RADIOACTIVITY OF PROMETHEUM
DISSERTATION
Presented in Partial Fulfillment of the Requirements
for the Degree Doctor of Philosophy in the
Graduate School of The Ohio State
University
By
John K‘. ^ngj B. S.
The Ohio State University
1963
Approved by* ACKNCWIEDGEMENTS
I wiah to acknowledge the assistance of my adviser. Professor
M. L. Pool, for his guidance and encouragement in the course of this work. I also wish to acknowledge the cooperation of ÎÆr. Paul Weiler
and other numbers of the staff for their operation of the cyclotron.
i i CONTENTS
Page
Introduction ...... 1
Apparatus ...... 7
Half-lives ...... 12
The 2,7-hour activity The 5.3-day activity The 40-day activity Activities of the order of one year Long activities
Radiations ...... 32
Radiations of the 2.7-hour activity Radiations of the 5.3-day activity Radiations of the 40-day activity Characteristics of the 1-year activities Characteristics of the 3.5-year activity
Discussion ...... 44
i i i INTRODUCTION
Prometheum is element number 61. In the table of the elements, prometheum stands between the rare earths neodymium, number 60, and samarium, number 62. Persistent efforts to discover traces of pro metheum in association with other rare-earths in nature have failed.
It is concluded that the element does not occur in nature (l), and all of its isotopes are unstable.
When the cyclotron became available as a tool for the transmu tation of isotopes, attempts were made to produce and identify isotopes of prometheum. One of the most successful of the early attempts occurred at Ohio State University (2). This early investi- 148 gation reported some charaoteristies of the 5.3-day Pm activity which have not required revision since that time.
Other prometheum activities which were established within the next few years included the 2.3-year Pm^^^ (S) and the 2-day Pm^^^
(4). Several reports on Pm^^^ were also made, but these showed some disagreement and rather inconclusive identification of the isotope involved. Wu and Segre (5) reported a 200-day activity resulting 141 from the bombardment of Pr withc(-partieles. Postulating that an
(*t,2n) reaction would be most likely to proceed under the conditions 143 of their experiment, they attributed the new activity to Pm .
Wilkinson and Hicks (6) performed a similar experiment and used ion- exchange chemistry to confirm that the observed activity belong to prometheum. They measured it as 285-days, however. It must be observed that the masses of the rare-earth isotopes in this region 2 are not known with enough precision to predict reaction thresholds.
The mass-assignment of the (200-285)-day activity therefore remained
in doubt.
This was all of the reasonably reliable information available on the pranetheum isotopes at the start of the present investigation. It is summarized in the section of the nuclear chart shown in Figure 1.
In addition, a great deal of misinformation about the prometheum
isotopes had been published. The contradictory reports are noted in
the article by Marinsky, Glendennin, and Coryell (4), These authors pointed out that some 15 activities had been reported in the prome
theum region, most of them unconfirmed, and many of them oontradiototy.
The early investigators were confronted with a situation compli
cated by nature . Neodymium, the most useful element for the production
of prometheum, has seven naturally occurring isotopes. The natural abundances of these isotopes are given in Figure 1, All of the
abundances are greater than and less than ZOf»» If a prometheum activity is formed from bombardment of the natural mixture of neodym
ium isotopes, the abundances give no clue concerning which isotope might be the parent. None of the isotopes can be said to predominate
in the natural mixture, and on the other hand, none can be said to be
present in a negligibly small amount.
In some sections of the periodic table, the masses, or at least
relative masses of the various isotopes are known with extreme pre
cision. In these regions, the threshold energy requirements for given
nuclear reactions can be accurately computed. This is often an aid in 142 145 W4 145 146 147 148 149 150
6 0 Nd 12.2 25.9 17.2
50d 5.5d 6 1 Pm
.23
62 Sm 3.2
Fi[ure 1. I'uclear chart in prometheum region, 19^0
M IÉ 4 the identification of and mass assignment of new activities:.
Such is not the case in the neodymium and samarium region. Mass spectrographs are not capable of measuring masses with the degree of precision necessary for predicting reaction thresholds in this region.
Chemical purification of neodymium is a difficult task. The end result is never entirely free of the other rare earths. When chemi cally purified neodymium is used as a starting material, stray activities due to impurities invariably are formed.
In view of the above difficulties it is not surprising that so few prometheum activities were accurately known in 1950, In the spring of 1951, however, enrichments of the neodymium isotopes became avail able from the AEG in quantities sufficient for cyclotron research.
These enrichments are made on the large mass-spectrographs known as
Calutrons, The AEG furnishes the isotopic analysis and a spoctro- graphic analysis with each enrichment. The materials are of a chemical purity previously unavailable in chemically separated neodymium. Each enrichment features one of the isotopes of neodymium in high abundance, but the enrichments also contain small amounts of the other isotopes.
Samples of each enrichment were loaned to this laboratory for research.
The analyses of these samples as given by the ABC are shown in Table 1,
These analyses made it appear that the enriched samples would furnish the key to the prometheum activities, A research progreun was designed with the purpose of verifying the reported prometheum activities and adding as much as possible to the nuclear data on pro metheum, The proposed program was to consist of 7-Mev proton bombardments on each of the enriched samples as well as the natural Table 1
Analyses of Neodymium Samples
Per oent
Principal Natural Isotope 142 143 144 145 146 148 150 Mixture
Ingredient
93.0 4.04 0.7 1.2 0.6 1.2 1.1 27.1 3.2 83.9 0.94 0.8 0.3 0.6 0.5 12.2 ::: 2.9 8.6 93.4 4.8 0.9 1.3 1.0 23.9 Nd^^® 0.4 1.8 2.3 78.6 1.0 0.7 0.4 8.3 Ndl^ 0.4 1.2 2.4 13.7 95.6 2.5 1.0 17.2 NdjJ® 0.08 0.15 0.15 0,6 1.4 89.8 1.2 5.7 Nd^®° 0.07 0.11 0.07 0.2 0.2 3.9 94.8 5.6 La 0.15 0.15 0.15 0.15 0.15 -- Mg 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 Ni 0.08 0.08 0.08 0.08 0.08 - Pt 0.04 - -- - -— Sm -- 0.31 0.31 0.31 - - Ba ---- - 0.04 - Fe - -- - - 0.08 - Ho - -- - - 0.31 - Mn - -- - - 0.04 - Si —--——— 0.08
mixture. By comparison with other elements in the same region, it was expected that 7-Mev protons would probably be sufficient to bring about (p,n), (p,Y)» possibly, (p,oC) reactions. The (p,zn) reaction would not be expected at all with energies less than 8.5-Mev and usually is not prominent in rare earth bombardments unless 10 or 11-
Mev protons are used. The 7-Me? protons should therefore produce a small number of new isotopes which should be identifiable from the isotopio analyses of the starting material.
Short bombardments (10 minutes) would be tried first in order to find short (20 second - 2 hour) activities. Longer bombardments
(3 - 10 hour) were then to be tried for the study of activities longer 6 than 2-hours. It was planned that these proton bombardments on neodymium could then be followed by whatever other bombardments
( oi-particles, neutrons, etc. on Ed, Pr, Sm) were indicated as cross checks of the results. In general this scheme has been carried out.
Some of the activities found have been followed for three years and are s till being followed. APPARATUS
The hombardmen'ts were performed on the Ohio State University
cyclotron. This instrument has a 42-inoh magnet pole face diameter.
It will accelerate protons to slightly over 7-Mev at about 13.8 megacycles.
Counting equipment included shielded G-M tubes, both the fh -sensitive (Tracerlab TGC-2) and x-ray sensitive (Tracerlab TGO-3) types. The long half-lives were followed on a Wulf electrometer as
long as their activity was high enough. This is an extremely stable
instrument over long periods of time. It therefore served as a valu able check on the more variable G-M tubes y^en following long half-
lives.
The magnetic ^-ray spectrometer used in the study of has
been described by Sooville (12). It is a single focusing, 180° deflec
tion instrument with a battery powered magnet. Its use is limited to
samples with a rather high activity.
A scintillation spectrometer employing a sodium iodide crystal was 146. used for the measurement of the -ray energy of Rn * In this in
strument, a scintillation caused by the passage of a y -ray through a
crystal is amplified by a photo-multiplier tube. The pulse from the
photo-multiplier is transmitted to an electronic gating circuit which
rejects all pulses except those within a narrow band. Those pulses which pass the gate are counted with conventional scaling circuits.
By altering the gate threshold, the y -ray spectrum can be scanned.
In other phases of the project, conventional counting equipment was
used. 8
EQUATIONS OF RADIQA.CTIVE DECAY
A few of the radioactive decay equations indicate the possibili ties of the enriched isotopes. The differential equation governing the simultaneous production (in the cyclotron) and decay of a radio active substance is
(1) dN . (R - NA ) dt
N - number of radioactive nuclei present in a sample
t a time
R a rate of formation of radioactive nuclei by cyclotron,
assumed constant
s decay constant, 0.693 divided by half-life
Upon integration, equation (l) gives
(2) N - _R_ = K e A where K is the integration constant. If time is counted from the moment bombardment is begun, then at t b o, N ■ o, and K b -R « A Therefore
(3) N = R (1 - e
Now denote by the subscript b the conditions which obtain at the moment bombardment is complete; and by the subscript oo , conditions approached in an extremely long bombardment at the rate R.
(4) Ny A . R (1 - e
(5) N«A s R
Combining 4 and 5 we can eliminate R and determine Noo in terms of
Nb* (6) Roc = Nb 1 - J-Aib 9
Now R will be proportional to the abundance in the bombarded sam ple of the parent isotope, Aj to the average orosa-seotion over the spread of proton energies for the formation of the radioactive nuclide observed, to the cyclotron beam strength, and to various geometri cal aspects of the bombardment, lumped together and denoted by B. This is expressed by (7) R = A CT"B
The saturation intensity, N ^ ^ is proportional to each of these factors. Therefore, if the cross-section,0", is the same for two different bombardments, the quantity N^^/A is proportional to B in each case. Different samples may therefore be bombarded and the rela tive beam strengths, B, for the bombardments determined, if the abun dances of the parent of any observed activity are known. This fact was used to correct the bombardments of the different enrichments for slight variations in beam strength and geometry. The 5.3-day activity of was observed in a ll the longer (3 - 10 hour) proton bombard ments of neodymium enrichments. Under 7-Mev proton bombardments, the
5.3-day activity can be formed only by the reaction Nd^^^(p,n)PM^^^, 148 Since the abundance of the parent, Nd is known, and the saturation intensity of the 5.3-day activity can be computed, the relative beam strength for each bombardment can be determined. In this way, the
5.3-day activity is used as a monitor to correct for beam strength.
If any other activity is than observed and its saturation inten sity determined, it is again true that
^ o o = A^O-'b where the primes refer to the new activity. The B is the same as before and its relative value is known from the monitor activity. 10
The quantities ÎT^ooA^B oan therefore be oaloulated, and the numbers so determined will be proportional to the abundances of the parent isotope. If from the analyses of the samples, arçr isotope oan be found whose abundances are proportional to the quantities for some activity, then this isotope must surely be the parent of the observed activity.
The foregoing treatment serves to identify the parent of an activity if only a single parent exists. In the application of these concepts to the prometheum activities, the possibility of a nuclide being produced from several parent isotopes must be considered. Thus for instance, might be produced from by a (p,V ) reaction "\A*Z and from Nd by a (p,n) reaction. An analytical expression can be obtained in a similar manner for the saturation activity in terms of the abundances of the two parents and the two corresponding cross- sections. The use of this expression has not proved of value in this investigation because of certain limitations on the accuracy of the method to be discussed next.
It was assumed in the development that the cross-section for the formation of an activity, was the same in a ll bombardments. This is true only under carefully reproduced conditions of sample prepara tion and bombardment. The present investigation was spread out over
several years, during which time adjustments and changes were made to the cyclotron. As a consequence, the maximum beam energy varied from about 6 to 7-Mev between the bombardments. With this much variation,
some changes in
In counting the activity of the samples every effort was made to use a standardized counting geometry, Nevertheless, certain samples prepared for special purposes, required somewhat different mounting and may have had slightly different geometrical factors.
The cyclotron "beam strength apparently does not remain constant throughout a long bcmbardment. The assumption that the factor R was a constant is not precisely correct.
Because of these possible deviations from the simple theory, the saturation intensity is expected to correlate with the abundance of the principle parent only as to order of magnitude. The further re finement of trying to correlate the activity with a secondary parent was unsuccessful. 12
HALF-LIVES
/ N 18 For the short C10-minute ; bombardments, the activity of F
(1.9-hour was used as a monitor. This activity is produced from oxygen (the oxides of neodymium were bombarded in each case). A 10- minute and a 50-minute positron activity were observed in each case.
After normalizing these activities by the use of the P activity, no correlation could be drawn with the abundance of any of the neodymium
isotopes. It was therefore concluded that these short activities must be due to trace impurities in the samples. Traces of carbon may ac count for the 10-minute activity by the reaction C^®(p,n)N^®. The
source of the 50-minute activity was not determined.
The longer bombardments proved more fruitful in producing the
known prometheum activities as well as new activities. A 2.7-hour activity, a 40-day activity, several activities of the order of one year, and a longer, perhaps 3.5-year activity were observed. Each of
these activities and their mass assignments will be considered in the
following sections.
The 2.7-hour Activity
The 2,7-hour activity has been observed previously as a result of
bombardments of natural neodymium with protons and douterons (2). No mass assignment has previously been made. In the present investigation
it was produced from three neodymium samples containing different iso
topio abundances. The observed intensities of the 2,7-hour activity
in the samples were normalized for variations in sample size, beam
strength and bombardment time by the use of the 5.3—day activity. 13
With these adjustments, the saturation intensities of the 2.7-hour aotivity in the three samples corresponded more closely to the concen trations of Nd^^*^ in these samples than to the concentrations of any other neodymium isotope. The adjusted intensities of the 2.7-hour activity were 3.8, 7.4, and 20 in samples where the per cent ahund- 150 ances of Nd wore 3.9, 5.6, and 94.8 respectively. See Table 2. It was therefore concluded that the 2.7-hour activity was formed by a
(p,n), (p, ir )» or (p, ) reaction on Nd^^^.
The (p, pt ) reaction on Nd^^^ is immediately ruled out as i t would 147 necessarily yield the known 10.8-day Nd activity, which was not ob served in any of the proton bombardments.
During the course of this investigation, a 27.5-hour activity was discovered through the study of fission products (7). This 27.6-hour activity was assigned to Under bombardment of Nd^^^ with pro tons, this 27.5-hour activity would be formed by a (p, V) reaction.
The 27.5-hour activity was formed in very low intensity, if at all, under proton bombardments. This observation would indicate that the
(p, ) reaction with 7-Mev protons on Nd^^^ is not a prominent one. 150 The 2.7-hour activity may therefore be assigned to Pm in accordance with the reaction Nd^^^(p,n)Pm^^^. A curve showing the decay of a 150 sample enriched in Nd and bombarded with 7-Mev protons is given in
Figure 2. No chemistry was performed on the sample. The 2.7-hour activity is prominent even in the presence of the easily formed F ^ activity. 14
Table 2
Corrected Saturation Intensities of the 2.7-Hour Half-life
Laboratory number Abundanoe of N^o ^ /B of sample Nd
4263 3.9 3.8
614 5.6 7.4
4305 94.8 20 lOOOË
100 agfcfe
■o c 8 (U tf> \ <0 c 3 O o
>% M
Hours after bombardment
_ l'-urn 2. Enriched bobbarded by protons. Decay. 16
The 5.3-day Aotivity
This aoti-vity was observed in bombardmants of a ll the enrichments as wall as the natural mixture. It is easily formed under the bombard ment conditions used, and is of course identified with the previously 148 known Pm . As pointed out earlier, this activity was used as a monitor to measure the relative strengths of the bombardments. The 148 decay curve for a proton bombardment of enriched Nd , in which the
5.3-day activity was very prominent, is given in Figure 3.
The 40-day Activity
In the course of these bcmbardments, a 40-day activity was discov ered and reported for the first time (8). At the time of this first report, the half-life appeared to be 46-days. As the measurements con tinued, it became apparent that 48-days is somewhat too long. After following the activity through 8 half-lives on several samples, it is now apparent that the correct half-life should be 40 - 2 days. Chemi cal separation showed that the activity belonged with the rare earth group. The activity was most prominent in bombardments of the material 148 enriched in Nd . However, the activity was formed in measurable strength in three other enrichments as well as in the natural mixture, as shewn in Table 3. In each case, the saturation intensity of the
40-day activity was about 10% of the saturation intensity of the 5.3-
day activity. Its corrected saturation intensity corresponds to the 148 abundance of Nd in the target material. Because of this correspon
dence, i t is concluded that the 40-day activity and the 5.3-day
activity were both produced from the same parent nuclide, namely Nd^^®.
In determining the mass assignment of the 40-day activity, the 17
100
DECAY CURVE
Nd'48 + p
•o c 5.3-d 0 o Q) (A \ (A 4- c 3 0 o 40" d >
Ü <
100 Days after bombardment
" in:..: aru a i-ro'Goro. 18 possibilities of (p, V)» (p#0^ ) and (p,n) reactions on were again considered. If the (p, (X ) reaction on Nd^^^ occurred, the 145 4.6-hour Pr would also appear in the sample. Since this activity was not observed, i t is concluded that the (p, 0<) reaction on Nd^^^ does not proceed with 7-Mev protons. Similarly, a (p,V) reaction 149 would produce the 2-day Pm . Since this activity was not observed, 148 a Cp, Y') reaction on Nd is not likely. The (p,n) reaction is there fore the only possible mechanism of formation of the 40-day activity, so that the 40-day activity must be associated with an isotope of mass
148. S till two possibilities remain, namely, that the 40-day activity could be either a Pm^^® activity or a Sm^^® excited state. The latter possibility is eliminated since the activity is not formed by neutron or Y-bombardments of sEimarium, and since the particle energy is too high (l.7-Mev3 to be an internal conversion particle from an excited state of samarium. It is concluded that the 40-day activity must be assigned to Figure 4 is a decay curve for a sançle of enriched 148 Nd bombarded with 7-Mev protons. It shows the 40-day activity clearly.
Table 3
Corrected Saturation Intensity of 40-day Activity
Laboratory number Abundance ^ o o h /B of sGtmple Ndl48 (40-day activity)
4201 89.9% 89.9
4251 .08% .09
4305 1.25% 1.2
N-17 1.42% 1.5 19 lOOOOOl
10000 DECAY CURVE MW 148 4 n
5.3“ d 4 0 “ d
200 400 600 800 Days after bombardment
111 0 Doer; rurvo of crriched '' bonbardod by protons. 20
Activities of the order of one year
An aotivity of 300-days was found from bombardments of enriched
Nd^. Bombardments of enriched Nd^^^ revealed an activity of 330- 144 days h alf-life. Bombardments of Nd resulted in a 350-day h alf-life.
All three of these activities have been followed for over two half- lives, as shewn in Figure 5, 6, and 7. At first i t might seem that these three observations are merely manifestations of the same activity with some small discrepancy due to experimental error. Close examina tion indicates that this is not so, however. For example, each of those samples while in its characteristic period of about one year, was counted on an X-ray sensitive Geiger counter, with and ^vithout a mag
netic field. The magnetic field, when applied, was strong enough to
deflect all -particles from the counter. Table 4 shows the results
of these counts.
Table 4
Electromagnetic Counts Relative to
Total Counts Ratio Sample Enrichment Counts per second e-m counts magnetized demagnetized particle counts
4251 Nd^42 0.10 0.17 1,4
4254 Ndi*: 0.19 0.25 3,2
4128 Nd"^ 0,21 0.61 0,5 10
DECAY CURVE
142
1.0 00 *0 c o o 0) V) \ V) 4- c 5.3-d D 3 0 0 - d O ü
>> 4- '> 0.1 o 4 0 “d <
200 4 0 0 60 0 800 Days after bombardment
,1):; o:ya:\ Activity, counts/second
o
Ô
/ DECAY CURVE 10 — \ mh I4 4 + n
3 5 0 - d
1.0
4 0
0.1 200 4 0 0 600 800 Days after bombardment
D-10' 24
The ooun-bing rates wore quite low on this apparatus. Long count ing periods wore used to obtain some reliability in the second decimal place. ïhe ratio of electromagnetic counts to particle counts is so different in the three cases that it seems probable that three differ ent activities are involved.
The corrected saturation intensities given in Table 5 offer an independent check on this conclusion. Reference to Table 1 shows that none of the abundances of any single neodymium isotope correspond to these relative saturation intensities. It is concluded therefore that the observed activities are not manifestations of the same activity, but actually represent three different activities.
Table 5
Corrected Saturation Intensities of 1-year Activity
Sample Enrichment A/B ( 1-year activity) 142 4264 Nd 1
4254 Wd^^® 2.04
4128 Nd^'^ 2.73
Positive identification is not possible because of the low level of the activities formed. Crude absorption measurements indicate that ■ the 300-day electromagnetic radiation is of the correct order of magni tude for a neodymium x-ray, however. Tentatively, the 300-day activity is attributed to the 330-day activity to Pm^^®, and the 14-4- 350-day activity to Pm 25
Long Aotivi'faies
A SO-yaar electron capture activity has been reported (9) since this investigation began. It was discovered as a result of the neutron 144 capture reaction on Sm , and establishment of the genetic relation
ship between and The 30-year activity is too long to
measure at this stage of the present investigation, particularly since, 145 as reference to Table 1 w ill show, the sample enriched in Nd con- 144 146 tains appreciable amounts of Nd and Nd . The latter two nuclides 145 also produce isotopes of a year or more, and the 30-year Rn could
not be expected to be observed until these other activities have sub
stantially died out. 146 Bombardments of enriched Nd resulted in an activity produced
in good yield and originally reported (10) to have a half-life from
1* to 2^ years. Subsequent to this original report, i t has become 148 apparent that some of the 40-day activity from Pm was present in
the samples. After more complete decay of the 40-day activity, it is
evident that the newly observed activity has at least a 3.5-year half-
life, and is perhaps considerably longer. For convenience i t will be
referred to here as the 3.5-year activity.
The 3.5-year activity can be produced easily and in fair yield.
Its mass assignment can be made with certainty using 7-Mev proton bom
bardments on enriched Nd^^^. The deuteron bombardment would be
expected to yield the 2.3-year activity by (d,n) and (d,p) re
actions, in addition to any Pm^^ activity formed by the (d,2n)
reaction. Actually observed is the 2.3-year activity and a longer
(3.5-year) activity in good yield. These can be distinguished by 26 aluminum absorption measurements -without waiting for complete decay of
the 2.3-year activity. The 2.3-year activity is known to be accom
panied by a soft (o.23-Mev)/^ -particle, while the 3.5-year activity
is accompanied by a harder (p.7S-Mev)/3-particle. Figure 8 shows two
aluminum absorption curves on a deuteron bombarded sample of enriched 146 Nd , taken 15 months apart. They clearly show the decay of the soft fit -component, while the harder component remains relatively constant. 146 On the other hand, bombardment of enriched Nd with protons
yields, after decay of shorter components, the 3.5-year, 0.75-Mev
^ -particle without any traces of the softer 2.3-year activity. 146 An aluminum absorption of the proton bombarded sample of Nd is
shown in Figure 9.
The only possible explanation of the deuteron and proton bombard
ments is that in the former case, the 3.5-year activity was formed by
a (d,2n) reaction on Nd^^^, and in the latter case, by a (p,n) re- 146 ^ 146 action on Nd . The 3.5-year activity thus belongs to Ra .
Decay curves of the deuteron and proton bombardments of enriched
Nd^^^ are shown in Figures 10 and 11. Although neither of these decay
curves has been followed long enough to be certain of the half-life of
it is evident that this isotope has a half-life of at least 3.5-
years .
As a result of this investigation, therefore, it is possible to
assign a half-life, at least tentatively to all the missing prometheum
isotopes between mass number 142 and 150. The firs t published reports
of the 40-day Pm^^® and the 3.5-year Pm^^®, as well as the first pub
lished mass assignment of the 2.7-hour Pm^®° resulted from this 10
ABSORPTIONS
Days after bombardment 1,0 D C 0.23 Mev 0 Ü a) > 0.75 Mev > +" u < 1- 0—— 0,01 Absorber thickness, mg/cm' 'Wiitio,!:. Mched:r'%DRb,iTcodij; dentci-ons. 28 10 ALUMINUM ABSORPTION 0 .2 3 Mev Nd^G + p 270 doys ofter bombardment 1,0 r c 0 u 0.75-Mev (D VI \ V) +• c 1 oO .i ü < 200 2 300 Absorber thjckness, nig/cm 'irovons. CD ë le, Activity, counts/Second CD fjo o o ô o If 1------1— I—r f" rrr T T—I I ri T T r 1 / CD O o ro s. o o o p, CD 5- o o ;-î o '«c 4^ O cr> âco o O or cz O DD 3 or o n> CT> - S CD O O 30 1000 ... 1... ,. .. ' ' f f-; 4- r 100 m #-: ^_i *o K J . 1 ; oc r.-‘-:T:-r:-t—t-H—.r i l V ■; 1 o o> r-'i ri'.-T=>5 u> (f) .... j .. t i- 4- H 'i—4- *• -—“ c f ' ■ "" ' f- • ♦ * 3 O o ^ \ 0 ’> o < 200 400 600 Days after bombardment 11. Decay of enriched boribarded by protons. 31 inves-bigation. These isotopic assigiœionts are now accepted as sub stantially correct (ll). The first publication of the approximate 142 144 one year half lives of Pm. and Pm also resulted from this inves tigation. These, however, are s till regarded as tentative, since the weak level of activity formed makes a detailed identification study 143 impossible. The half-life of Pm , which previously had been only tentatively established, was confirmed in this investigation by the use of the enriched isotopes, and now must be regarded as quite certain. After the establishment of these half-lives it is desirable to know the character of the radiations in some detail. The activities will again be considered in the order of increasing length. 32 RADIATIONS Radiations of the 2.7-hour half-life The 2.7-hour half-life was first associated with mass 150 when a routine aluminum absorption following a bombardment of enriched showed particles much too energetic to be confused with the soft posi- IQ trons of F , Magnetic analysis showed these energetic particles to be negatively charged. The spectrum of the particles was determined using a magnetic (3 -spectrometer. This instrument has been described previously (l2). In the present investigation a rather thick source (about 20-milligrams per square centimeter) was used, mounted in an aluminum holder. Because of the possible effects of self-absorption and back-scattering resulting from this procedure, counts correspond ing to energies less than around 150-kev are subject to possible errors. Such counts were therefore disregarded in this spectrum. The spectrum showed a clear internal conversion peak with a 2,7-hour period at 270-kev. This agrees with a 0.3-Mev y -ray reported by absorption measurements (13). The internal conversion peak, together with the rest of the spectrum is shown in Figure 12. The maximum count rate of the continuous spectrum was over 50 times background, and occurred at 630-kev. A Fenni plot of the data is shown in Figure 13. The maximum energy of the ^ -particles is at least 2.8-Mev and may be as high as 3-Mev. Points above 2.7-Mev are so close to background that the exact end point is hard to establish. The method of constructing this graph is explained by Fermi (14). Values of the function f ^ ^ were taken from the tables of I. Feister (l5). In the analysis of this type of plot it is customary to look for 32 Energy, Mev 5000 G ouss- cm i5o :ï'-iiro 12. PiirticlG spectrm from Pir 34 0.4 FERMI PLOT 2.7-hr Pm'50 0.3 0.1 1.0 2.0 3.0 Energy, Mev F iju r e 1 3 . j'erv.ii p lo t o f ^pectrvun 35 a straight-line portion at the high energy end. This straight line is extrapolated haok and subtracted, yielding a new Fermi plot. This process is continued until the end points of the several components of the continuous^ -spectrum are found. In the present instance, this procedure could not be used, since there is apparently no straight line portion at the end of the plot. Thus only the maximum ^ -particle energy can be determined from the spectrum. The value of the function log (ft) can be calculated by the method of Moszkowski (16). This function is a measure of the forbiddenness of the transition. The calculated ?alue is 7.4 to 8.6, depending on the percentage of the total decay attributed to the 3-Mev component. This indicates a forbidden transition. It is therefore not surprising that the Kurie plot is curved throughout its length. In connection with the 2.7-hour activity, the first published re ports of the 270-kev conversion electron and the forbidden log (ft) value resulted from this investigation. Radiations of the 6.3-day activity The use of the enriched isotopes has so far added nothing to the knowledge of this activity. The activity can be formed easily from the natural mixture of isotopes. Radiations of the 40-day activity This activity was studied principally by absorption measurements in aluminum and lead. Figure 14 shows four aluminum absorptions taken at 50, 77, 101, and 127-days after proton bombardment of a sairçle en riched in Nd^^®. They show two components of the ^ -spectrum, with energies 1.7 é 0.1-Mev and 0.6 6 0.1-Mev, both decaying with a 40-day Activity, counts/second o 5 CD ; i O lu.: 37 half-life. Two lead absorptions on the same sample are shown in Figure 15, taken 46 and 98 days after bombardment. They show a y-ray whose half-thiokness is 1.8 inches of lead decaying with approximately a 40-day h alf-life. This corresponds to an energy of 0.54-Mov. It should be noted that the half-thickness reported here is corrected by comparison with a known V -ray under the same conditions of geometry before computing the energy. Such a correction is necessary in lieu of an exact reproduction of the geometry under which the absorption coefficients were determined. It is significant to note that these ^ and V-energies can be linked in a consistent scheme, if it is realized that the measured Y -ray energy might represent two Y -rays in cascade, both of about 0.54-Mev. The sum of the energies of the two f -rays would then equal the difference between the -energies. The first published observation of the radiations from the 40-day activity resulted from this study. Characteristics of the 1-year activities These activities were all made in rather low yields. Detailed study of the radiations was therefore not possible in some cases. The ratio of electromagnetic to ^^rticle counts for three 1-year activities is given in Table 4. These data were obtained on an argon-filled G-M tube (Tracerlab TGC-3). These tubes have an effi ciency for electromagnetic radiation not greater than 5%, while their efficiency for -particles is neai* 100%. We can therefore multiply the observed ratio of electromagnetic to particle counts by a factor of at least 20 to approximate the actual ratio. When this is done, it # 7-T- n ' x-i • I- ]- — • * , 4*1 4-t 'f \ : •- -4-'•-• -f- I Ij9 -. ■ ‘ j .! --T ■ ' ■ -“■■ -f- ■ -r . | —j. - t .4^ M i i H i S W ih# - 4- t • i -4- f 4}. M ! riT kY ri. FT# . J ■ t«4 i- • 1. r i .: Î 1 ‘ ; i ^1-t rM B .îfilîS tti î; ’. : i -, J- -.1- ' ■ -?r-r-t 4--.--'r* V-■ V , ! -4 t ,0 %: {. -fF ÿ . - . -Ltv ♦ -i rjî , 4- : I — • -1- - 1 ' I H4-; I ;■ r.i ' -! -1 jLTl- Ti= - n f rTTTT-i-^iTTi TT T-rr ; I;? î l î l ‘ : i kf | :ki k k ki.' T^kkr^}; l i f 444:4: 4R 4 '■ T k : I.' Ljk " Ti':4 ■I't i j n i ' i l S I;k-I iy'i^ 4 44-4 ;.l T44 : : r .\ - f ' ' 1 ■+*■ ■ I - •I iTl i a m #44 f i : ‘ r ;• I iilliiS iiliil: • ■ • : m 10 2 0 30 Absorber thickness, groms/cm^ T.:r]l':G 1;, n r d c d 1': lire 14 Leod Ansorpbions. kiiriched nrotons. 39 is Been that in a ll three oases, the eleotroioagnetio events outnumber the particle events by at least 10 to 1. It is concluded fr The activity was sufficient to perform a copper absorption on one of the samples of the 300-day Pm^^^ activity. Aluminum absorptioas had already indicated that almost all particles would be stopped by 192 milligrams per square centimeter, although the activity was too weak to determine an exact end-point. A berylium absorber, 192 mg/cm^, was therefore placed over the sample to stop the particle radiation. Copper foils were then successively placed over the beryllium and the count rates recorded. The result is shown in the copper absorption curve of Figure 16. Subtraction of the base line leaves a component with a ha If-thickness of 3.5-mils of copper. This would correspond to an x-ray of 0.S8A, which is in good agreement with the 0.336A K x-ray of neodymium. 144 The 350-day period of Pm evidently has associated with it a negative particle of energy 0.6 ^ 0.1 Mev, as shown by absorption in aluminum, 400-days after bombardment (Figure 17). The shape of the absorption curve indicates that this is probably an internal conver- 144 sion electron. This is only reasonable, since Pm would not be ex pected to decay by ^"-emission if the reported decay of Pm^^^ by 144 electron capture (9) is correct. The 350-day period of Pm is there fore interpreted as electron capture and internal conversion. Other components of the 1-year activities were too weak to be identified. rr, . ^ ^ j... » - -- • I## T.-TT- • •'. -r-'* i - . 1 ...... ►• « 4 --4 • • ; i::t 'f i -4-T 1 - .r . f., 4 *------t. r-'Jpi-Tt.-r-l--i--f +- . , . . 4^ 4-- 4 2 - * .-54*- i .>-:r' » *•- ::;-f ■"? - j- 4 » ' ■ «•I ♦--* - fip' >—r~~* ■ • V ? - *.-'t • liW illl — , 4. ... . l.,-i t_4 I f I , I ' - 4 4 - 4— - — ^ 1 ' * I f-j |- - . —- • . I 4- •• -1 I ' 4 .' f , « i■ * V • ^ J. - p . ^ - .4 ■ I - _ f.. : i t ■ r . t j--- ' - — t )- , - ' ' j -j-|-44--.j-i t i I —t ■“t— - Y - ‘0. # ; : . - ..... ;. ; : • ; r * : ' 1 : 4 ^ T- » - . .. ■ :-: ,!, ; .1 '. I t. : J. . ' % ' ;; --r-; ' T: r": T T ' :'T'. f. ' " .. . 1 t ,-T:' l-T- { : 0.01 0.02 0.04 0.06 Absorber thickness, inches of copper obove 0.041 inches of beryllium 1 ] t ^ ire l6 .. Jopper resorption. Enriched I Id bomoarded by protons. o 10 '4' r-i 4',-' - T - ., . .... ______vs"'r> & ^ ^ 3 .-13%: I—r-*1 :.. • * '7^"^ ' -f • -»i -T - I i- . ■ t. " : n l.. . .4 i m g T3 ■ ' f -a ----a a— -a - a . .a . . w- - a .. % 4 ' -ata —— ■I » .la lài — .^ ^ 4^ FT-TTH-1 C ' —- - — . J — 1- —i - . L • • "a , . . J -F> ..a :.J 4- - > ' . 7* “ " V . •a f ■’- f-“ r-.î -T- -3 3 = --t-'- J -t - « r- a- 4 - ‘ - *— » • f - ' t - i a ' a -a# - t ^ -a .4 a- ^ * . ( a aaa . . ^ i - f l î - I ^ ' f -a-'V i II I — a. — 4——*• * * “ - 49 * ,-^a , * , ^ ' , aa a, . a 4 a* r-i- • k ^ “*T a-aa.#—a|*a. 4- -a -a a a a . a ' a*. 4 f a^aa*.!---- *- * -a^ a- -^ •. ... i 4 —.^— * - a t . - 4 - •» —a . .4.a - 4 i — ? t -a---» - .44 — ; t". ^ 4 3 —i • - .- -. . a . -p . - -4 ... .1.. ..I _ 4 ._a 4 4 - - a . - .. a- j. a . .a- .a- - 4——t - 4——4_ . .v- »- -t— -4— i a-J * . .... — 8 ' a ... a ..4 . ... 4 . a * ^ _ *-.*4 — -.y-., a . - a — i — — a - • ''“ **'f'''t ^ 4 a4. * -4- —— a.a. a-. , U. a a a —a --a* T -a---' ' —a.» •—a •» 4 —a • ' - f - 4-a Q) (/> \ ,. .;iir I.J.; 1 '.i. 1 T j:.j^.44.4.-_î .; . 1 - : T ;.-T)-. ji.î7 .i3 '-; Il -.-.^ } T i i n .'ï 3 1 - 3 3 t- rj-, "T-rzjzzk-: = X 0> t r. : .ÜlZjzLL ■Z3i NË- *• 4 » ' t [-a-a à —{ -♦ < - -a - 4 ' j à t . a i î»t * -» a- a a-L -I . .— * a- ' -» —i - a. 4. 4 • ^ . | 4 ^ . . 4-- C ' - * 4 - 4 —i —. 4 . * - ^ ^ •4-» — ai.. 4 . » « .4 » ) r - f-a -4- -4-. —f . - 4 -a. - . a ^ _.,... 4. ^ .J— •*•' ■ TTI' •■•*' t - . .. 4. (- -- i - ^ {-i-.A.. ï a * . a 4 . j . la»_I ,., , - f 4-4 -| t - - r a • t ' ' n -a... i .L_l... _» 4 .4. 4...^. y ..*... i . a « r ï * T ( ! 4 • (----4- 4 i “ r--- r - -V- «• 4 —4 .a-.j-i- -*.-;•- 3 ... , r —t t--"" *.»* -Ja —:--t T-4 - - - I i - { - 4 - f— -i 4 - r 4 - ' * * 4 a >- 1 a ' f—‘ O .; .î -:r .-.i1 r T... 3 Ti.q . ! i. -#4i- -i':' - 4 - r*-■ -.i-l;’. t vj- -+ -% ; I ; -+ • • “TJ*— + • -4 — ■ " » ' ■> ^ —f-—r'^-44— 4 • I - I —» ' '«a. 7 • .* Ÿ -f a. .iaa. ».—.»■ aa.-^a -».a|, i t. — --..a- L - a a a a * 4-- 4 a-aa—4 ada^a 4" 4 . i a a m a - i — 4 f. 1 i _ , _ , ^ . 1 . . 4 » v , . ^ , a ,.4 . . a ü ; . . , • i--a r ; -r : - T - I - - -a | , ' i - | i- ' I . .. t a ....,.,.; J 4 l j i.. ' I 4 -•» - - t i' 4' 4 - p i— r “ l -f- - - • •. I - . -a .j. 1 ag- 1. i— i « a , La. i , . ^ l , - . - , — 1 - ' ... i.-Y-i- - i. 4 * 4- ^ ‘ • ! - 1 f t - i . ; .4- .â. ^ ! ‘ ' ( i . - . J ; ; ; ; j. .;. :4.j 4. ■ - ! 0 .6 Mev .»■ ;fr-r.-.-4;--j - I- . r.-^ f; î ït - J - n - i 3 g V\ :; _ 1 r-'.L ^ * L"_: 7 ^ ^ h-T- - M^T'jr.- h-TTTTTTTr rrfc^trrrrir-- i" i: 4' 4 - a 'a* a . -A “V ■ " fr-a*4.*T"*'* 4.— a .a i...... '■ ■ h L u i-.p-r .r- ; -| .. : : - i. 1 --..T.ti-i.. 1 .i.-. ■. r t.- ; t-r-.j.-:'.T. V r J. .r._f-v-r-^''4"-T'T-- " • -ï--^ -T-+. - - - f — r— ■•:- r.-riT i-f .1-..'T -. T -a——r — r i-.n-trfT4 • l-.- * ■ w «_»►.» a — I aaa* .4 . — i...* » i II > a« fc a ■ I l — f . :* il .;, : 1 ! ' i .. ] r '- r : .1 .4. : -4 -- .'. ! : t l.:*-.;ix r.tvT-- ■ •.r 1-T.-.rJF: 4z_wn..- 4— - — 4 4 * ' * a » - a ► - .i— f • a».. . a.. ^ .fc , 4 - . .. . . - a. . . 1 ' a V- * • f. + L' *. I ^ ' a r ---- •• '■*“ ■ i ' i—r ^-4 4 a. a — f »--| -—A. a-.. 4 ► .4- . fa. I . i . a a_^. - .. 4 . a 4 ' *am# J. —4I - a ,. L- - 4 a a -I , 1 ». . à - _ J * • i • f ‘ I I » 4 ... f— t I ' -I i » a a I L f- 4 a f. a— -i a . i . -*a — rt* » 4 f --' 4. a. a 4 .4- . .a. 1 a-a. 4 : a A 4 . #4-4:4441 t"!K .T-rt-NaFtablri-1 m * -^#?^rW#ln=3zr # # # # 1 ' '. ; . n#:-! :.c ; : - p Ki r,r% # # ! . . ' !.a , . , . ': : ■ I 't. il ; .! ■) 1 ; : .z; i t ' 4 ! i 200 400 600 ABSORBER THICKNESS, mg/cm^ ji-urc 17. Aliüânui;! al soi Lon. Inriched bornearded by protons, 42 Characteris-bios of the 5 «5-year aotivity This activity has associated with i t a negative particle of energy 0.75 0.1-Mev as shown by aluminum absorption. Figure 9. It has a If-ray of 0.45-Mev. The ratio of electromagnetic to particle radiation appears to be about one to one. It would be of particular interest to know whether the S.5-year activity of Pm^^^ decays toward higher or lower atomic number. Essentially, the problem is to deter mine whether the 0.75-Mev ne^tive particle is the maximum of a con tinuous fh -spectrum, or a discrete energy internal conversion electron. The aotivity has not been produced in large enough quantity for spectrometer measurements. A certain amount of indirect evidence in dicates that this is a negative ^ -particle, however. An internal oonversicn electron of 0.75-Mev and B.5-year half-life would necessar ily be associated with a high multipole transition and a low conver sion coefficient (18). The approximate one to one ratio of electro magnetic to particle radiation is inccnsistent with a low conversion coefficient. Furthermore, the only observable X ~ray appears at 0.45-Mev using a scintillation spectrometer. The particle energy is definitely higher than this, indicating that the particles are not conversion electrons associated with the only observable V -ray. Finally, the shape of the absorption curve (concave upward) is charac teristic of a ^ -spectrum, whereas conversion electrons often result in an absorption curve which is concave downward. See Figure 17, for example. It is concluded therefore that the particle is most probably a negative ^ -particle. Log (ft) for the transition is about 9.3 (forbidden). 43 If decays by negabiv© -particle emission, to then Sm^^® ought to decay to Nd^^^ by -emission. This would be consis tent with the (ji -decay already known from Gd^^^ to Sm^^^ (l?) and would complete the chain of (X -decay from Gd^^^ to the magic number isotope Nd^^^'. The anticipated^ -particle was not observed, however. 146 It is possible that the - half-life of Sm is too long for obser vation of the Of -particles. It is also possible that the tC -particle 146 146 from Sm may be observed after the Pm has had more time to decay and build up the concentration of Sm^^^. Samples of radioactive Pm^^® have been observed for only about two years in this study. 146 The first published report of the radiation from Pin appeared as a result of this study. 44 DISCUSSION The nerw nuclear data first observed in the course of this study are summarized in Table 6. Table 6 Prometheum Nuclear Data Obtained in This Investigation Mass Half life Particles Electromagnetic Mode of Radiations Decay 142 300 d NdK x-ray capture 143 330 d I capture 144 350 d O.eCo.l Mev capture ocnversi^ electron 146 3.5 yr 0.75 / 0.1 Mev (3' 0.45 Mev f (3 ■ 148 40 d 0.6 to . 1 Mev ^ ” 0.54 Mev V" ra ■ 1.7 L0,1 Mev ^ — 150 2.7 hr 3.0 Mev 0 ~ 0.27 Mev (3" conversion electron Independently of the present investigation, a similar program of proton and deuteron bambardraents was carried out at the University of California. The California results were published (13) soon after the principal results of this study were published. Substantial agree ment is evident on many points. The California study is summarized in 146 Table 7. It is believed that the longer estimate of the Pm half- life reported in Table 6 is the more accurate. This aotivity has the appearance of decaying rapidly at first, actually due to traces of the 40-day aotivity. Since the California results were published only little more than a year after the enriched neodymium isotopes became available, it is possible that these authors underestimated the half- life due to the presence of the 40-day activity in their samples. It is also believed that the California study may not have reported the 45 T able 7 Principal Results of California Study of Prometheum Nuclear Data (is) Mass Half life Particles Electromagnetic radiations 142 nothing between 2 min and 200 yr 143 200-400 days 0.6 e" K, L x-rays 144 0.9 0.17 0.44 0,65 X 146 1 yr 0.7^ or e” 148 42 1 d 0.7, 2.7^'" 1.0 X K x-rays 150 161 1 min 2.01 (S" 0.3, 1.4 X 3.00 ^ " K x-rays 142 1-year half life of Pm because of the low level and consequent uncertainty of this activity. The most recent nuclear chart published by Hollander, Perlman and Seaborg (11) gives considerable credit to both studies in the clarification of the prometheum radioactivity. The activities reported for Pm^^^ and (40-day) are listed as B. Pm“^^ is listed A. and Pta^^ are listed as D# On this scale, A indicates element and nass number certain. B indicates element certain and mass number probable. D indicates element certain and mass number not well established. Figure 18 shows a section of the nuclear chart in the prometheum region as it is known today. The principal nuclear data resulting from this project are given in heavy print. Comparison with Figure 1 shows the increase in the knowledge of this region which has been gained in the last three years. I-'. i4 .1 i-r ; " r 60 Nd 300 D Y33C D 2.7 H Pm .2 3 62 Sm I 3 .8 15. I'uclear chart in the pronethe’jii region, hover,iher, 19o3* O) 47 Bibliography 1. Bethe, H. A. Elementary Nuclear Physics. New York* John Wiley and Son, 1947, p. 1. 2. Kurbatov, J. D., and Pool, M. L. Phys. Rev., 63 (1943), p. 463. 3. Price, H. C., Motz, J ,, and Langer, L. M. Phys. Rev., 77 (1950), p. 744. Lidofsky, L., Macklin, P, and Wu, G. S., Phys. Rev., 76 (1949), p. 1888. Marinsky, J. A., Glendenin, L. E., and Coryell, C. D. J. Am. Chem. 3oc., 69,2 (1947), p. 2781. 4. Bothe, W. Z. Naturforsoh, 1 (1946), p. 179. Mandeville, 0. E., and Scheib, M. V. Phys. Rev., 76, ( 1949), p. 186. Inghram, M. G., Hayden, R. J., Hess, D. G., and Parker, G. W., Phys. Rev., 71 (1947), p. 743. 5. Wu, 0. S., and Segre, E. Phys. Rev., 61 (1942) p. 203. 6. Wilkinson, G., and Hioks, H. Rev. Modern Phys., 20 (1948), p. 685. 7. Rutledge, W. G., Cork, J. M., and Burson, S. B. Bull. Am. Phys. Soc. 26 (1951) p. 38. 8. Long, J. K., and Pool, M. L. Phys. Rev., 85 (1952), p. 137. 9. Buternent, P. D. S. Nature, 167 (l95l), p. 400. 10. Long, J. K., Pool, M. L., and Kundu, D. N. Phys. Rev. 88 (1952), p. 171. 11. Hollander, J. M., Perlman, I., and Seaborg, G. T. Rev. Modern Phys. 25 (1953), p. 469, 12. Sooville, G. L. "The Beta Activities of Some Ruthenium and Rhodium Isotopes" Masters Thesis, The Ohio State University, (1951). 13. Kistiokowski, V. Phys. Rev., 87 (1952) p. 859. 14. Fermi, Enrico, Nuclear Physics (Revised). Chicago* University of Chicago Press, 1950, pp. 72-83. 16. Feister, I. Fermi Function Table, Preliminary copy. Washington, D. C.* National Bureau of Standards (l95l). 16. Moszkowski, S. A. Phys. Rev. 82 (l95l), p.35. 17. Mack, R. G., Doctoral Dissertation, The Ohio State University, (1953). 18. Rose, M. S., Goertzel, G. H., Spinrad, B. I., Harr, J., and Strcng, P. Phys. Rev., 76 (1949), 184. 48 Autobiography If John K. Long, was bom in New Rochelle, New York, December 12, 1921. My elementary and secondary school education was in the public schools of New York, N. Y. In June 1942, I received the degree of Bachelor of Science in Chemical Engineering from Columbia University. I have been employed as a chemist and engineer by the Hercules Powder Company of Wilmington, Delaware, and by the Wri^it- Patterson Air Force Base, Dayton, Ohio. I have studied in the Department of Physics of The Ohio State University since September, 1947. During the academic year 1951-1952 I was the recipient of a fellowship from the Graduate School of The Ohio State University.