out of the deta.i 1 s of thu cornp 1 oto docay scheme of j3b^^ is bo i ng done by Campbell"** vino made the gamma-ray studies on this isotope at this laboratory.

a . rt. Campbell, to be published. CHATTY II

TH3 I IhiTiiUUlhJT AIID IT J PlihJhi J E

1. History and Qholoe of Instrument:

The two principal kinds of magnetlo speotrometers are the flat type and the helical type. In the flat type tho electrons that are detected movo approximately at right angles to the field, whereas in the helical type those elec­ trons that are detected make much smaller angles with the field. The helical type of spectrometer consists of a long cylindrical vacuum chamber around which is placed a coil of wire. Inside of the chamber are placed appropriate baffles.

The other necessary equipment is similar to that of the flat spectrometer. The construction of 1’ o helical type is usu­ ally iron free. ticking the instrument iron free has the advantage that the current can be taken to be strictly pro­ portional to the magnetic rigidity. The advantage of such an arrangement is that the current can be measured readily with greater precision than can the field strength. The constant connect!ug tho current and magnetic rigidity is determined by calibration with sources emitting conversion electrons whose energies have been accurately measured by other means. Iron free construction has the disadvantage that the power consumption of the instrument is greater than

5 6 that for an iron instrument of equivalent size. The power requirements for an iron instrument oan be adequately sup­ plied by a relatively inexpensive electronic power supply.

The greater power required by the iron free spectrometer is usually supplied by a do mo tor-sonerator set.

Tho main reason for not building the helical iron free type was the greater power consumption. A motor- gener&tor set was not available that could be used exclu­ sively with such an instrument, and funds were not available to purchase suoh a unit. .'mother important consideration was that of space. The iron free helical type is rather large in actual physical size. The most convenient space that was available in which to place suoh an instrument was deemed insufficient for the helical type. The flat type with iron pole pieces is more compact and could be readily used in the space available.

i'or the reasons indicated, the choice of the type of instrument to build was limited to one of two possible flat typos that are constructed with iron pole pieces. Of the two types that could have been built, the 100° flat, magnetic spectrometer was the simpler and easier to construct. How­ ever, this type does not compare favorably in terms of trans­ mission or resolution with the type that was chosen. Because of the fact that for the same resolution the transmission is greater, or for the same transmission the resolution is better, a flat, double-foeising magnetic spectrometer was built instead, of the 100° type. The only essential differ­ ence between the two typeB is the field distribution. In the 180° type the iron pole pieces are parallel, the field distribution where the focused electrons travel is uniform, and the olectrons are brought to a focus 100° away from the source. In this type of instrument there is foousing in

.just one direction. In the double-focusing type the pole pieces are shaped, the field is not uniform, and focusing occurs at Tf(2)^ radianB from the source.

Prior to the development of this type of spectro­ meter, it was realized from work, on the betatron and other

kinds of particle accelerators that the accelerated parti­

cles performed oscillations about an equilibrium orbit.

One can imagine that any mode of oscillation is composed of a vertical component and a horizontal component. If the periods of these two modes could be made equal, at certain points along the equilibrium orbit there would be nodes,

i. e ., a double-focusing of the particles at those points. g Siogbahn and dvertholm achieved this condition in a magnetic spectrometer by shaping the iron pole pieces of their magnet.

The radial dependence of the field strength in the median plane for their magnet is

B * B0 (r0 / r ) l (1 )

2 k. Siegbahn and N. Svartholm, Nature 157. 8 72 (1946); IT. Svartholm and k. 8iegbahn, ^rk. MatT iCstr. Fys. A33, No. 21 (1946). 8 where r Q is the radius of the equilibrium orbit at which the field strength is B0t and r is the radial distance where the field strength is li. The location of the median piano can be seen from Titrure 1. If a Taylor expansion is made around tho point r * rQ , the series obtained for this equation is

b b0 [i - 1/2 ( r ;or° ) + 3 / a . . . ] . (2)

3 Shull and -Dennison have shown that a somewhat more general approach can be made to the problem. They begin by consid­ ering the more general equation

Their analysis shows that for double-focusirigot must be -l/2, and that double-focusing occurs after an angular displace- mont of Tf (z)- radians. The value for p is somewhat arbi­ trary. The value of p will determine what is the best shape of defining baffle to use. Tor example, for P - r6/Q , the resolution is independent of the vertical divergence of the beam, while the value l/Q makes the resolution independent of the horizontal divergence. The different valuer of fl are obtained by shaping the inlc pieces in sli.tntly different ways. To out a inp = ^/O oiegbahn e_t al.^ have shown that tho pole pieces must be shaped so that if a vertical cross

3 T, ohull and D. Dennison, Thy s. *iov. 71_, 081; IT: . 0 50 (lcJ47).

iiedgran. T. biegbshn and U. ^vartholm, Iroc. Thys. Doc. AQ5. TOO (18 50). 9 section of the magnet is taken, the intersection with the pole pieces gives two parabolae whose vertioee lie at the center of the map-net. Tho equation of either parabola is z ■ c • (r)8, where z is the distance from the median pltmo , and c is a constant that is determined b y the air cap size desi red.

starting with £q. (3), hhull and Dennison show that if a particle is emitted from the source at the point z * dz, r ■ r +dr and makes an angle ^ in the median plane and an angle y* which is in a plane perpendicular to tne median plane, then the particle will strike the plane I? = f f (2)^ at the point given b37 the following equations:

r* = rQ - dr + (E ~ drE + <■«* ~ 51 dz2 + 3‘. 3r0 (4)

z * - - dz + S—J^L dr dz •+■ rQ / uf . (5 ) 3r0 3 i'rom 3q. (4 ) and noting that the dispersion

dr 4r (6) d (.Br ) hr the following approximate expression can be derived for the momentum resolution, -<, at half maximum:

H =f —=8 ■+■ ■ —- w — +• — (£------— (716 ) ^ (16ytf — 6 ) 6r0 8r0 £4 £4 where b is the source width and w the detector width. 10

2. The Components o f thc_ In str u m e n t:

a . The Magnet and Vacuum Chamber

In Pig. 1 is shown a cross section of the mar-not.

The magnet is symmetric about a vertical axis through the centers of C and P£ lor the vertical cross section shown

The iron yoke consisting: of , C and is made of soft iron. The exact physical and metallurgical properties of this iron are not known since it was purchased as scrap from a local concern. The hysteresis of this iron is such that for a current of 200rrta the difference in field inten­ sity on the two sides of the hysteresis loop is about1%.

The maximum current sent through the magnet was 075 ma. The various parts shown in Pig. 1 were machined in tho shop of the College of Chemistry and Physios.*

The shaping of the pole pieces to the biogbahu and uvcrtholm parabolic cross section could not be done with the facilities at this laboratory. >vn attempt was therefore made to approximate the required parabola with a pair of straight lines, so that the pole pieces would be conical in the region where the focused electrons travel. In order to determine the straight lines that would most closely approx­ imate the desired parabola, a full-scale drawing, similar to

Pig. 1, was made of the magnet. The air gap at a radial dis tance of 4t" was 2". Prom this information the constant in

^cknowledgment is made to Hr. ^Cdward Keel and his associates for their work on the items shown in Pig. 1 and also on t ae detector, Pigure 6. MEDIAN PLANE

H K

I’ig . 1 Jroas Petition ofIvlagnet anti Vaouurr. Chamber

11 the equation for the parabola is determined. The desired parabola was then plotted, and the best straight line approx imation was determined graphically. The results were also

checked analytioally. It was found that to achieve the desired pole piece shape it would be necessary to cut a 5.2° bevel on the pole pieces starting at a radial distance of

4.5 inches. rfhen the magnet was completed and assembled, the field strengths at various radii for a current of approx imately 200 me through the ma.-met coil were determined by means of a flip coil. The results are shown in1'ig. £ for the case that rQ = 5.85 inches md - l/4. These values of rQ and 0 gave the best fit to the data obtained. i/hile one might expect the value of 0 to bo closer to o/O , as in the case of Siegbahn1 s instrument, Ovartholm has shown that for conical pole pieces f i = 1/4. The data of 1'ig. 2 show that r0 should be fixed at about 5.8 inches since the field form is adequate over the desired region for this choice of rQ.

This value of rQ was established by proper placing of the source, central defining baffle and detector.

The vacuum chamber consists of parts of the top and bottom pole pieces, as indicated in i'ig. 1, and two concen­ tric aluminum rims, VQ and . The rims ere held together by pieces of aluminum cut to fit the gap between the pole pieces. The details are shown in I'igure 3. Appropriate rectangular holes are cut in some of these aluminum pieces ;

bvartholm, ^rk. f. Fys. 2, iJb. 14, 115 (184b). 7 CALCULATED experimental 6 RADIUS (INCHES) RADIUS 5 Pig. Plane Pig. £ Plot Pield in Median Radial too

I .025 I I I OOO I .075 I 1.050 O. 975 O. 0.9 5 0

O O 925 O CD CD M

SOURCE

S

R

±'ig. 3 Top View of Vacuum Chanter

14 15 thus, they also serve as the baffle system, The ones that have holes out In them are indicated in the figure. The hole in the central defining baffle, II, is a rectangle,

1 5/0" in the redial direction and 1 l/2" in the vertical direction. The present baffles are l/8" thick aluminum and readily removable. One-quarter inch thick aluminum baffles are available if there iB any need for their use, A vacuum tight seal is achieved by large O-rings at the tops and bot­ toms of the aluminum rims. These O-rings were made from l/8 M diameter neoprene cord. The n-ring grooves are cut in the aluminum rims. The source holder is attached to the chamber at the flange marked 8, the coupling to the vacuum pump at

P, and the detector at D. All connections are made by means of O-ring seals. The aluminum can, R, welded into the outer rim provides a speoe where a rotating coil is placed. This arrangement eliminates a vaouum seal through which the shaft of the rotating coil would hove to pasB.

A vaouum is maintained in the chamber by means of a

Genoo meohaniaal pump. The residual pressure in the chamber is about 5 microns. At this pressure, for the energy range covered in the experiments (75 kev to 1 Mev), gas scattering does not cause any noticeable distortion in either a contin­ uous spectrum or a line spectrum.

The cross-hatched section between and G in Pig. 1 is the space available for the magnet coil. This coil con­ sists of approximately 5000 turns of #24 formvar covered wire. This coil is wound on a spool made of l/l6" aluminum. 16

The leadB to the ooil are brought out through a vacuum tight tube between VQ and .

The pole pieces and the aluminum rims ere firmly

clamped to c-ether by moans of a large steel nut and bolt arrangement. The bolt runs through the magnet along the vertical axis. There are also two small steel aligning pins between C and , and two between C and Pg. The pins are

needed because the magnet can be assembled in only one way.

The large bolt, nut and pins are not shown in Fig. 1 in order to keep this drawing from becoming too confusing,

b. The Electronic Power Supply for the Magnet

The essential features of the power supply* for the magnet ooil are shown in Figure 4. The unregulated power

supply (T-ft-F) that provides the main current and the other

two regulated voltage power supplies aro not shown in detail

since there is nothing of particular interest to show about

them. The 200 volt screen supply for the d07Ts is mounted on the same chassis as the T-rt-F. The regulated voltage

supply with the 300 volt and 150 volt outputs is on a sepa­

rate chassis. The control circuit which is shown in detail

is also on a separate chassis. The entire circuit is fed by

a Model 1001 Sorensen A.G. Regulator. This regulator is

rated to control the ac output to within ±0.01^. However,

*The assistance of Mr. Leslie Edlen and Mr. G. L. Peacock, Jr. in building some of the electronic equipment is gratefully acknowledged. T - R - F + 150 + 3 0 0 2 0 0 V 4 5 0 V REGULATED REGULATED 4 0 0 MA

TO FOUR SOT'S 3 3 0 ^ A

IO K TO POTENTIOMETER

-AAA 4 7-A- J l S L Q A . MAGNET COIL (4 0 0 JA. ) 2.5 K 3 0 W

47 K 202 V — AAAAA O W V R I 50 IOO K H POT MEG 6 A N8 v R 150 5651

VR 150 I O K

10 K A \-----f\ H POT ■=£“ 6 V

S.K I O v-v w w STD.

f

4 Kegulated Current Supply

17 18 teats indicated that a more realistic estimate of the regu­

lation was ±0.Q5>, bince the regulator did not perform

as well as desired, the filament current to the 6AN8 was sup­ plied by a 6 volt automobile battery* This current to the

6AN8 must be as constant as can be obtained because any fluc­

tuations in It will introduce variations in the coil current*

The regulation of the control circuit results from the following actions of this cirouit. If for some reason

the current through the coil decreases, the current through rts, a 10 ohm standard resistor, decreases. As a result, the voltage on tne control grid of the pentode side of the 6AIJ3 decreases. This reduces the current through the pentode, and, consequently, its plate voltage increases. This volt­

age increase naturally causes the voltage on the control grids of the 807's to increase; therefore, the current through the 807* s and all elements that are in series with

them, which include the ooil, increases. On the other hand,

if the current through the ooil increases, there will be a

similar compensating action to decrease it. The voltage drop across Hs is effectively compared against that on the grid of the triodo side of the 6AIT8. The voltage on the

grid of the triode is held constant by the 5651 voltage ref­

erence tube. The current through the coil can be changed

from approximately 5 ma to 375 ma by means of the lOic Heli- pot which is used to change the grid voltage of the triode side of the 6AN8. 19

An Interesting test of the ability of the circuit to maintain the current through the ooil constant is the follow­ ing. If the load resistance is halved, the current changes by about 0.01;b. As a result of this teat it is evident that any relatively large changes that occur in the coil current are not due to the inability of the control olrcuit to ade­ quately compensate for external causes of change in the ooil current, but rather are a result of some instability associ­ ated with the SAN8 itself. With the arrangement shown in

1'ig. 4, the current through the coil can be maintained con­ stant to within ±0.01>o for a period of from 5 to 10 minutes.

This degree of stability has proven to be more than adequate for the work done this fur.

c. Oource Preparation

The difficulty caused by source thickness in the study of beta-spectra is well known. The preparation tech­ nique to be described is adequate far the study of beta- spectra down to an energy of about 50 kev for sources of relatively high specific activity. In the study of conver­ sion lines, source thickness and the consequent scattering is not so serious a problem as in the study of beta-spectra.

The technique to be described for preprring the kind of source under consideration is similar to that used by langer.^ i'or more refined techniques reference may profitably

L, M. langer, rtev* 3ci. Instr. hO, £16 (1942). 2 0

7 bo made to the comprehensive review by Blatis. A top view drawing of a typical source is shown in .Figure 5, Prepara­ tion of the source begins with the machining on a lathe of an aluminum rim. The inside diameter is 3.14 cm and the out­ side diameter 3.76 am. The rim is 1/8" thick. Two refer­ ence marks on the same diameter can be made with the cutting tool while the rim is still in the lathe. Another pair of reference marks can be made later at right angles to the first and also along a diameter. ./electing one pair of ref­ erence marks, two holes are drilled into the rides of the rim r.o that both holes are parallel to the reference marks.

These holes are threaded with a 4-40 tap. «Vhon the source is completed it is attached to a 4-40 screw at the tip of the source holder. Two holes are recommended in the event that it is desired to rotate the source lo0°.

^fter the rim is made, a piece of aluminum foil 0.2 mil thick is attacnod to tho rim with luco cement. Prior to attaching the aluminum foil, a rectangular section is cut from it that is 1 1/8" long and 3/32" wide. In attaching the aluminum foil to the rim, the other two reference marks are used to align the rectangular hole so that it is in the center of the rim and at right angles to the topped holes, as shown in i’l-mre 5. After the cement has dried, any excess aluminum foil is trimmed off of the rim. If it is seen that

Tt K. oiatis, Jhap. VIII (II), Beta- and Gamma-Bay dpeotro scopy. Interocience rublishors" I n c . ~ !7eu York, lb 5 3 . I CM \

5 Top View of oource

21 tho foil and rim do not make contact at any point, a small tab of foil can be folded over the Bide of tho rim and held in place against it by a piece of Scotch electrical tape.

This procedure is recommended to insure electrical contact between the foil and rim, and thereby lessen the effects of source charging. The rim is connected electrically to the vacuum chamber by means of the souroe holder.

The next step in the preparation of a source is to lay a thin, wet film of Lithgow LG 600 over the rim and foil arrangement. when this thin film is wet it clings very well to the aluminum and will not come off easily after drying.

The film is obtained by pipetting a diluted drop of LG 600 into a large dish of clean water. The film spreads over the water and may be picked up with a scoop. The LG 600 films used had average areal donsities s 0.1 mg/om^.

After the LG 600 film has dried, the area over the rectangular hole, which is cut out to reduce buckscattering, is treated with insulin diluted with distilled water. The dilution is approximately 20 parts water to 1 part insulin.

The insulin is applied in small drops from an eye-dropper that has a very fine point. The fine point is obtained by heating and drawing in a Leeker burner flame. The drops are drawn together by running the tip of the dropper between them. The tip of the dropper can also be used to spread the insulin so that the treated area will more closely approxi­ mate a rectangle. The excess insulin is next removed with the dropper. tfhile the film is still wot with insulin, a 23

drop or two of the radioactive solution is pipetted onto

the treated area with another eye-dropper with a fine point.

Tho insulin reduces the surface tension of the filmt and,

therefore, when the source dries the active material will

he more uniformly distributed over the treated area. The

drying process can be hastened by tho use of a heat lamp.

During the drying process, rocking the source from time to

time will also help to keep it from drying non-uniformly.

A source prepared in this manner can either be used as it

is or covered with another thin film of LG 600. i'or active matorial that i3 very stable chemically, leaving tho source

uncovered is the bettor thing to do since the covering will

increase the source scattering. For material that is not

very stable obemioally the souroe should be covered in

order to prevent the entire vacuum chamber from getting con-

taminat ed.

The completed souroe is attached to the source

holder by means of a 4-40 screw. Tho source holder consists

of an aluminum rod which passes through an 0-ring seal. The

screw is at the tip of this rod. It is possible with this

arrangement to change the radial position of the source

while maintaining a vacuum. The details of the source

holder are not shown since it was a temporary arrangement

used in setting up the instrument. Now that tho initial

work has been done on the instrument, this source holder,

which is rather bulky, should be replaced with a simpler and

more easily handled arrangement. 24

The souroe preparation technique discussed above yields sources that are reasonably sturdy and -*hich do not produce noticeable charging effects. The positioning of the source is reproducible to within about ± # mm* This possible variation in source position can introduce an uncertainty of about ±i $ in the value of Br for a conver­ sion line when different sources are used,

d. Detector

A horizontal cross section of the detector is shown in i’igure 6. The electrons being focused at the exit slit

strike the plastio phosphor which is l/3£" thick, l/8" wide and 1 l/8" high. The phosphor Is covered with 0.2 mil alu­ minum foil. The light piper is wrapped in 0.5 mil aluminum foil except where the 0-ring seal is located. A mu metal shield is placed over the photomultiplier. The arrangement is light-tight as it is shown so that no further measures need be taken to insure that the tube is shielded from exter­ nal sources of light. The entire apparatus shown in the figure ie surrounded with a large cast iron sleeve 7" long,

4#" inside diameter, and 5/l6,f thick. This sleeve is neces­ sary to prevent the fringing field of the magnet from being too great at the mu metal shield. Changes in the magnetic field strength do not produce noticeable changes in the per­ formance of the detector. Good optical coupling between the light piper end photomultiplier is achieved by using a small amount of petroleum Jelly at the Junction, The phosphor is shielded from gamma-rays from the source by approximately PHOSPHOR

i v- l Lj t ' - 1 DUMONT LIGHT PIPER 6 2 9 2 m 1 J - =

i"

6 Cross Section of Detector

£5 £6

1 inoh of lead. The deteotor Is attached to the flange D shown In Fig. 1 by means of large brass sorews and nuts.

The sorews pass through holes in the flange.

The regulated high voltage supply for the photomul­ tiplier was built in this laboratory. The usual voltage used has been 1000 volts. The signal from the photomulti­ plier is fed into a p re -amplifier built in this laboratory and then to an KGX Lionel 2205 linear amplifier. The pulses from the linear amplifier ore taken from the discriminator output. The discriminator on this amplifier is usually set to a reading of 75 when the full gain of the amplifier is used. The pulses from the amplifier go to an HCL Model

2006 scaler. A system of relays is employed so that the timer and scaler ore started together. After the scaler registers a pre-determined number of counts, the same relay system stops the timer.

When there is no source in the instrument and the detector Is operated under the conditions specified above, the background counting rate is about 0.7 counts/second.

When a source is in the spectrometer the background will depend upon the nature of the source and its total activity.

The largest background counting rate observed was with a strong Os ^ source. In this case t tie background was 2 counts/second. Usually the background rate was about 1 count/second.

From studies on the lower energy beta-spectrum of

Cs it has been determined that the deteotor cut-off is 27

approximately 30 kav and that the deteotor becomes 100/o

efficient around 125 kev. These values compare favorably

with those that could be obtained with a conventional mica

window goiger counter. Bata taken below 100 kev are not

very reliable since the discriminator in the amplifier can

drift from one set of measurements to another. This problem

is unavoidable, and it is one of the disadvantages of using

a scintillation detector. Mention should be made of the

fact that this problem was foreseen, and the anticipated

useful ra;ige of the magnetic spectrometer was about that

which it presently has. Because of the difficulty noted

with the discriminator for low energy measurements it is

recommended that a thin window geiger counter be built for

the spectrometer. The scintillation detector was built first since it could be put into operation sooner than a thin window geiger counter. iven when a thin window counter is available, the present scintillation detector will be the more useful device for certain kinds of measurements. For strong sources the sainti11ation detector is recommended since its counting loss is far less than that of a geiger counter* Also, when faster electronic circuits become avail­ able at this laboratory, it is anticipated that electron- gamma coincidence measurements will be attempted. The scintillation detector will be necessary for such measure­ ments. £8

e. Magnetio Rigidity Measurements

In a magnetic speotrometer, since r is fixed by the baffles, some quantity that is proportional to B is measured,

Binoe r is constant, this quantity will also be proportional to Br. Calibration of the instrument using standard sources is required since the proportionality constant is unknown.

In the initial work with the instrument the quantity meas­ ured was the emf induced in a rotating coil that was driven by a synchronous motor. The rotating ooil was located between the pole pieces in the aluminum can shown in Figure

2. The induced emf vues fed to a rotating primary trans­ former. The emf induced in the secondary of this trans­ former was then fed to a peak reading voltmeter. The rotating primary transformer was similar to that described s by Langer and Scott.

In the course of the work done with tho rotating coil several undesirable features of this system were discov­ ered. The calibration curve, Br vs. potentiometer reading, was non-linear. The departure from linearity was of the order of 1/5. Another difficulty experienced in using the system was caused by frequency fluctuations on the power line feeding the synchronous motor. These fluctuations made taking points close together on the spectrum a difficult task. In the course of thiB work the discovery was also made that the focusing aotion of the magnet depends to a

Q 1. Langer and F. Scott, Rev. Sci. Instr. 21, 522 (1950). 29

small extent on the previous history of the iron. The

e f f e c t o f this property of the magnet is to cause the shape

and position of a conversion peak to depend upon the mag­

netic history of the iron. Tho areas under the peaks are

the same to within ± 5/o which is the usual uncertainty in the

determination of the area under a peak. This effect causes

no problem in the determination of the intensities of con­

version lines, therefore, but it does lessen the accuracy of

the energy determination of these lines. In order to lesson

the uncertainty in energy caused by this effect the magnet

is cycled in the same manner before each experiment. The

cycle which gives the most consistent results is to increase

and decrease the current for one direction through the coil,

reverse the direction of the current and repeat the proce­

dure. The maximum current used is 375 ma. This limit ia

set by tho power supply. The cycle is repeated five times

before each experiment. This necessity of cycling the

magnet in order to obtain the best results possible was

another factor which diotated the abandonment of the rota­

ting coil system.

Because of the difficulties noted above in using the

rotating coil, the magnetic rigidity determinations were made by measuring the current. The conversion electron spec­

tra of a number of standard sources were run. The data col­

lected were counts/second vs. potentiometer readings. The potentiometer reading in each case was proportional to the

current since the potential difference applied to the 30 potentiometer was that across tho 10 ohm Btandard resistor shown in Figure 4. Tho potentiometer, which was made in this laboratory, could detect ourrent changes of O.Ol^S.

The ourrent values, expressed in arbitrary units, at which various conversion lines of known magnetic rigidity were found were plotted as a function of Br. The Br vs. ourrent plot was linear as far as could be determined from the data.

Any non-linearity is estimated to be of the order of 0.1;£ or loss. This calibration curve is shown in Figure 7.

The points on the line are from conversion peaks of known Br values that are found in Cs 131t Bal*^ and Bal^^.

The uncertainty of the Br value of each of these conversion peaks lsso.l;5 of t iie value given. A least squares fit was made of the points, and the best straight line so determined is given by the equation

Br = 1.513 I - 53.0, (8) where I i3 the current expressed in the same arbitrary units.

«nfhen operating the instrument in the manner stated , the potentiometer readings at which the 3ame conversion line would appear on different runs were reproducible to within i 0.3/tf. This uncertainty plus that caused by source posi­ tioning requires attaching an uncertainty of about ±0.5;o to the Br values of unknown lines.

3. The Gs^7 Test Spectrum: 1 ^7 A Gs electron spectrum obtained with the instru­ ment is shown in Figure 8. This isotope has been studied by Br (GAUSS-cm) 2000 0 0 0 4 0 0 0 3 0 0 0 5 'g 7 i-ionentum 7i'ig. Calibration Curve Spectrometer for 1500 URN (RIRR UNITS) (ARBITRARY CURRENT 0 0 5 2 SI

3500 COUNTS/SECOND 0 0 6 200 0 0 4 0 0 8 ’ g 8 Gs-'-*7''7i’ig. 8 Electron Gpectruni C U R R E(A N T R B I T R AUN R Y I T S ) .200 32 1600 L P 2000 O . V.8 66 K L + M 661 33 a number of worlcers.9-^ Tho generally aooepted deaay aoheme la shown in Figure 9. This spectrum was studied in order to test the performance of the instrument, to obtain a very reliable momentum oalibration point, and to obtain the inten­ sity of the ^-conversion peak. This last piece of informa­ tion is used to determine the conversion coefficients of transitions in Bo134 and Gs131. Actually, several Gs137 sources were used, and the spectrum shown is from Just one of these.

Cesium 137 was chosen for testing the performance of the instrument since it provides both a continuum and conver­ sion peaks. Since this isotope has been thoroughly investi­ gated by the other workers mentioned, their results can be compared with those obtained from this instrument. Cesium

137 is a good isotope for testing purposes since it is long- lived and is obtainable with a high specific activity.

The spectrum shown in Fig. 9 was obtained from a thin souroe prepared in the manner described previously. The £ areal density of this source was —0.1 mg/cm . The source was approximately o/lu" wide and 1” high. A Fermi plot was made ol the data shown in Figure Q. This plot is shown in

9 G, L. Peacook and A. G. iwit oh ell, Phyc. dev. 75. 1272 (1949).

X(^L. II. Langer and H. G. Price, Phys. Fev. 76^, 641 (1949); L. ti. Langer and rl. J. i^oflat , ibid . 82, 635 (1951); Graves, Lunger and loffat , ibid. 68 . 344 (195 2 1".

1Xl:. A. Waggoner, Phys. lev. 82_, 906 (1951). 137 137 CS Ba

i *

.51 ME V 0 2 •/•)

156 SEC.

2

1.17 ME V (8 °/o)

.661 MEV

3 _ t 4 - + C.

i'ig. 9 Decay Scheme for Cs^7

D4 35 i'igure 10. The upper ourve resulted from first assuming

that Tooth the 1.17 Mev and 0.51 Idov beta-speotra have

allowed shapes. It is obvious that the lower energy beta

group is notrallowed. The energy of this beta transition

was found to be 511 2 0 kev, in good agreement with the pre­

vious results. It is known that the i’ermi plot of this

lower energy oeta group oan be made linear by using the

unique f irst-forbi a den shape factor, a-^ , whioh is

&1 ( w ) * az - l + fwQ - w)2 , (s)

p where W is the total energy in nrc units and »iL is the end - o o point energy in the same units. The result of applying this

factor to the upper curve is shown in the lower curve.

Before applying this factor tho higher energy beta group was

subtracted from tho total spectrum. The result is a straight

line down to ■ xi energy of about 130 lev. The bending over

of tho line at energies lower than this results from less

than lOO/o detection efficiency at these lo .v energies. hnae

the lower curve is a straight line , in agreement with gener­

ally accented experimental results of numerous investigators,

this test indicates that the slight dependence of focusing

on the previous history of the iron does not causea notice­

able error in intensity measurements.

several of the investigators previously mentioned

have reported values lor the x»./(l+ld) ratio of the G61 kov 137 transition in Ba • Waggoner obtained a value of approxi­

mately 4.3. Graves , Langer and Loffet using a magnetic 6

6

4

2

MEV

OL_ 1.0 I . 4 I 8 2.2

O I (i + p ) 5

Pig. 10 Permi Plot for

36 37 spectrometer with greater resolution obtained 4.6 ±0.3, The experimentally determined values of the k/fL+ll) ratio, the half-life, enci the conversion coefficient for this transi­ tion are all compatible with an 114 assignment* for this tran­

sition.

In testing the spectrometer, the h/fL+i.) ratio of the 661 kev transition was determined in order to compare the value obtained with the above mentioned results. This ratio was found by measuring the area3 under the if and L+M peaks. The value obtained was 4.5* 0.5, in good agreement with otiier work.

Using liq. (7), one can a 1 oo compute the expected momentum resolution. The expected resolution ior the source si i'.e used is about 0.006. As shown in i'ig. 8, the resolu­ tion obtained experimentally is about O.JOQ.

One can also make an estimate of the trnnsmission of the instrument from the data shown in I'ig. 8, using the known A-coaversion coefiioient of the 661 kev transition which is 0.U84. One can write

a = (paafcl. , do) */1 o t al where a^ is the k-conversion coefficient, T is the desired transmission of the instrument, total i® total number of 661 kev gamme-rays emitted from the source per second in

4ff storadians, and Ne(peak) is ‘t'he P°alc counting rate of the K-conversion line. The total gamma-ray activity from the source waB calculated from measurements made with a 38 scintillation spectrometer. solving ^q. (10) for T and putting in tho known values for the remaining termsf one obtains a transmission of 0*2^, The theoretical value for

the transmission is also about Q.2,o.

Those figures for the resolution and transmission

compure favorably with those of other instruments of this type. CHAPTER I I I

the r :t jh;.iihatijn of goijversioii cosfiici >:irro

1* Importanoe o1 Conversion Coefficients and K/L Ratios:

The A-oonversion coefficient, a^ , b 1 defined as

, where is the number of K-eleotrons emitted per second for a particular transition and Uy is the number of gamma-rays emitted per second. Entirely analogous defi­ nitions oan be made for the U and H-oonversion coefficients.

The H/L ratio is defined as Fl/L = a^/a-^.

The K and L-oonversion coefficients hr.ve been calou- 12 lated by Rose et_ al. The theoretical conversion coefficients are calculated as a function of the transition energy, the atomio number, the atomic orbital involved, and the multi­ pole character of the associated radiation. from experiment one knows the conversion coefficient, the transition energy, the atomic number and the orbital for the conversion elec­ trons. The relore, comparison of the experimental value of, say, the A-conversion coefficient for a particular transi­ tion with the theoretical values will, in principle, permit one to determine the multipole character of the radiation.

12 Rose, Goertzel, opinrad, Harr and strong, Phys. Rev. 93, 79 (1951) ; id. E. Rose, Appendix IV, Beta- and Gatnma~=iTay Bpeotro soopy . Interscience Publishers, Inc. , l/ew York, 1955.

39 40

Having determined the multipolarity oi the radiation, the

selection rules resulting irom conservation of parity and total anrular momentum for the system, nucleus plus photon, permits one to make some statement about the total angular momentum change and parity change, if any, experienced by

the nucleus for the particular transition involved* If the total angular momentum and parity of the ground state of the product nucleus is known, and if the changes between levels

can be determined, then a definite total angular momentum and parity can be assigned to each of the excited levels in the product nucleus. The ability to make these assignments to each oi the excited states by comparison of the experi­ mental conversion coefficients with the theoretical values

is the reason why such studies are made. 1'rom the point of view of theory, the angular momenta and parities of the vari­ ous exalted levels are of as much importance as is the knowl­ edge of the energies of the various levels.

The JC/L ratio of a nuclear transition also depends upon the multipolarity of the radiation. The advantage in

using the ii/l* ratio to determine the multipolarity of a transition over using the 1-conversion coefficient is that gamma-ray intensities do not have to be determined. To determine the ix/L ratio all that is required Is that the

intensity of the 1-oonversion peak be compared to the inten­

sity of the 1-oonversion peak. while, in principle, use of the I-/ii ratio in determining the multipolarity of a transi­ tion appears to be the better way to make this determination. 41

In practice, diffioulties are met because of the finite

resolution of the instrument used in such studies. At ener­

gies in excess of 300 kev and for medium 2, isotopes, the ii-oonvereion electron peak cannot be resolved from the L

peak lor a momentum resolution of about O.B^. Therefore the

K/(L + Li) ratio is the quantity that is determined experi­ mentally under the ciroumstanoes specified. While reliable

theoretical X and L-oonversion coefficients are available,

there are no reliable theoretical U-conversion coefficients,

i’or this reason the K/(L + I.I) ratio is not as useful a quan­

tity as the h/l ratio. Because of this circumstance, in the

work reported the h-conversion coefficient was the quantity

most used to determine the multipolarities of the radiation

from the various transitions investigated. The £./fh + i.i)

ratios were used as a check on the conclusions reached from

the comparison of the experimental and theoretical 11-

conversion coefficients.

2. Method o f Determining Conversion Co efficient s;

In determining the conversion coefficients associated

with various nuclear transitions, one begins with the rela­

tive intensities of the gamma-rays involved in the decay and

the relative intensities of the corresponding conversion

electron lines* Borne means must be found to relate the con­

version electron and gamma-ray intensities for each transi­

tion so that the conversion coefficients may be determined.

The relative gamma-ray intensities for the radio­

isotopes studied were obtained with a scintillation 42 spectrometer. The method used in determining the relative intensities oi various conversion electron lines can be ill­ ustrated by considering the way in which one can determine the Ic/fL+ii) ratio ior the 661 kev transition in The determination of the K./fL + 1-l) ratio is begun by plotting from the experimental data the K peak and the L + M composite peak on linear graph paper to suitable scales. The areas under the L peak and the L + H peak are determined with c planimeter. The area of the L peak, , is next divided by the average magnetio rigidity value appropriate to the posi­ tion of the peak in the momentum spectrum. Likewise the area of the L ■+11 peak, » is divided by the appropriate magnetio rigidity. The reason for dividing the area by Br is that the spread in momentum of the electrons that pass through the exit slit of the spectrometer increases linearly with the flux density. Therefore, dividing the area deter­ mined for each peak by B, or Br, since in the instrument used in these studies r is constant, yields areas that are essentially normalized to unit momentum interval. Areas so calculated will in the future be referred to as normalized areas. Low each normalized area is proportional to the corresponding conversion electron line intensity. fherelore , for the L/fL+Id) ratio, one can write

' ( 1 1 )

L + H since the proportionality constant for each normalized area is the same and will cancel in the ratio. 43

The .^-conversion coefficient for the 661 kev, M4 transition has been determined with good accuracy by

.naggoner.1'*' Waggoner's value of is 0.097*0.005. 1'he theoretical value of ior1*14 radiation of this energy from

Ba^7 is 0.094. The theoretical value was used in the calcu­ lations which were performed in the analysis of the data reported in this paper.

The use of the 661 kev transition in Ba1^7 to deter­ mine the conversion coefficients of the various transitions in a radioisotope under investigation involves essentially f our independent measurements , two measurements being made on each of two sources. The two sources used are an elec­ tron source of the radioisotope being studied and an elec- 137 tron source of Cs • The two sources are made as nearly alike as possible. The two measurements made on each of the two sources consist of determining the normalized areas of the various conversion electron lines with the magnetic spectrometer and the relative gamma-ray intensities with the scintillation spectrometer.

In order to see more clearly what is involved, consider the following general case. Involved in the decay of a radioisotope under investigation is a certain transi­ tion whose energy is X kev. The number of ± -oonversior electrons associated with this transition and emitted per second from the source is The number of gamma-rays associated with this transition and emitted per second from the source is H(Y)X. The corresponding terms lor the 06^37 44 source are and 11 ^^661* ^rom the magnetic spectro­ meter data, the normalized areas for the t-conTersion peaks 1 rS 7 in Ba and the X kev transition in the radioisotope under study are obtained. The relationships between the normalized areas and Ii(e..)v and B(e, ),,,, are n x k 6 o l

Nfejj.)^. r kA* (e^^, and (12)

N^ek^661 = ^ ^°k ^ 661 * (13) where A' (e, )_ is now to be used for the normalized areas, k x 1'rom the scintillation spectrometer, one can obtain if (‘f ) £5 i/l* (^ )x. the ratio of the framma-ray intensity of the

661 kev transition from the source to the gamma-ray intensity of the .a. kev transition irom the source under investigation. By definition,

•‘X = • (14)

Eq. (14) can also be written as,

a v - J jJ e jtJ -X Xif y >661 ^ek)661 or (15)

l U ' f )x i a y ) 661 N(ek )661 ’

ak = lUe)JnfVl s;______Li { * J661 # or f1G)

ak_ = (0.094) A'.(.e^ ------(17 ) 661

Prom Bq. (17) it is evident that akx* the X-conversion coef­ ficient , lor an X kev transition in an isotope under study can be determined from the results of the measurements 45 indicated and the theoretical K-conversion coefficient for the 661 kev transition in

The method discussed above for the determination of conversion coefficients is somewhat of a departure from tho usual methods employed in determining these quantities, For the mere common methods reference may he made to Hitohell.^

3. Teat of Method for Petemining Conversion Coefficients:

The method described above for determining conver­ sion coefficients was used in tho determination of the conversion coefficients ol the two principal transitions in

Ba^-*^ in order to teat the method. The two principal transi- 134 tions in Ba have energies of 605 kev and 797 kev. These two particular transitions were chosen because of their high intensities, and because the various investigators^4"^^ who have reoently studied the disintegration of Os are in reasonable agreement on the conversion coefficients for these transitions,

13 •a., C, G. Mitchell, Chap. VII, Beta- and Gamma-Bay Spect ro sconj . Interecience Publishers, Inc., Hew York, 1965.

■*"^Bashilov, Antonieva, Blinov and Dzhelepov, Izvest. Akad, Uauk. Ser. Fiz. 18., 43 (1954).

•^Gr. Bertolini , 11. Bettoni and B. Lazzarini, Uuovo Cim. 2, 273; 1., 746 (1955).

Forster and J. Wiggins, Phys. kev. 99., (1955). 17 G. L. Keister, B. B. Lee and F. H. Schmidt, Phys. Kev. 97, 451 (1955). IQ Cork, Le Blanc, Hester, Martin and Price, Phys. Kev. 90, 444 (1953). 46

The work of leister et_ al. wan oho sen as the main work lor comparison purposes because their results are the most reoent , their method o± determining the ooeflioients seems reliable, and the values obtained are in reasonable agree­ ment .i«i th those of other investigators. i-eister et_ al. _ 3 report a ^-conversion ooeffioient of 5.1ax 10 lor the

605 kev transition and (2.5 ±0.15) x 10*3 for the 797 kev transition. The value of a ^ ■ 5.19 x 1Q~^ for the 605 kev transition was arrived a t by noting that the 12/l ratio and other data lor this transition were compatible with an i£2 assignment for the radiation. The value of 5.19 x 1^“^ is the theoretical K-conversion ooelficient for this transition, with the E£ assignment to the 605 kev radiation, the conver­ sion coefficients for the other transitions can be deter­ mined independently of any knowledge of the decay scheme, leister et al. could determine the K/l ratio ol tho 005 kev transition since the momentum resolution of their instrument was 0.5/o. if’or this resolution, the L and k-conversion peaks are practically resolved at about 600 kev. The relative conversion electron intensities were determined in the usual way, and the relative gamma-ray intensities were determined by using a thorium radiator and studying the photoelectrons produced by the various monoenergetic gamma-ray groups. As noted previousl;/ , lor gamma-ray energies in excess of 0.5

Ikev, this method of determining relative gamma-ray inten­ sities is reasonably accurate. In fact, leister et_ al. performed experiments with their instrument in order to 47 verify the feet that «uoh measurements of gamma-ray intensi­

ties were reliable above 0.5 ilav. With a knowledge of the

relative conversion electron and gamma-ray intensities, the

various conversion coefficients could be calculated since

the 605 kev, 62 transition served as a medium of comparison

for the two sets of intensities.

The electron spectrum resulting from the decay of

*1 Os ^ obtained in the present work is shown in .Figure 11.

The continuous electron spectrum results from at least four

beta continue that are involved in the dieintegretion of

Os124. The two principal bets-spectra in this composite

spectrum have end-point energies of approximately 8U kev and

655 kev. There iE some lock of agreement among the various

investigators as to the number and end-point energies of the

less intense beta-spoctra. ilore will be said about the var­

ious beta-spectra later. The present discussion is concerned primarily with the conversion peaks of the 605 kev and 797

kev transitions. While from the data shown in i'ig. 11 it is not evident that there is a weak satellite resulting from a

801 kev transition making up part of the 797 kev conversion peaks, other investigators who have studied the decay of

with instruments of greater resolving power have observed this weaker transition. Because of this fact, the

resulting conversion peaks are labeled 797 & 801. T 6k A Two Gs spectra, one of which is shown in lig. 11,

were run, and for all practical purposes the two were iden­ tical. The same source .va3 studied with a scintillation COUNTS/SECOND O O 2 2 0 0 2 2 0 0 0 2 K 563 569 K BACKGROUND 605 i. 1 ata Electron Spectrum Partial 11Pig. + 605 0 6 L+M 0 0 0 2 0 0 6 2 0 0 4 2 URN (RIRR UNITS) (ARBITRARY CURRENT 48 7 7( 801) (a 7 79 K L + (0i 797 801)M 0 0 0 3 0 0 2 3 1040 49 spectrometer. Of the data obtained with the scintillation spectrometer, only inlormation on the 797 and 801 kev tran­ sitions was used in the calculations to be reported on here since the intensities of the other gamma-rays relative to 19 these are known from the work of Gabro and Leister et al. 1 *57 Two Os sources were prepared and studied with the magnetic spectrometer and the sol ntillation spectrometer.

All of these data are shown in Figures 12 through 1G. Only the ~-coaversion lines for the Cs^3^ sources were obtained with the magnetic spectrometer since this is all of the data needed to determine conversion coefficients. For the same reason, only the photopeaks of the GG1 kev gam: a-roys were obtained with the scintillation spectrometer. 1 37 One of the Os sources studied 'with the magnetic spectrometer was ’i” wide and 1" high, while the other was l/l6" wide and 1” high. The Os'1'34 source studied was l/8" wide and 1” high. The reason for using different width sources was to determine experimentally what effect source width has on the areas under the corresponding conversion 137 electron lines. Both of the Os sources were covered with 134 a film of LG 600, while the Cs source was not covered.

The result of covering a source is to increase the amount of scattering. Oource scattering of conversion electrons mani­ fests itself as a tail on the low energy side of the peak.

19 A. Oabro , id. s. Thesis (unpublished), Louisiana state University, 1955. COUNTS/SECOND 0 0 5 £ ata I3a^^ Oamma-Ka.y Partial 1£ Opoctrum (l/8,T Source) PULSE PULSE 0 0 8 0 0 6 569 a 605 KEV 5 0 6 a 9 6 ,5 3 6 5 HEIGHT 0 0 7 50 ABTAY UNITS) (ARBITRARY 0 KEV 801 a 7 9 7 0 0 9 IOOO COUNTS/SECOND 10 , — — i’ig. 13 0 0 5 ata B Gamma-riay Ba bpectrum Partial (1/16" bource) US HIH (RIRR UNITS) (ARBITRARY HEIGHT PULSE 0 0 9 0 0 8 0 0 7 0 0 6 1 ^*7 6 KEV 661 51 COUNTS/SECOND O O 400 2 I __ i. 4 ata Ba Partial 14 iig. 500 P UHE L S I E G H T(A R b l T RUN A R Y I T S ) 137 (lamma-Ba;/ Bpectrum {*" Source) P O 6 KEV661 700600 800 900 COUNTS/SECOND 0 0 4 0 0 6 0 0 8 200 O ' g 1 K-Conversioni'ig. 15 Peak Ba^*^ of { 0 8 0 2 C U R R E N(A T R B I T R AUN R Y I T S ) 2160 0 4 2 2 4 Source)" 0 2 3 2 COUNTS/SECOND 300 200 100 i . 6 il-ConversiBig. 16 on Peak BaT37 ol fl/lG” Source) URN (RIRR UNITS) (ARBITRARY CURRENT 2180 54 2260 2340 55

Such a tail is clcr vly evident in Pig. 15 which shows the

K-conversion peak of the 561 kev transition for the wide

souroe.

The photopeoks of the various gamma-ray transitions needed in the computations are shown in i'igures 12 through

14. The number of counts per second in the various photo­ peaks were determined by calculating the normalised areas under the photopeakc, i’rom the number of counts per second

in the photopeak, the number of gamma-ruys that strike the 20 crystal per second was calculated by using Bell1 s data lor

intrinsic peak efficiency. In the different gamma-ray meas­ urements the same 14-" x 1 " Uni (Th) crystal and spectrometer

were used. The source to crystal distance was the same in

each experiment. This distance was 53.5 cm.

The results obtained from the conversion electron and gamma-ray measurements are shown in Table I. The A-

convorsion coefficients calculated from the data shown in

Table I are tabulated in Table II. The values of given by Keister et al. are also shown in Table II along with the

expected theoretical values.

Concerning the data shown in Table II, a, for the

797 kev transition was not determined entirely from the data

in Table I. As is seen in Table I, data are reported for

the composite conversion line and photopeak resulting from

the 797 kev and 301 kev transitions. To determine n value

it. Bell (private communication). 56

TABLE I 1 34- 3UAG.IAHY OF L/VT/i FOB DZTEBLO. UIIIO a^’ B OF Ba

Source Transition # 1 s/sec Used (kev) A* (k) x 10 3 on ital

Cs1*^, 1/8 " 797 & 801 1 0 0 4030 uncovered 605 218 3700

Ga137 tl/4" covered 661 138 0 1370

CB1 3 7 ,l/lon covered 661 394 464

TABLE II

ll-couversioii o o eff ig i ints of Ba1 3 4

Energy ak x 1 0 3 a,IV x 1 0 3 a^ x 1 0 3 x 1 0 3 ak x 1 0 3 G8137^ h (kerr) Os137 fl/l 6 " (ave rage) Kei ater theory

797 2.6 3. 0 £.8 ±0.3 2.5 ±0.15 2.7

605 5.3 6.2 5 • 8 ± O . 6 5.2 5. 2 57 of for the 7 97 kev transit ion alone, the data of Keister et a1 . had to be used for the ratios of conversion peaks and gamma-ray intensities of the 797 kev and ‘301 kev transi­ tions. Actually, the correction to the data in Table I is

small, since the 801 kev transition is only about 15/o as

intense as the 797 kev transition. If the 901 kev transi­ tion were neglected, the resulting ^-conversion coefficient

for the 797 kev transition would be 10yo less than that given.

.From the results shown in Table II It Is evident

that the coefficients obtained in this ,.ork compare favor­

ably with those obtained by leister et_ al_. Source width appears to cause some difference in the results obtained,

but not enough to cause a qy concern especially since the two 137 Cs sources used were purposely made considerably differ- 1 ^4- ent in width from the Os source. In applying the method

developed to the study of the decay, source dimensions

were made as nearly alike as possible.

In conclusion, it is felt that the work done on

determining the two conversion ooelficients discussed for 134 Ba illustrates that the method developed does yield

reliable values for these coefficients. CHAPTER IV

THE Cb13 4 B3TA-3PECTRA

1. Previous Work:

Table III summarizes the end-point energies others have found for the beta-spectra resulting from the decay of

Os134.

TABLE III

MAXIMUM ENERGIES OP Gs1 3 4 BETA-SPECTRA fKEV)

Bertolini Bashilov Porster & Leister Cork et al. et al. Wiggins et al. et al.

06 88 70 83 80

£10 ~5T0 " 335 310 H o 410 645 654 657 655 657 6 S0 68 3

Prom Table III It is evident that there is agreement on a low energy beta-spectrum of approximately 83 kev maxi­ mum energy and on a high energy spectrum witha maximum energy of about 655 kev. There is disagreement on how many weaker components lie between the 83 kev and 655 kev bata- speotra. There is also disagreement on whether a weak beta-

58 59 spectrum exists with an end-point energy of from 68 3 to 690

kev. Keister et^ al. rather carefully investigated the part 134 of the Gs beta-spectrum in the region of 690 kev. These 134 147 investigators compared the Cs results with a Pm spec­ trum obtained with their instrument which was operated under the same conditions in obtaining both speotra. By comparing 147 134 the behavoir of the Pm ana the Ca beta-speotra near

their respective end-points. Keister et_al. concluded that

there is a higher energy beta-spectrum. In making a Permi 134 plot of the Gs beta-spectrum, these investigators found a maximum energy of 683 kev for this high energy spectrum.

£. Present Work: 134 The Gs electron spectrum obtained in the work

reported here is shown in Pigure 1 1 . The source material was obtained from the Oak iiidge National Laboratory. No

chemistry was done on this material. In studying this mate­

rial over approximately a six month period, no changes were 1 rx,A observed that could not be attributed to the decay of Gs

In Pig. 11 the portion of the spectrum from approxi­ mately 500 kev to 1 klev is shown* Por that part of the

spectrum above 10 counts/sec, 1 statistics were usually obtained, ana for that part below 10 counts/sec, Z>fo. The portion of the spectrum from 500 kev to1 Mev was analyzed because from the figure it is apparent that there is a high

energy, low intensity continuum with a maximum energy in

excess of 800 kev. 60

In fig. 11 the background counting rate has not been subtracted. One can readily see how much above background the high energy component lies. A fermi plot made of the beta continuum of fig. 11 is shown in figure 17. from the fermi plot, the beta continuum can be analyzed into two components, the familiar 660 kev component and another beta- spectrum of low intensity and a maximum energy of 0.95 ±0.15

Mev. from the fermi plot there is no reason for saying that a beta-spectrum exists with a maximum energy of 683 kev.

The discrepancy between this work and that of leister et_ al■ can be explained by noting that Keister et_ a_l. used a source with an activity of l/5Q or less than that used in the work reported here.

655 kev spectrum and produces a counting rate greater than background that is statisticolly significant in the region from 660 kev to 690 kev.

The beta-spectrum with a maximum energy of 0.95 ±0.15

Mqv is of importance in working out the decay scheme of Os^^ because the scheme proposed by Keister eJL al. cannot accom­ modate a beta transition with this energy, and, while the scheme oi Cork et_ al. can accommodate such a transition, there are other difficulties presented by the latter scheme. 0.660 MEV 0 .9 5 MEV

2.0 2.2 2 .4 2.6 2.8 3.0

{ I + P 2 )2

fig. 17 Fermi Plot for Gs^34 62

The writer feels, therefore, that the work reported here 17 A indicates that further study of the decay of G s is warranted, CHAP TAB 7

thb gon 7bbgij ;; blsctbgn gpbgtbuh of cs131

1• Introduoiton:

Barium 131 is known to decay to Gs131 by electron capture with a half-life of approximately 11.5 days. The various investigators^-3^ who have studied this radioiso­ tope have found no evidence ior positron decay that might compete with electron capture. hhen Ba^3^ decays to Cs^3^ , the product nucleus may be left in one of several exei ted states. Gonsi der^-ble work has been done in this laboratory 1 in order to determine these excited Btates in Gs and the transitions that occur between them. All of the previous work done in this laboratory in investigating this decay was periormed with scintillation spectrometers* The work

* * 1 Yu. Gideon and xwurbatov , PhyB. Bev. 7^., 392 (1CJ47).

22S. Katcoff, Phys. Bev. 72., 1160 (1947).

23Dale, Biohert, Bedfield and Kurbatov, Phys. Bev. BO. 763 (1950). 24 Bimmertnan, Bale, Thomas and Kurbatov, Phys. Bev. 80. 908 (1950).

25B. Kondiah, Ark. f. F'ys. 2., 2 95 (1950). 26 W. H. Guffey, Phys. Bev. 82., 461 (1951).

27B. Canada and A. G. Liitchell, Phys. Bev. 8j3, 76 (1951).

63 64 reported in this ohapter, whioh is concerned ^ith the mag­ netio spectrometer studies of the decay, was done in order to confirm and possibly expand upon the results that were obtained with the scintillation spectrometers, and also to determine the multipolarity of as many of the tran­

sitions as possible, primarily by means of measuring the h-

converaion coefficients of these transitions.

2 . Previ ous dprk :

i«io st of the previous work, done on the decay of

i s summarized in Tables IV and V* In Table IV are shown the 1 ^"1 numerous transitions in Gs that were reported by the

investigators indicated. The numbers in parentheses are the

relative intensities reported for the various gamma-ray tran­

sitions. All reported intensities are referred to that of

the 497 kev transition. i'or the work done with magnetic

spectrometers, the gamma—ray intensities were determined by

£9 Elliott, Gheiig, Haskins and Kurbatov, Phys. Hev. 88 . 263 (1952). 29 Cork, Be Blanc, Hester and Brice, Phys. hev. 91. 76 (1953). 30 Vv. Payne end K. Goodrich, Phys. rtev. 9l_, 497 (1953). 31 L. A. Jeffries, ti. G. Thesis (unpublished ) , Louisiana Gtate University, 1954.

^2Lu, Kelly and ^iedenbeok, Phys. Hev. 97_, 139 (1955). 'Z'X iieggB, Hobinaon and Pink, Phys. ^ev. 101. 149 (1956) . 34 H* <<. Campbell, to be published. 65

TABLE IV

RETORTED ENERGIES OI’ TRANSITIONS IN Cs131 (LEV)

(A) MAGNETIC 5P K C T A O LET S A3

Tran­ sit i ons Refererioe 25Reference 27 Reference 28 Reference 29 1 43 55 O *5 65 78__._ 3 108 92 4 122 122 (1.3) 122 124 5 153 6 196 7 r,06~f2 0 ) 8 215 9 241 ^i:L' £41 fl4) 239 10 242 11 372 (25 J 371 (12) 370 (7) 374 12 488 13 494 (100) 497 (100) 494 TlOO) 497 14 58 5 15 62 0 66

TABLE IV (CONTINUED)

(B) SCINTILLATION SP_CT.t0i£STER3

Tran­ sitions Referenoe 30 Reference 31 Reference 33 Reference 34 1 83 O 90 ( 6 * .61 3 100 4 f57j 123 (57) 122 (55“) ’123 (73.) 5 160 6 r > c A (44 J 215 » £14 (41) ’215 (55) 7 240 I OX l 240 (10.3) 9 a250 9 370 (29 ) 373 (36) 372 (£9! (365 10 *4Q_5._ (5.4, 11 500 (100 ) 497 (100) 496 (100) 497 (100 12 588 (3*6; 13 620 (8 ) 620 (9) 620 T9.3) 620 (6^4; 14 600 10*11 15 750 16 [Q> 5.6.7 17 900 Ta7 900 917 ;2aoi . 918 18 1020 74) lu20 (4 )-— 10 32 3 ,1 J 1039 (3.1J *Va lues from reference £9 used as calibrMiion points. 67

T/'.JB LE V

itEPuHTEL CONVSi\oIOi; GOEPPICIENTS AMD K/L RATIOS i'O li Gs131

E lliott ©t_ al Cork, et^ a l ®v K/L K/L K/L

124 6 . 0 i 0 • 5 3 ± 0 . 5 0 .1 3 1

8 ± 0. 6

15 0. 19 y.o ± 3. 3 ± 0. 5 0. 013

£49

374 0.010 6 .0 1 0 ,5 a= 0.013

0.0045 2 .5 ± 0.0497

620

1020 8=0.0008 68 measuring the photoelectrons ejected from a radiator. It is interesting to note the great disparity between the relative intensities reported lor a particular transition as deter­ mined by workers using magnetic spectrometers as opposed to those using scintillation spectrometers. lor the 124 kev transition, lor example, the investigations eone with scin­ tillation spectrometers yield relative intensities oi from

55;j to 73/&, whereas Canada and Ilitchell, using a magnetic spectrometer, found a value of 1.3/i*. As the energy of the transition increases, it is to be noted from the table that the disagreement between the magnetic and scintillation spectrometer work lessens. It is also interesting to note that the reported intensities lor the various transitions as determined with scintillation spectrometers are in reason­ able agreement, whereas the intensities as determined by magnetic spectrometers are not In good agreement. I'or exam­ ple, for the 373 kev transition, scintillation spectrometer data yield relative intensities of from 28^ to 36,j, whereas the magnetic spectrometer data yield intensities of Irom 7,j to 257S.

In Table Y is summarized the previously reported information obtained from studies’ of the conversion electron 1 spectrum of Cs . Irom the table two facts can be noticed, lirst , not all of the desired quantities have been measured, and second, where several measurements are reported for the same quantity, considerable disagreement exists. I’or example* for the 215 kev transition, Illliott et_ al. report a 69 value of of 0.19, whereas Jeffries finds a value of 0.042.

ITor the same transition, Hlliott et a l. report a K/L ratio of 0.8, while Cork et. al. found a value of 9 for the ratio, and Jeffries, a value of o.3. This laok of complete Infor­ mation and the disagreement on those results that were obtained oan be attributed. In part at least, to the diffi­ culty of making accurate measurements on the spectrum from a source of low specific activity, particularly with a mag­ netic spectrometer. Concerning the scintillation spectro­ meter work on electrons, provision was not made to correct or reduce the error introduced by electrons scattering out of the crystal. Scattering in the source could nlso affect the results obtained with the scintillation spectrometer.

-Both of these last mentioned effects would tend to lower the values of the conversion coefficients and K./L ratios, partic­ ularly at relatively low energies.

The data collected in Tables IV and V show why fur­ ther studies were made at this laboratory on the decay of 1^1 Ba • Campbell, some of whose work is included In Table IV and whose work was done concurrently with that reported in this paper, studied the garni: a-ray spectrum with n scintil­ lation spectrometer in an effort to resolve the obvious inconsistencies in intensities. Campbell also performed numerous gamma-gamma coincidence measurements in order to determine the sequences of the various transitions. The magnetic spectrometer studies on the Cs conversion elec­ tron spectrum were performed in an effort to resolve some 70 remaining questions about the decay of Ba^*'’’^.

3. Present rtork;

Earlier magnetic spoctromct'jr studies of the conver­ sion electron spectrum of were severely hampered by the very low specific activity of the sources then available.

Barium 131 is produoed by an n,y reaction on one of the least abundant of the stable barium isotopes. Of the stable barium found in nature, approximately O.l/o is 130 This scarcity of the 3a isotope, coupled with the fact that Ba13^ has a low cross section for thermal neutrons, make it difficult to obtain high specific activity sources 1 of Ba . In order to obtain souroo material with as high specific activity as possible, 10 mg of BaOO^ that had been enriched in the Ba^ v isotope were obtained from the Stable

Isotopes Division of the Oak Ridge National Laboratory. Of i ^ n the barium present in this sample, 27.5 yo was Ba . This sample was irradiated in a thermal neutron flux of approxi­ mately 10^^ n/sea/om^ for a period of ten days.

An analysis of the BaCO^ made at the Oak Ridge

National Laboratory showed, in addition to the carbon and oxygen, small amounts of Ca, Pe, tig, Na and or along with the various barium isotopes. Aince no extensive chemistry was to be done on the sample, calculations were made to determine if any other activities might cause difficulty in working with the sample. i'rom the calculations, the conclu­ sion was reached that no other activity should cause confu­ sion in the work to be done on the Ba^1^*. This conclusion 71 was borne out, ior the most purl, by the subsequent experi­ ments. However, in the first run (Fig. 18) with the magnetic

spectrometer, using a strong source, indi oal ions of several

weak transitions were observed that might be caused by other

eotivit i es.

The initial activity of the 10 mg of BaCO^ was

approximately 100 me. -a. small amount of distilled water was

added to the BaCO^ and the sources for the magnetio spectro­ meter were prepared from this solution. A number of sources

were prepared at different times and were numbered according

to their chronologioul order. Bourse I was1/8'* wide and

1 l/Q" high and was uncovered. The average surface dens ity

/ P was approximately 0.1 mg/cm . n conversion electron spectrum

obtained from this source is shown in Figure 10. In this

figure the abscissas are given as voltages since in this run

the field measurements were made with the rotating coil

system. The units are arbitrary. Of the various data taken 1 *51 In the study of the Cs conversion electron spectrum, only

that shown in this figure were obtained using the rotating

coil. In subsequent runs, the current through the field

coil, measured in arbitrary units, was determined as a func­ tion of the counting rate.

The purpose of this first run was to survey the

complete conversion electron spectrum. In subsequent runs those parts of the spectrum that showed certain interesting details were to be repeated if to do so was deemed profit­ able. But, because of the considerable length of time COUNTS/SECOND 10 3 g. ig F K 124 8 s nvrin ecron setu Suc I) I (Source spectrum n o ctr le K version on C Cs 18 00 00 00 5000 4000 3000 2000 M124 I -*i r r ■I OTG (RIRR UNITS) (ARBITRARY VOLTAGE K 215 K 249 L215 239 & 239 249 L -t-M K290 72 7000 K 373 _L 373 L+M K830 TK435 L 405 L405 OR K 497 8000 K915 49 49 7 L+M * 8 585 K585 K1040 K 620 6000 9000 L + M A _L L+M L + M 1040 73 required to colleat the data and the relatively short hall- life of decay of the souroe limited the more detailed study to the most significant regions of the spectrum.

Cinoe the souroe was already thicker than desired, nothing could be gained by making a now and stronger source,

Concerning the details of the spectrum shown in

Fig, 11 , a number oi facts should be noted. To begin with, the data give no clear indication of transitions with ener­ gies less than 124 kev. This is* not to be interpreted to mean that such transitions do not exist. They probably do, since a number of invectigators have observed such transi­ tions. The main reason why such transitions were not seen is that the efficiency of t lie detector falls off rapidly below 100 kev.

The conversion peaks for the main transitions, i.e., the 1£4, 215, .573 and 497 kev transitions, were all observed in this initial run of the spectrum. Conversion perks for a number oi weaker transitions were also observed. The h. peak for a 133 kev transition is clearly seen. In subsequent runs t he 1 and id peaks for this transition were also observed.

Confirming this transition is of some importance since only

Cork et al. had previously reported such a transition. Con­ version peaks corresponding to £39 and 249 kev transitions were also observed. That two transitions exist was shown also in subsequent experiments. In some previous work these two transitions were reported as one. The 515 end 6£Q kev transitions were also confirmed by the conversion peaks 74 shown. The 650 kev transition had been reported in a number ol prior investigations. The 535 Icev transition was previ­ ously reported only by Cork et_aJL. and Campbell. The three highest energy transitions, all orsome of which had previ­ ously been observed only with scintillation spectrometers, also produced h-couver sio11 peaks that were observable. In order to more easily observe the highest energy peaks, the di s criminal or setting of the amplifier used ’with the detec­ tor was, increased until the background counting rate dropped from 1 count/'sec to 0.3 count/sec. The indicated energies of these transitions are those determined in subsequent runs in which the current was measured as a function of counting rate. The energies determined for these transitions were

350, 915 and 1040 kev. The uncertainty in these values is

In addition to the transitions discussed above, the data iihow two more weak transitions whose energies are 590 and 405 or 435 kev, the uncertainty in these energies being fcyo. The data shown are all of the information that was obtained on these transitions. As previously stated, it is not certain whether these tr nsitions are involved in the 1 decay of Csx , since the data on them are quite sketchy.

In addition to the information on these transitions, the data obteined lor -Fig. 13 and that from some subsequent runs give some indication of weak transitions at roughly 90, 160,

490, 630 and 750 kev. These facts are mentioned since tran­ sitions of these energies have been reported. 75

After making the initial survey of the electron spectrum shown in I’ig. 18, a number of subsequent runs were made over certain parts )i the spectrum using various pre­ pared sources. In these subsequent runs the points of the spectrum were taken more closely so that- better measures of the areas under the conversion peaks could be made. home of the data obtained in these subsequent runs showning typical results are presented in T'igures 19 through £4.

In Tig. L9 is shown a repeat of the lowenergy part oi the conversion electron spool rum. This spo drum wee obtained from source I one and a half months alter the run shown in 1’igure 18. One added lecture to notice is the 1. peak of the 153 kev transition. j^Iso , the complexity to the right of the A peak of the £15 kev transition is resolved into the A peaks of the 159 and £49 kev transitions and the

L peak of the ‘15 kev transition*

if pure £0 shows the port.ion of too spectrum indicated that was obtained from source V. The source was l/8" wide and 1 l/QM high and covered. This source was quite thick, the average surlace density being approximately £ mg/on^.

A considerable amount of scattering is evident. It is to be realised that in this end in other figures where the vertical axis Is logarithmic that the lower part of the spectrum appears exaggerated In size relative to the higher parts.

This spectrum was run in order to get a better estimate of the A/ (L + i.x) ratios of the 373 and 497 kev transitions. An attempt was also made to use this source to produce photo- i. 9 ata O Conversion opecstrum electron Os (Source Partial 19 I) iig. COUNTS/SECOND IO 0 0 7 URN (RIRR UNITS) (ARBITRARY CURRENT 124 133 L 0 0 9 76

IIOO 2 39 K 215 M 249 4 2 K i; 2 Partial Co eso El ron detu (ore V) (Oouroe dpectrum n o tr c le E version on C l a i t r a P 20 Pip;. COUNTS/SECOND IO O 2 0 0 4 1 373 7 3 K URN (RIRR UNITS) (ARBITRARY CURRENT 373 7 3 L + M 0 0 6 1 77 I 800 7 9 4 + 7 9 4 L+M 78 electrons from a Pb radiator. The activity, however, was insufficient to produce enough photo electrons to make reli­ able measurements.

In Pigs. 21 and 22 are shown conversion electron spectra obtained from source III. This source was 3/lG" wide and 3/4" high and was uncovered. The average surface density of this source was only several micrograms per square centimeter. It is to be noted that very little scattering is to be observed. Two new features are to be observed.

There appears to be a weak peak to the left of the X peak of the 135 kev transition. If this is taken to be an fi-conver- sion peak, the energy of the corresponding transition is 94 kev. There is also what appears to be another weak peak which, if considered as an 1-conversion peak, corresponds to an energy of 158 kev. These interpretations would bring the results into agreement with those of Campbell.

In Pigs. 23 and 24 are shown conversion spectra obtained from source IV. This source was spiked with 137 a small amount of Cs , the comparison standard used in these studies. This spiking was done as a further check on the effect of source geometry. .source IV was l/8" wide 'nd

1" high. The average surface density of the source was p approximately 0.01 mg/cm . Ho new features were observed in the spectrum obtained. The conversion peaks shown in Pig. 24 belong, of course, to Bal37.

All of the sources used were also studied with the scintillation spectrometer it; order to make the necessary Pig* 21 P a r t i a l Os C on version E le c tr o n bpeotrum (source (source bpeotrum n o tr c le E version on C Os l a i t r a P 21 Pig* COUMTS'SECOND 120 0 4 eo 0 5 6 URN (RIRR UNITS) (ARBITRARY CURRENT 0 0 7 0 5 7 79 oo eo 0 5 6 0 0 9 III) ii'iC- 2? P a r t i a l Cs131 C on version E lec tro n Spectrum (Source (Source Spectrum n tro lec E version on C Cs131 l a i t r a P 2? ii'iC- COUNTS/SECOND 20 0 3 1050 K 2 2 K39, URN (RBT RY UNITS) Y AR BITR (AR CURRENT 249 9 4 2 K M 2 15 2 M 0 5 4 S ^ 0 5 H 80 497 L + M 7 9 4 1900 III) P ig . . ig P COUNTS/SECOND 12 8 6 70 4 'l‘ 2 80 > 00 1080 1040 > ' 860 B20 'Sl>‘ 740 700 660 2.3 s neso El ron Setu (ore IV) (Source Spectrum n o tr c le E onversion C Gs l a i t r a P 124 K URN (RIRR UNITS) (ARBITRARY CURRENT "1 —— 740 81 124 L J __ L 2(3 K T COUNTS/SECOND 0 3 20 i . 4 iv 24Pig. and (Source L 17)+ l.L Conversion Peaks of 2250 661 K ) S T I N U Y R A R T I B R A ( T N E R R U C 0 9 2 2 82 2330 661 LM + 0 7 3 2 83 measurements lor calculating the conversion coefficients in 137 the manner previously described. The Gs source used in

conjunction with the sources that were not spiked was the one from which the spectrum shown ini’ig» 8 was obtained.

The information obtained from the various measure­ ments is summarised in Table VI. i'rom the table it is to be

noted that a definite multi pole assignment iB made for the majority of the transitions studied. The ambiguities that

exist for the 830 kev and 1040 kev transitions probably

result from the difficulty in making accurate intensity measurements on the weak conversion lines. The ambiguity

that exists in the case of the 139 kev transition probably

results from the fact that the gamma-ray intensity dotermi-

nation is uifficult to make accurately because of the posi­

tion of the photopeak in the gamma-ray spectrum. The 115

lev transition presents another problem, however, since it

is one of the principal transitions and is rather highly

converted. Both conversion electron and gamma-ray inten­

sities for this trsns,it ion s.hould be reasonably accurate.

XI the intensity measurements are accurate to within about

2 0 ,0, then the differences between the experimental values of

a^ and the theoretical ones for the only possible rultlpole

assignments are difficult to explain.

A possible explanation is suggested, however, by

some very recent developments on the theory of internal

conversion that were published after the completion of the

work reported in this paper. Bose et a 1. in calculating 84

TABL‘D 71

OUUUAKY OP Gs131 DATA

f -day A &/or L iinero Kelati ve average Theoretical Multipole fkevj Intensity a x 10*3 K/L K/L a x 10*3 Assignment

*124 L: 290 ± 20 150 ± 10 3.21.3 7.6 Ml 59 E2 + Ml 3.2 £2 195 7.7 El 7.6 Ml *133 A: 25 ±5 6.511 3.3 E2 E2 ♦ Ml 5.3 M2 Ml 102 *215 A: loo 68 ± 15 22 108 E2 & /or Ml 8.0 El 7.8 Ml 75 A; 10 ± 1 36 ± 10 5.1 E2 75 *239 -5 L> * 2 M2 E2 i/or Ml Ml 9.4 L: 2.0 ± . 6 7.5 ± 2 E2 14 Ml 8.2 *249 L ; 2. J i • 6 7.5 12 E2 12 Ml 7.8 Ml 25 *373 A: 17.2±.5 17 ± 2 "*3.5 6.2 £2 20 E2 6 . 1 £1 7.9 Ml 11.7 *497 A : 30.1 ± . 5 7" -L. * H 10.51 1 -7.5 6.7 E2 9 . 7 * mi. 7. 2 M2 Ml 8.0 * 56 5 A: 0.5G±.10 5.8 1 1.5 E2 E2 5. 7 El *620 A: 0.471.15 2.8 ± 1.2 1.8 ------— E2 4.9 El O 94 950 A: 0.035± 1.7 ± .8 Ml 3.5 El >r Ei. .015 E2 7 . 4 Ml 2.7 915 A: 0.08-*02 1.6 — • 6 E2 E2 1.85 Ml 2.0 1040 A: u.14±.03 1.7 ± .5 E2 A/or Ml E2 1.3

Values from reference 29 used in calibrating spectrometer. 85 their table of conversion coefficients have made tho assump­ tion that the nucleus can be treated as a point source of rz c virtual photons. rieoentlj, however. Church and «

into account the finite size of the nucleus and considering the charges and currents as being distributed uniformly throughout the nucleus. One result of these calculations

is that observable deviations can occur from nose et_ al. 1 s values for various convert, ion coefficients at C = 55 and

even lower atomic numbers. Church and ..eneser have estimated

that for = 55 and 1*11 transitions that deviations as large

as 20,0 can sometimes result. These deviations can be either

increases or decreases, depending upon the particular transi - tions involved. The discrepancy that apparently exists for

the 215 kev transition in Csx^ would be explained if the

theoretical values lor the h-oonversion coefficients arc£0,.*

smaller than the values from the calculations ofnose et a 1.

In spite of the faot that deviations from --tose

et a 1. * s values can exist, it was ustially possible to make 1 '~-i definite multipole assignments for the transitions in Cs

since the maximum possible deviations of the conversion

coefficients are approximately equal to or less than the

estimated experimental errors. Because of the possibility

of such deviations, however, when a particular transition

35 il. L. Church and J.

L. W, Campbell , to be published.

E* L. Churoh and J. Weneser, Phys. Lev. 104. 1382 (1956).

Cork, Le Blanc, Keater and Brice, Phys. Lev. 91_, 76 (1953).

Craves, Danger and Moffat, Phys. Lev. 88. 344 (1952).

Leister, Lee and Schmidt, Phys. Kev. 97., 451 (1955).

L. M. Longer and B. C. Price, Phys. Lev. J76, 641 (1949); L. M, Langer and L. J. Moffat, ibid. 82. 635 (1951).

C, L. Peacock and A, C. Mitchell, Phys. Lev. 75. 1272 (1949).

M. E, Lose, Appendix IT, Beta- and Camma-Hay Spectroscopy. Interscience Publlshers, Inc." Lew York, 1955.

E. Siegbahn and II. Svartholm, Nature 157 . 372 (1946); N. Svartholm and L. Siegbahn, Ark. Mat. 1'ys. A5 3 . No. 21 (1946).

P. Shull and D. Dennison, Phys. hev • 71. 681; 72. 2 56 (1947).

N. Svartholm, Ark. f. i?ys. £., No. 14, 115 (1949).

M. A. Waggoner, Phys. Lev. 8£_, 806 (1951).

(JENELAL LIABLE 1JC OS

J.

L. D. Evans, The ^tomi o Nucleus. MoSraw-Hi11 Book Co. , Inc. , New Yo rk , 1955.

L. Siegbahn, Beta- and Camma-Lay Speotro noopy. Interecience Publishers, Inc., New York, 1955.

8 7 VITA

Loon otanley August was born in Hew Orleans,

Louisiana, In 19£6. He attended grammar and high school

the_e, graduating irom St. AloysiusHigh School In 1944.

In i otober of 1944 he was called to active duty in the

Uni ed States Unval neeerve and served until August, 1946 whe i he was honorably discharged as a Yeoman Third Jlasa.

In jeptember of 1946 he entered Louisiana ^tate University and received his B. S. degree in Physics In June of 1950.

In September of 1950 he entered the Graduate uohool of

ane University and received his 91, 0, degree in June of 195&. In June of 1955 he married Lorraine Donnelly of

He. Orleans, Louisiana. In 1'ebruary of 1953 he entered

th j Graduate school of Louisiana otate University, end he

if now a candidate for the Doctor of Philosophy degree in

tie Department oi Physics.

86 EXAMINATION AND THESIS REPORT

Candidate: l e a n * u t*- y ~ ^ « J ^ * f~

Major Field: A y s / r J I „ />(3 V fD 13 1 Til lo of Thesis: T~A e E } e c\ r 0 h- ^ p ** <- ^ h <2 * ^

Approved:

Major Professor and Chairman

\/ a P en n of tJje’T^ rm u n te School

EXAMINING COMMITTEE:

i /

\

& Q \ ?

L—

Date of Examination:

:~Tt j ; / h i /