AUTOBIOGRAPHT
I, Bichard Lewis Woodward, was horn in Kansas City,
Missouri, December 11, 1913 - My elementary and secondary school
education was in the public schools of Kansas City and St . Louis,
Missouri. In June, 1935. I received the degree Bachelor of Science in Civil Engineering from Washington University.
I received the degree Master of Science from Harvard University
in June, 1948. Since 1937 I have been an officer of tie
United States Public Health Service . Under the auspices of
this organisation, I have studied in the Department of Physics
at the Ohio State University since June, 1949-
- 42 - ACKNOWLEDGMENTS
I wish to acknowledge the assistance of my adviser Prof
M. L. Pool, of my coworker Mr. S . C. Fultz, of our chemist
Mr. James McGlotten, and of Mr. Claude McWhlrt and other members of the staff of the Physics Department shops.
- hi - BLBLIOGEAPHT
1. J. H. Buck, Fhys. Rev., 59, 1025 (1938)
2. A. Mukerji. P. Preiswerk, Felv . Phys. Acta, 23, 516 (1950)
3* G • T. Seahorg, I. Perlman, Rev. Mod. Phys., 20, 585 (1998)
9-. G. E. Valley, R. 1. McCreary, Phys. Rev., 5 6, 863 (1939)
5 . S. Sag&ne, S . Kojina, G. Miyamoto, Fhys. Math. Soc . Japan, Proc., 21, 728 (1939)
6 . E. C. Barker, Plutonium Proj . Report Mon. P-269, 8 (199-7)
7. S . E. Haynes, Phys. Rev., 79, 9-23 (1998)
8 . I*. M. Langer, R. D. Moffat, Ihys . Rev., 80, 6 51 (1950)
9. A. C. Helmholz, Phys. Rev., 60, 9-15 (1991)
10. J. M. Cork, L. N. Hadley, Jr., C. V. Kent, Phys. Rev., 61, 388(A), (1942)
11. L. Lloyd, Masters Thesis, 0 .S .U. (1951)
12. K . Metropolis, G. Reitwiesner, U.S.A.B.C., RP-1Q80, March, 1950
13- W. B. Kann, Phys. Rev., 52, 905 (1937)
19. L. H. Ridenour, V. J. Henderson, Phys. Rev., 5 2, 889 (193?)
15. H. H. Eopkins, Jr., 3. B. Cunningham, Fhy s .Rev ., 7 3, I9c6 (1998)
16. E. Segre, A. C. Helmholz, Rev .Mod .Phys ., 21, 281 (1999)
17. M. E . Rose, G. H. Goertzel, C. L. Perry. U.S.A.B.C., ORJiL - 1023, June 25. 1951) 18. M. G. Mayer. Phys. Rev., 78, 16 (1950)
19. J. A. Hunter, Doctoral Dissertation, 0 .S.U. (1950)
- 90 - Correction of this difficulty will probably require a slit
system or a more efficient ion source . A higher accelerating voltage would also he helpful. Time was not available to complete the necessary revisions to the vacuum system which installation of the slit system requires.
- 39 - The Isotopes of lead were resolved satisfactorily. Other elements tested included zinc, antimony, tin, arsenic, selenium, indium, aluminum and cadmium. Salts of several elements of interest,ihich have too hi£h melting points in the elemental form, were also tested.
These were ruthenium chloride, molybdenum oxide and a complex fluo ride of niobium. None of these provided satisfactory ion beams, either because of decomposition of the salts before appreciable amounts were vaporized or because of fragmentation of the molecules in the electron beam used to ionize the molecules with attendant reduction in the amount of material deposited at any one mass number.
Zinc was chosen as the first active element to work with.
A number of tests were made on inactive material to determine opti mum oven temperature, electron voltage and filament temperature.
Over-all efficiency of approximately 1/1500 was obtained under the best conditions, i.e*. one ion was collected for every 1500 atoms vaporized.
Two attempts to separate the radioactive isotopes were failures. In both cases, although there was apparently good reso lution, the activities of Zn^ which are made abundantly by deu- teron bombardment of zinc were found on all parts of the receiving plate. Molecular contamination was suspected and this was con firmed by finding activity on the receiving plate even when no electron beam was used to ionize the molecules. Such an effect would not be detected using inactive materials.
- 38 - APPENDIX A
MASS SPECTROGRAPH FOR SEPARATING RADIOACTIVE ISOTOPES
The mass spectrograph for separating radioactive isotopes described by Hunter (19) was found in 1951 to be giving poor reso lution. A fluorescent screen was devised to permit direct visual observation of the lines produced at the receiving plate and poor focusing was observed. Various adjustments made no appreciable improvement .
It was found that the arm of the spectrograph had incorrect dimensions for satisfying the geometrical requirement for first- order focusing, i.e., that the source, the apex of the sector mag netic field and the receiving plate be in a straight line . It was also found that the pole pieces of the magnet did not remain parallel because of the unsymmetric arrangement of tie pole pieces with respect to the main body of the magnet. A new arm was designed which overcame these difficulties and this was built in the Physics
Department shops. The power supply which provides the accelerating voltage for the ions and the current for the filaments in the ion source was rebuilt to eliminate some overloaded elements, to pro vide for more convenient operation and to improve the stability of the high voltage supply.
The rebuilt instrument was tested on Inactive samples of a number of relatively low melting point metals with good results.
- 37 - The relati-rely large cross section found for a (He^,2n) reaction suggests that a "diproton" may exist momentarily during bombardment with He^ .
- 3 6 - Summary
G a ^ decays %ritb a half life of 1 5 *2 2 +. 0.08 minutes to 250 day Zn6-5 by emission of a positron with a maximum energy of 2.2 mev
(determined by aluminum absorption). K capture also occurs * No gamma ray is found. The $6 kev gamma ray presently assigned to this isotope results from decay of Ga?®.
The assignment of a 48 minute half life to G a ^ is almost cer tainly in error. No such activity was found in numerous bombardments of enriched Znfi** with protons of 7 -5 a*td. 3 .^ 5 maximum energy.
Buck (l), who reported this activity, used natural zinc and reported a threshold of 4.1 mev for production of the Isotope by proton bom bardment .
A (p.He ) reaction is produced in Zn by 7.5 ®ev protons.
The half life of G a ^ has been accurately determined to be
9.55 ±. 0.03 hours. / n The 93 kev gamma ray accompanying the decay of Gu 1 results from decay of an isomer of Zn^ with a half life of 0 .3 5 ± 0 .1 5 milli seconds . The 180 kev and 297 kev gamma rays are in case side and lead directly to the ground state of Zn * . The 93 kev gamma ray is probably magnetic quadrupole radiation and the spin of the isomeric state is probably 9/2 . A decay scheme for G a ^ is proposed.
He^ bombardments of copper produced the following reactions:
(He^t He**), (He^.n) and either (He^.gaassa) or (He^,2n) or both.
Relative cross sections for the various reactions have been measured.
- 35 - particle entered into formation of a compound nucleus, a probable reaction would be the ejection of a neutron. This would appear to be a (He^,2n) reaction. Bombardment of enriched Cu^-5 with He^ would be desirable to eliminate the possible interference of a cap ture reaction on Cu63.
- Jit - 15 minute positrons could not be determined accurately but they were relatively energetic. It is reasonable to ascribe this activity to GeA-* produced by a (He^,n) reaction on Cu®-^ . The 3.26 day activity is due to Ga®7 produced by a (He-^.n) reaction on Cu®^ .
The reaction by which Ga®® was produced could be either Cu®-^
(He-^, gamma) or Cu®-* (He^,2n) . The fact that Ga®® was apparently not produced in any measureable amount tends to favor the latter reaction as the more likely. However the relatively large cross section for this reaction computed on the assumption of only the (He-^,2n) reaction occurring indicates that both reactions may take p l a c e .
The relative cross sections for the production of the vari ous activities are as follows:
Relative Product Hu c I c u m Cross Section
Ga®7 9
Ga6 6 6
Ga®5 1
Cu6if 1
Cu6 2 1
A possible explanation for the apparently large cross sec
tion for the (He^f2n) reaction postulates the existence of a
"diproton" which could be formed as a result of polarization of
the He^ ion in the coulomb field of the target nucleus. If such a
- 33 - Activity - Arbitrary U n i 13 0 0 0 0 1 0 0 0 1 0 0 1 _ o i - - - 0 4 Fig* I 0 tooo- GU V - u He - + Cu XVI 0 or atr bombardment after Hours _ 80 iue Atr Bombardrent After Minutes cr - 32 — S h o r t Ac t S h o r t ■ ^ : 3 ea Cur^e Deca* 0 2 1 A ? S 160 level with the Mine oscillator quantum nuater at the ground state would have the same parity . Thus the most likely spin assignment
for the isomeric state of Zn^? is 9/2 . Cu + He^ Bon>>^T-A^»Tit.a r Copper was bombarded with He-^ of
16 mev maximum energy. Since He^ has not previously been used as a bombarding particle in cyclotrons, the bombardment served the dual purpose of determining the nuclear reactions induced by He^ and of assisting in making the proper mass assignment to G a ^ and Ga^->. Three samples were bombarded. All three followed similar
decays. The activities produced had half lives of 10 minutes, 15
minutes, 1.9 hours, 9*5 hours, 12.9 hours and 3 *2 6 days (Figure XVI) . The decay of the activities was followed through various aluminum absorbers to obtain measurements of the energy of the particle radiations. Positron activities predominated. The 9«5 hour activity had positrons with an energy charac teristic of Ga^. The 1.9 hour positrons showed an energy of about 0.6 mev and are probably due to produced by a (He^.p) reaction
on O 16 . Bombardments of other elements which oxidize readily showed this reaction to be quite probable and there may have been a small amount of oxide film on the targets . The 10 minute activity
showed a positron of about 3 maximum energy and is attributed to C u ^ produced by & (He-^, He^) reaction on C u ^ . The 12.9 hour activity showed only low energy particles and is attributed to Cu64 produced by a (He-^, He**) reaction on C u ^ . The energy of the
- 31 - 0 1 2 3
Log E ( kev )
Fig. XV - Half Life us. Energy for Ganma Rays of Various Multipolarities (From Segre and Helmholz Mod. Ph.ys.2l, 281 (19 49)
- 30 - Figure XT shows curves from Segre and. HeImhoIs review of nuclear Isomerism (1 6) relating the half life of gamma rays to energy and order of transition. It is seen that the 93 kev tran sition corresponds to an Jt * 3 transition which could he either
electric octupole or magnetic quadrupole radiation. Rose et al.(l7) have computed conversion coefficients for the K shell for various energies down to 153 kev and for various Z and multipolarity of
transition for b>th magnetic and electric radiation. A linear extrapolation of these results to lower energy (which would give a result s omewhat lower than the correct one) gives conversion coef
ficients of 3 *5 for electric octupole radiation and 0 .t7 for mag
netic quadrupole radiation. These values should he increased by the amount of conversion in the L shell to obtain the total conversion coefficient. Helmholz (9) found the X/L ratio to be 8 and the total
conversion coefficient to be 0.75 * The available data seem to in
dicate that the 93 kev transition is magnetic quadrupole radiation.
The measured spin of the ground stats of Zn^^ is 5/2 . The
Mayer shell model (18) predicts that N = 37 all but one of the
*5/2 will *»• filled. The next levels in order are P^yg and
Cqy2 * Either of these levels would permit the necessary spin change
to the ground state of Zn^?. However such a trams it ion requires a
change in parity. The C^y2 1®TS* *lth an oscillator quantum number
of four would have different parity from the f^y2 £r Zn67 with an oscillator quantum number of three, whereas the P^y2 Ga6 ' Decay Scheme 3 . 2 6 day Ga67 100 kev S 7 297 kev GM kev STABLE Fig. XIV -- Proposed Decay Scheme fo" Ga^ - 28 pulses were in the Initial inch of the sweep and that tie number thereafter was essentially constant. As a further cheek, the sweep was triggered by the 93 kev gamma ray and pulses from the G-M tube fed to the vertical input of the oscilloscope . A random distribu tion was found. To determine whether the gamma rays in cascade lead tc the 93 kev energy level or pass directly to the ground state of Zn^, the pulses from the 180 kev gamma ray were used to trigger the oscilloscope sweep and pulses from a G-M tube insensitive to x-rays were ibd to the vertical input. Approximately 90$ of the pulses from this tube were due to conversion electrons of the 93 kev gamma ray. If the gamma rays in cascade led to the 93 kev energy level, a decay similar to that observed in the x-9 3 kev gamma experiment would be apparent. If the gamma rays in cascade led directly to the ground state of Zn^?» a random pulse distribution would result. The random distribution was found. Thus the decay scheme for Ga^? is as shown in Figure XIY. This is the first isomer of a stable nucleus reported with a half life of this order of magnitude. A number are known with much shorter half lives and a number with longer ones but there have been none previously reported with half lives longer than the 22 micro second Ta1®^ but shorter than 5 *5 second . There is no apparent theoretical reason for this but the greater ease of working in the microsecond region or the region of times from seconds upward may account for the absence of such isoswrs heretofore. - 27 - Act iv ity 100 10 Fig. I T, = 2 5 0 in i c sc sc cion ind-=. 0 i 5 c 2 = T, XIII - Hal - i i f Time -Microseconds ife feterninat:ons Isomer of of - 26 I 0 00 - I I 500 Z n 67 Tabla IV Data for Datermination of Half Life of Zn ? Isomer (Sweep Rate 500 microseconds per inch - Triggered hy X-ray) Vertical Counts t x ^r inch- 2i minutes 1 Counts nor inch - x minutes Input 1 st Inch 2nd inch 3rd inch 1st inch 2nd inch 3rd inch 94 ker 81 90 80 11 9 6 117 76 76 17 6 3 95 78 62 10 10 4 99 92 74 8 7 5 94 90 78 12 10 4 .Total 486 426 370 _ 58 _ __ 41 22 180 ker 31 31 27 4 2 3 30 33 30 6 3 5 28 23 22 2 4 4 24 32 32 5 6 5 26 29 29 3 6 6 Total 139 148 14© 20 21 23 Mean of Totala _____ 142.3 _ _ 21.3 Count x-ray 2 2 .8 x-ray 2 2 .6 Rates 94 ker 252 94 ker 31.0 180 ker 102 180 ker 23,1 Random 252. , 31.9 , e 142.3 - 352 c 21.3 * 2 ? .2 Counts 1 0 .2 . I 23 a Vertical Counts i»r 1 /2 iiich — 2 ailnutes __ _ Input 1 st 1 /2 " 2nd 1 /2" 3rd 1/2" 4th 1 /2 " 5th 1/2" 6th 1 /2 " 94 ker 32 23 ! 21 23 22 17 29 22 20 20 18 17 30 25 23 19 19 16 35 27 25 24 21 20 26 24 22 25 17 18 31 29 23 22 18 15 Total 183 150 134 133 115 103 180 ker 8 12 9 6 9 9 10 7 7 8 6 9 6 11 8 8 7 8 6 6 7 7 7 7 8 8 6 9 8 8 9 _ _ 6 ____ -6 _ 6 9 6 Total 47 50 45 44 46 47. Mean oi Totals _ .46^5. ___ Count x-ray 2 3 .5 Rates 94 ker 137 180 ksv 59.8 Random 137 x 46.5 - 107 Counts . . 5 9 .8 . - 25 - for the 93 kev gamma ray provided the vertical deflection. By vary ing the sweep rate, the apparatus becomes essentially a coincidence mixer with a variable resolving time. The number of pulses per unit length of sweep was counted at several places along the sweep. At a sweep rate of $00 microseconds per inch, it was found that the num ber of pulses per inch was greater in the initial part of the sweep than in the latter part . The correction for random pulses was made by counting the pulses in a similar way with the 180 kev gamma ray providing the vertical input. By avoiding the origin of the sweep where prompt coincidences would be found, a random distribution of pulses is obtained. Correcting the rate thus found by the ratio of the count rate of the 93 tcev gamma ray to that of the 180 kev gamma ray, one obtains the expected random rate to be subtracted from the observed rate to obtain the net count rate. Since this is a novel method of determining a disintegration rate, the data obtained are included in Table IT. Figure XIII shows three decay determinations made in this way. The average of the three determinations was 0.35 milliseconds. The precision of the measurements is poor, partly because of the large random correction which must be made and partly because of the uncertainty in measurement of the sweep rate . The average result obtained is considered to have a standard error of approximately 0 .1 5 milliseconds. As a rough check on this determination, a sweep rate of 2500 microseconds per inch was used. It was found that most of the - 24 - PHOTOMULT i PLI E * N* 1 PULSE A ft k ‘ Y 2 E R CATHODE ft AY OS CI L L OS C 0 PE {IQ. XII - Diagram of Apparatus for Measuring She r t Ha If Life - 23 - there ware gamma-ganma coincidences between the 180 ker gamma ray and the 29? kev gamma ray. The resolving time of the coincidence apparatus was approximately one microsecond. It is apparent from the above results that the 180 kev and the 297 kev gamma rays are in cascade and t hat t he 93 kev gamma ray represents a transition from a metastable state. Since this me testable state would be an isomer of Zn^, sev eral attempts were made to isolate it by making chemical separations of the sine and gallium. To an ether solution of Ga^Cl^ a few milliliters of 6N HC1 containing Zn carrier was added and the mix ture shaken in a separatory funnel. After the acid and ether layers had separated, the acid portion containing the zinc was removed and the decay of its activity followed. A small amount of the Ga^? activity was invariably present but no shorter half life was found in any of the sine fractions. The separation can be made quickly and in one instance the zinc ffaction was being counted three min utes after addition of the acid to the ether solution containing the gallium activity. Considering the activity of the sample used, a half life of one minute or longer would have been detected had it been present. To investigate the region between a few microseconds and one minute, an apparatus using an oscilloscope with a variable sweep rate was used. A schematic diagram of the apparatus is shown in Figure XII. Pulses from an x-ray sensitive G—M tube triggered the horizontal sweep and pulses from the scintillation spectrometer set CiA t N S a A 1 K - | >* i ► I 00 Mlse Heljkt - Volts Pu1»e He i ght - Vr 11 I to Ga67- t 80 Kev y > »- Ga*7 h* - 197 kev y K O U <*L / < 2,0 8.0 Pvlte Height - Volte Pul»£ He : gnt * i r 1 ts Fig. XI - Ga67 - Typ icul Spectra from Gamma Ray Scintillation Spectrometer * RUSTIC Pb Fe plastic o M « s M — 500 1000 1500 2000 2500 mg/cm2 — Lead Absorber i n i::kne-i j Fig. X - Ga67 -X-y Coincidences per y count vs. Lead Absorber Thickneso apparatus was approxlnately two microseconds . Both x-ray - g u m a ray coincidences and ganna-gasBA coincidences were found. Figure X shows the variation in the x~gaana coincidence rate per gamma ray counted as the gamma rays are absorbed by lead. If all gamma rays followed promptly after K capture, the coincidence rate per gamma ray detected would remain constant as the gmma rays are removed progressively by the lead absorbers . The rise in the curve shows that one of the gamma rays does not contribute t o the coincidence ate. This gamma ray therefore has a half life rela tively long as compared with the resolving time of the apparatus. From the rate at which the curve rises, the 93 kev gamma ray is con sidered to be the one not in coincidence with the x—rays . The gamma rays of Ga^? were also examined on a scintillation spectrometer. Typical spectra are shown in Figure XI. This figure also shows the peaks of the 660 kev gassaa ray which accompanies the decay of C a ^ ? * These serve as mklibration lines . The three gasuna rays of Ga^? are well resolved and the peaks rise above relatively low background. This facilitates coincidence measurements using the scintillation spectrometer. With the scintillation spectrometer and an x-ray sensitive G—M tube in a coincidence apparatus, it was found that there were true coincidences between the x-rays and the 180 kev gamma ray and between the x—rays and the 297 kev gamma ray but no coincidences between the 93 kev gammm, ray and the x-rays. It was also found that - 1 9 - _ -O c. tr> k- i (0 _ I ) Act ivity i. IXFic. 10 mo/cm Ga Lead Absot pt ori Curve 00 67 Lead - Absorption Cu've G .7 H Absorber - Hb 1 - 18 - 15000 iiOOOO <•5 000 too c D <0 l_ -© I— t 10— X-Ray Act 1v i t y > ■*-« u Gamma Act i v.i ty 25 50 75 100 125 150 175 mo/cm.2 Absorber Fig. V I 11 — Ga67-AIuminum and Po I ystyrene Absorption Curves - 17 - bardments and since the energy used during this work was greater All than that used by Buck, it is quite doubtful that Ga was produced In his experiments. Gallium - 6 6: Copper was bombarded with 20 mev alpha parti- cles. The activities of Gaf i f i and Ga were produced. Four measure ments of the half life of G a ^ over a total of 28 half lives showed a mean value of 9>55 hours with a standard error of 0.03 hours. It is believed that these measurements are more accurate than those previously reported (l) (13) (19-) (15) . 67 Gallium - 67: Ga was produced by deuteron and proton bom bardment of natural sine and by proton bombardments of enriched Z n ^ O . The gallium fraction from the bombardments of natural sine was separated chemically from the other target material. After de cay of the shorter-lived activities, pure Ga^? activity was obtained. It deoayed by orbital electron capture with the well known 3 .2 6 day half life. Figure Till is a typical aluminum absorption curve for this activity, showing the soft conversion electrons from the 93 hev g*""* ray and the characteristic zinc x-ray accompanying K capture . Figure IX is a lead absorption curve for Ga^? . The curve cannot be well resolved into the three components reported by previ ous investigators on the basis of beta ray spectrograms. Helmhols (9) mentioned tils same difficulty. Coincidence me&surements were made on Ga^? using an apparatus with two x-ray sensitive G—M tubes . The resolving time of the - 16 - tides used. Even though such energy estimates are subject to con siderable error, the relative cross sections for the two activities and the manner in which they vary with the energy of the bombarding particle Indicate that the two activities are produced by different types of nuclear reactions . Thus the 15 minute activity should be assi^ied to Ga^ produced by a (p, gamma) reaction on Z n ^ . The average of 26 determinations of the half life of this activity is 1 5.2.2 minutes with a standard error of 0.08 minutes. — 64; Xn none of the proton bombardments of Z n ^ has the h8 minute activity reported by Buck (l) been found. He reported a threshold of h-.l mev for production of this activity. Since re peated efforts even with 7*5 mev protons showed no such activity, it is concluded that the original assignment was in error. Table III shows the energy differences between gallium and zinc isotopes of various mass numbers as computed from the Fermi semi-empirical mass formula (13). These energy levels are in rea sonably good agreement with those computed from the decay energy for Ga^ and Gn^®. For Ge^t which decays by K capture, no such direct check is available but the small ensrgy difference computed from the formula is consistent with the observed mode of decay. The computed energy difference also checks reasonably well with the decay energy of G a ^ as reported above . If the computed mass difference for G a ^ is approximately correct, it is unlikely that tiis isotope was produced in our bom- - 1 5 - herded for 30 minutes with 7*5 mev protons and in foils of natural sino 'bombarded in 1951 by Lloyd (ll) . This proves that the (p.gaama) reaction on Zn is produced at both the higher and lower proton energies used. Table II shows the relative cross section for production of Ga activity to that of the 15 minute activity. These were com puted from the &tlos of measured activities corrected to infinite bombardment time and adjusted for the difference in isotopic abun dance of Zn®**" and Zn**®. The influence of the (p, gamma) reaction on Zn®? in producing Ga®® was neglected, and since the radiations from the two activities are similar in energy, no correction was made for difference in counting efficiency for the two Isotopes. At both energies the cross section for formatlon of Ga6ft ° was much greater than the cross section for production of the shorter half life and the relative cross section increases by more than 11 times as the energy is increased from 3*^5 mev to 7*5 mev. Buck (l) gives a threshold of 3.7 mev for production of Ga®® by proton bombardment and, regardless of possible inaccuracies in such measurements, it is certain that the 3*^5 mev proton energy used in the present work is not far above the threshold. The Q, for a (p,n) reaction producing a 1.8 mev positron emitter is at least —3*3 mev. To produce the 15 minute activity with its 2.2 mev posi trons by a (p. n) reaction would require protons with more than 3*7 mev,which is greater thatn the estimated energy of the bombarding par- - 1 4 Activity - Arbitrary Units 2 3 — U - U — 7 Kev 67 Tpcl pcr rm am a cnilto Spectrometer Scintillation Ray Gamma from Spectra Typical - 0 0 0 0 0 100 90 80 70 60 50 us Hih -VcIts HeightPuIse Curve A- ICnictes rcmove-J viCi 1120mQ/cr' 5 - Curve B- Particles -emoveJ m; ^ not ‘a -A ! Curve C- X-ray Activity from Curve A. D- X- 4 - C => 3 - ♦j > u -a 50 100 1 5 0 200 2 5 0 oiq/ cm.* - Aluminum Fi q. VI — Zn«*0 + P - A Iu m nun Abaerpt i on o f • . i Get 'omu ^fi .* t . c fJa j .i t t cn > M 15 minute Gallium Activity - 12 - Si®liar families of decay curves were obtained of electro magnetic radiations from the short gallin® activities produced by 3 *65 mev protons on Zn^O . In one instance the positrons were re moved by 1120 mg/cii^ of beryllium. In another instance the parti cles were removed magnetically and only electromagnetic radiation counted. In both cases aluainum absorption measurements showed the presence of characteristic zinc x-rays (Figure TI). The gamma rays from a Zn^O target bombarded with 3 .6 3 mev protons were examined on a scintillation spectrometer to check for the presence of the gamma rays presently assigned to Ga^5 . Except for annihilation radiation, no gamma ray peak was found. The region between 40 kev and 140 kev was scanned a number of t imes but no peak was found in this region. A target of Zn?®0 similarly bombarded was examined on the same instrument and a peak found corresponding to an energy of 67 kev. This is probably the same gamma ray as the one reported by Valley and McCreary (4) with an energy of 56 kev. The peak decays with a half life of 20 minutes. It Bhould be reassigned to Ga?°. Figure VII shows typical spectra of the region between 40 and 140 kev for both Zn^O and Zn^®0 targets bombarded with 3*65 mev protons. Boreal sine was bombarded for six hours with 7*5 mev protons and for five hours with 3*&5 mev protons. The sine fraction from both bombardments vss found to contain the 230 day activity of Zn6^. 64 A small amount of this aotlvity was found in a sample of Zn 0 bom- - 11 - Activity - Arbitrary Units (00 (00 - - 0 1 F 0 0 2 i g . V -- Zne O O U GOO GOO (Hfl/cm.1 LMNM ABSORPTION ALUMINUM 15 Gallium A^iiv.tyminute 0 0 8 Aluminum - OO 20 UO iGOO IUOO 1200 lOOO 10 - Aluminum mg/ 0 8 0 1 mev. 2 . 2 " ^ Absorption cm 2 Aiu*.': cm2 Aiu*.': J3 >< rt CO L_ Activity I 100 10 Fig. IV 50 2 12 8 Zn**0 Chemistry hog various Through iue Atr Bombardment After Minutes + p (3 (3 p 100 Tikes n mg/cm in Thickness , Numbers are Aluminum Absorber .65 9 - 9 - -Gallium mev) 15- mi 150 lmnm Absorbers. Aluminum Frac rac t i Dec.iy - on nute 200 bomb.irdme.i 2 250 100 ALUMINUM ABSORPTION c Cu61 5 5 0 mg/cm. 1.2 mev Fig-III — Zi*6^ + p (7.5 mev) - Aluminum Absorption of 3.4 hour Copper Activity showing 1.2 mev Positrons 100 - x> I Gallium Fraction i Copper F ractio n ^1 - 3.1* hours 0 2 6 8 10 12 u Hours after Bombardment Fig. II - Zrr^O + p ( 7.5 mev ) 10 Minute Bombardment - Chemistry Decay of Positron Activities Zn 0 v&> bombarded with 7*5 protons for 10 ainutes and & chemical separation parformed on a portion of the target. The gallium fraction showed positron activities with half lives of 15 minutes, 68 ainutes and 9.5 hours (Figure II). The copper and zinc fraction showed a positron activity with a hour half life. No negatrons were found. These measurement a were made on an apparatus which dif ferentiates magnetically between positively and negatively charged particles. Chemical separation of the activity from a 20 minute bombard ment of Z n ^ O with 7 »5 protons Bhowed that the 3 -^ hour activity was due to copper. An aluminum absorption measurement (Figure III) on this activity showed that the positrons have a maximum energy of 1.2 mev. The activity is obviously Cu^^ formed by a (p,alpha) reaction on Z n . This is the heaviest element on which such a reaction has been reported. 64 . Bombardments of Zn 0 with protons of 3 *65 meT maximum energy produaed gallium activities with 15 minute and 68 minute half lives . Both activities docay by positron emission. The decay of these activities was followed with various aluminum absorbers between the sample and a G—M tube (Figure IV) . The contribution of the 15 minute activity to the total activity was found from these decay curves . Figure 7 shows an aluminum absorption curve obtained from these data. The positron energy so determined is 2.2 mev. - 6 - Table I Percentage Abundance of Stable Zinc Isotopes in Natural and Enriched Zinc Material 64 66 6? 68 70 Natural Zn 48.87 2? .62 4.12 18.71 0.69 Enriched Zn^ 93 .12 6.29 0 .1 6 0 .4 3 < 0.01 Enriched Zn8? 18.7 10.5 5 6 .0 8.1 6 .7 Enriched Zn88 2.7 2.8 0.4 93.9 0 .3 Enriched Zn?® 29.8 19 .4 3.5 13.5 32.0 Table II Relative Cross Sectionslone for IProduction of Ga^8 and 15 minute Ga Activity by Proton Bombardments Relative Cross Section for Production Activity by Proton Bombardment of Zinc 7.5 mev Protons______3.65 mev Protons Ga68 260 18 15 min. Ga 1 1 Table III Energy Differences between Various Gallium and Zinc Isotopes Mass Ho .______64______6i_ _ . _ 66______62______6§_ Computed Energy Diff. 7.48 3.45 5 .1 1.1 2.84 m e v . Decay Energy Experimental T 3*2 5*1 * 2.9 - 5 - daunt and lowsr-snergy positrons as well as at least three gamma rays. There is also evidence of K capture . Mukerji and Prelswerk (2) have studied the positron spectrum of Ga and found positron groups of 1.8 mev and 0.8 mev maximum energy with a gamma ray of 1.0 mev. The radiations of G a ^ were studied hy Helmholz in 1941 (9) and by Cork et al.in 1942 (10) . Both found gamma rays of about 93 kev. 180 kev and 297 kev. Cork et al. also reported a 172 kev gamma ray. Helmholz gave a value of the K/L conversion ratio of 8 and a conversion coefficient of 0.75- He proposed a decay scheme based largely on intensity measurements . No coincidence measure ments were made. [Experimental Be suits foYlHV1!?1 - 6*i: Natural zinc bombarded with 7*5 mev protons showed such a small proportion of activities with half lives shorter than one hour, that it was necessary to use electromagnetically en riched Zn^* for subsequent work. Table I shows the percentage abun dance of the various stable Isotopes of zinc in natural and enriched zinc. Although the enrichment with respect to Zn is less than two. the abundance of Zn^® which gives rise to the 68 minute activity of Ga**® is reduced to less than one—fortieth of its natural abundance in enriched Zn^\and Zn?° from which the 20 minute negatron activity of Ga?® is produced is virtually eliminated. 4 - found to have a half llfo of 20 minutes and to decay by negatron emission (5 ) (6) (7 ) • The negatron spectrum of Ga?® has been studied by Haynes (7 ) and found to have an end point of 1.65 mev. No gamma rays with energies greater than 600 key were found but the lower energy region was not explored. The other three neutron-deficient gallium isotopes can be pro duced by bombarding other elements than sine and their mass numbers can thus be determined. Alpha particle bombardment of copper pro duces predominantly positron activities with half lives of 68 min utes and 9*5 hours. Fast neutron and hl^i energy gamma rey bombard ments of gallium produce the 68 minute activity. All of these activ ities are also produced by proton and deutron bombardment of zinc,and all of them follow gallium chemistry. From this it is apparent that the 9*5 hour activity is due to G a ^ and the 68 minute activity to Gaw68 . Another gallium isotope produced by both proton and deuteron bombardment of sine and in quite small amounts by alpha particle bom bardment of copper decays by K capture with a half life of 3.26 days . It is also produced by alpha particle bombardment of zinc but not by neutron bombardment of gallium. It is obviously Ga**? . Ga^* and G a ^ are by no means certainly placed. One of the objects of this research has been to clarify this situation. The mode of decay of G a ^ has been studied by Langer and Moffat (8) and by MuterJi and Preiswerk (2 ). It has a complex spec trum with a positron of h.l mev maximum energy and three less abun- - 3 - Mass Number € 2 63 64 65 66 67 6 8 6 9 7 0 Nl 2 H 2 8 4 1M 2 9 Cu ■ • 4 OT» * 30 Zn 40 l ] N 20 3M 3 IGa fi*. I — Portion of Chart of the Nuclides K fLftCTItOM (IRTURI ( N l U C * 0^ B* I* M E V Radioactive Nucleus { , N * i r L< Ft CtaCtfGr Or paom'namt r 1 *• mev \ r N C * H * or 0 ' fA WEv 4 e* T 3 t Stable Nucleus i NuCcU* inn ii* uR'M of 0 If SlO* muTDON CAfTijttf 0 '* BCMl - 2 - HAD 10ISOTOPESS OF GALLIUM Introduction Although the radioactive Isotopes of gallium were among the early ones studied after the advent of particle accelerators, sev eral of the neutron-deficient isotopes have heen inadequately inves tigated. Figure I shows the pertinent section of the chart of the nuclides. £■ * Ga was reported by Buck (1 J in 1933 to be a positron emitter with a half life of 48 minutes . There has been no subsequent pub lished confirmation of this activity. Recently Mukerji and Preis- werlc (2) have r eported failure to find this activity. G a ^ is listed by Seaborg and Parisian (3 ) as having a half life of 15 minutes and decaying by K capture with gamma rays of 56 kev and 116 kev. The basis for this assignment is not clear. The references cited Include little evidence which would support such an assignment. Virtually all of the published information on this iso tope is contained in a paper by Valley and McCreary (4 ) . Using a beta ray spectrograph, they studied the conversion electrons from proton and deutsron bombarded zinc targets. They found lines indica tive of gamma rays of 5*> hev and 116 kev with a half life of 18 minutes from the deuteron bombarded targets. The proton bombard ments gave rise to the 56 keT line but did not produce the 116 kev line. They attributed the activity to Ga?®. This Isotope, which can be produced by slow neutron irradiation of gallium, has been LIST OF FIGURES Figure I. Portion of Chart of the Nuclides ------2 Ah Figure II. Zn 0 + p — Decay of Positron Activities - — 7 Figure III. Zn 6A0 + p - Aluminum Absorption of 3 hour Cu. Activity ------8 Ah. Figure IV. Zn 0 + p - Decay through Various Aluminum Absorbers ------9 Ah Figure V. Zn 0 + p - Aluminum Absorption of 15 minute Ga Activity ------10 Ah Figure VI. Zn 0 + p - Aluminum Absorption ofElectro magnetic Radiations of 15 minute Ga Activity — 12 Figure VII. Zn^O + p - and Z b 7 ° 0 + p - Typical Spectra from Gamma Ray Scintillation Spectrometer ------13 Figure VIII. G a ^ - Aluminum and Polystyrene Absorption Curves ------17 Figure DC. G a ^ - Lead Absorption Curves ------18 Figure X . G a ^ — X-Gamma Coincidences per Gamma Count vs. Lead Absorber Thickness ------20 Figure XI. G a ^ - Typical Spectra from Gamma Ray Scintillation Spectrometer ------21 Figure XII. Diagram of Apparatus for Measuring Short Half L i f e ------23 Figure XIII. Half Life Determination of Isomer of Zn^? - - 26 Figure XIV. Proposed Decay Scheme for G a ^ ------28 Figure X V . Half Life v s . Energy for Gamma Rays of Various Multipolarities ------30 Figure XVI. Cu + He^ - Decay Curve ------32 - ii - TABLES OF CONTENTS Introduction ------1 Exper imantal Result* ------4 Gn65 ------4 Ga6 4 ------______----- 15 ---______------______16 Ga6? ------16 Copper + He-^ Bombardment s - 31 Summary ------35 Appendix A. Mass Spectrograph for Separating Radioactive Isotopes - — 37 Bibliography ------40 Acknowledgment s ------4l Autobiography ------42 LIST OF TABLES Table I Percentage Abundance of Stable Zinc Isotopes in Natural and Enriched Zinc — - - - — 5 Table II. Relative Cross Sections for Production of Ga®® and 15 Minute Ga Activity by Proton Bombardment ------5 Table III. Xnergy Differences between Various Gallium and Zinc Isotopes ------5 Table IV. Data for Determination of Half Life of Zn67 Is on e r ------25 - 1 - 9 & 1 G 3 4 RADIOISOTOPES OF GALLIUM DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Ifeilosophy in the Graduate School of The Ohio State University By Richard Lewis Woodward, B.S ,, M.S . The Ohio State University 1952 Approved hy _ A Adviser