Some neutron-deficient isotopes of thulium and erbium

J. Inorg. Nucl. Chem., 1961. Vol. 21. pp. 193 I~ 200. Pergamon Press Ltd. Printed in Northern Ireland

SOME NEUTRON-DEFICIENT ISOTOPES OF T H U L I U M A N D ERBIUM A SURVEY

OF HALF-LIVES AND y-RAY SPECTRA USE OF MASS-SEPARATED SAMPLES

WITH THE

S. BJORNHOLM, H. L. NIELSEN, O. B. NIELSEN, G . SIDENIUS, O. SKILBREID, a n d ,~. SVANHEDEN* Universitets Institut for Teoretisk Fysik, Copenhagen (Received 28 November 1960; in revisedfi~rm 13 March 1961)

Abstract---Neutron-deficient isotopes of Tm and Er were produced in Er(p, xn)-reactions. Chemical separation, as well as mass-separation of the products, was performed, and the half-lives and ),-ray spectra of the individual samples were measured. Definite mass assignments of the activities were made, and previously reported results essentially confirmed. THE p r e s e n t p a p e r r e p o r t s o n a n a t t e m p t to p r o v i d e m o r e d e t a i l e d i n f o r m a t i o n a b o u t the n e u t r o n - d e f i c i e n t i s o t o p e s o f t h u l i u m a n d e r b i u m . T h e d a t a e x i s t i n g b e f o r e F e b r u a r y 1958 o n these n u c l i d e s is s u m m a r i z e d in the T a b l e o f I s o t o p e s . tx~ Since t h e n , t h e w o r k s o f HARMATZ et al. ~z~ a n d JACOB et al. cJ~ h a v e a d d e d c o n s i d e r a b l e n e w i n f o r m a t i o n t h e r e t o . A l t h o u g h m a s s d e t e r m i n a t i o n s by m a s s s p e c t r o s c o p y h a v e b e e n m a d e o n several T m a n d E r isotopes,t4, 51 mass assignm e n t s o f s o m e activities in this r e g i o n are still u n c e r t a i n . t In this w o r k we h a v e p e r f o r m e d c h e m i c a l p u r i f i c a t i o n as well as i s o t o p e s e p a r a t i o n o f t h u l i u m i s o t o p e s p r o d u c e d in Er(p, x n ) r e a c t i o n s with x r a n g i n g f r o m z e r o to 4 o r 5. E a c h m a s s n u m b e r has b e e n c h a r a c t e r i z e d by its d e c a y c o n s t a n t s a n d 7-spectra. EXPERIMENTAL

PROCEDURE

A more detailed description of the experimental procedure applied in this type of research has been published in an earlier paper.'" Two hundred and twenty milligrams of natural erbium oxide were irradiated for 5-5 hr with 65 MeV protons from the Uppsala synchro-cyclotron. Two to three milligrams of the target material were mass-separated in the electromagnetic isotope separator in Copenhagen. This set of samples, which was labelled "A-samples", was ready for counting 8 hr after termination of the bombardment. From ca. 30 mg of the target the thulium fraction was separated by cation-exchange chromatography. " Permanent address: The Gustaf Werner Institute for Nuclear Chemistry, University of Uppsala, Uppsala, Sweden. * After the conclusion of this work, research on the isotopes l~Tm and l~;2TrnC~(1960) and t':~Tm and t63Tm~?~and 16aHo~8~has been published. ~t~ D. STROMINGER,J. M. HOLLANDERand G. T. SEABORG,Rev. Mod. Phys. 30, 585 (1958). t2~ B. HARMA'rz,T. H. HANDLEYand J. W. MtHELICH,Phys. Ret'. 114, 1082 (1959). ca~ K . P. JACOB, J. W. MINELICH, B, HARMATZ a n d T. H. HANDLEr, Phys. Rev. 117, 1102 (1960).

~4~M. C. MICHELand D. H. TE~IPLE'rON,Phys. Rev. 93, 1422 (1954). I~ D. R. NEIHAWAY,M. C. MICHELand W. E. NERV[K.Phys. Rev. 103, 147 (1956). t6J R. G. WILSONand M. L. POOL, Phys. Rev. 120, 1827 (1960). tTJ F. D. S. BUTEMENTand P. GLENTWORTH,J. Inorg. Nucl. Chem. 15, 205 (1960). ts~ R. A. NAUMAN,M. C. MICHELand J. L. POWER,J. lnorg. Nut'l. Chem. 15, 195 (1960). t~

K. S. TOTH, S. BJORNHOLM, M. 1-I. JORGENSEN, O. B. NIELSEN, O. SKILBREH) a n d A. SVANHEDI-N, J. htorg.

Nucl. Chem. 14, I (1960). 1

193

194

S. BJI3RNHOLMet al.

Subsequently this fraction was mass-separated. The mass-separated samples were labelled "Bsamples" and were ready 11 hr after the bombardment. A 3 × 3 in. NaI(TI) scintillation crystal was used in connexion with a 256-channel analyser to record the y-spectra of the two sets of samples. The decay of each mass number was followed, using a windowless, flow-type proportional counter operated on the /Y-plateau, as well as using a singlechannel scintillation spectrometer with a 1 × 1 in. NaI(Tl)-crystal. RESULTS T h e d e c a y o f e a c h s a m p l e was f o l l o w e d f o r a b o u t f o u r m o n t h s . Fig. 1 s h o w s t h e v a r i a t i o n in t h e activities o f t h e t w o series o f s a m p l e s , " A " a n d " B " , w i t h respect to mass number and time after termination of the bombardment. The values of the c o u n t i n g rates w e r e t a k e n f r o m t h e f l o w - c o u n t e r m e a s u r e m e n t s , a n d c o r r e c t e d f o r g e n e r a l b a c k g r o u n d only.

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FIo. l(a). FIG. l.--Radioactive decay of mass-separated samples. Part A shows the proton irradiated erbium oxide target resolved in mass numbers 159-171. Part B (next page) shows the thulium fraction of the same target resolved in the same mass numbers. Points representing activities measured simultaneously are connected with curves. Each curve is labelled with the time after termination of the bombardment. The time intervals between the curves increase with a factor of four.

Some neutron-deficient isotopes of thulium and erbium

195

The predominant reactions occurring in the erbium oxide target irradiated with 65 MeV protons are (p, xn) reactions with x = 3, 4 and 5. The distribution of resultant activities versus mass number shown in Fig. 1 illustrates this fact, taking the abundances of stable erbium isotopes into consideration. Every mass number showed a complex decay. Besides the activity of the proper mass number each sample contained background activities of different origin: ( 1) Part of the radioactive material placed in the ion source of the isotope separator was almost uniformly distributed over the collector during the separation, thus giving rise to a "smear-our' activity having the same relative composition as thc A- and B-samples before mass separation. The magnitude of the "smear-out" activity in a set of samples was about one per cent of the total activity in the set. (2) In every sample corresponding to a given mass number a certain part of the activities of the two neighbouring mass numbers was found, usually a few per cent. r I i i r i

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This contamination was due to the finite resolving power of the isotope separator, which generally yields a separation factor of the order of one hundred. The efficiency of the chemical separation can be checked by means of the activity of mass number 169 in the two series, " A " and "B". l~gTm is stable; hence mass number 169 in the B-series should not display any activity when corrected for background

196

S. BJORNHOLM et al.

effects. However, an amount of 169Er was found to be present in the B-sample, too. Comparison between the relative amount of leaEr in the A- and B-samples yields a value of approximately twenty-five for the chemical purification factor. A sample of the target taken out before mass separation was tested for z-activity with the flow-counter operated on the ~-plateau. No 0c-activity was detected, the ratio of 0~- to 8- counts being less than 10-5.

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Table 1 lists the half-lives of the nuclides belonging to a given mass number as determined from the corrected decay curves. Comparison between corresponding A-samples (not chemically separated) and B-samples (thulium fraction) provides element assignment for the thulium activities, but in general it is not possible to assign any atomic number to the daughter activities. However, daughter activities * Note that unambiguous element assignments only can be made on masses 163 and 165. The y-spectra of masses 162 and 164 are evidently due to contamination from other mass numbers.

Some neutron-deficient isotopes of thulium and erbium

197

TABLE 1.--HALF-LIVES Sample A

Sample 13

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AND RELATIVE INTENSITIES

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Element assignment Ho Ho Ho Er Er Er Tm Tm Tm Tm Tm Tm Tm Tm Tm Tm Tm Tm Tm Tm

Reference 2, 12 2 2, 12 2, 13 13 13 2 2 2 2 2 2 2 2 2 2 2 2 2 2

* Intensities are given as per cent of total X-ray intensity. t Part of the intensity could be due to the sum line of 2 K--X-rays. ~/ Intensities are given as per cent of the part of the X-ray intensity which was calculated to belong to the l~>Tm decay.

S. BJORNHOLM et al.

198

could in all cases be identified with previously reported isotopes of erbium, holmium or dysprosium. We have listed these isotopes in Table 1 with the symbols in parentheses. Fig. 2 shows a series of ?'-spectra recorded with the same setting of the scintillation spectrometer. The X-ray peaks have been left out. Table 2 lists the intensities of the photopeaks in per cent of the X-ray intensity for some of the nuclides. All intensities were corrected for crystal efficiency. 10 4

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FIG. 3.--Decay of mass number 163 of the thulium fraction, recorded with a windowless flow-type proportional counter. The part of the curve with a small slope is due to "smearout" activity, proportional to the total activity of the thulium fraction before mass separation. As such, it is extrapolated and subtracted from the gross decay curve. Remaining is a 1.8 -~0.I hr half-life belonging to te~q'm (in transient equilibrium with the t " E r daughter).

In the following each mass number is discussed separately. 159. The A-sample contained a 130 ± 20 clays half-life, which can be ascribed to 144 days ~59Dy.(1°) 160. The A-sample decayed with a 28.7 ± 0.5 hr half-life. The ?'-spectrum of this mass number showed peaks at 95 and 200 keV and some less pronounced lines at 620, 720 and 900 keV, besides the 810 keV peak due to contamination with mass number 161. NERXaI~and SE~ORGm) have reported 29.4 hr 16°Er to decay by pure K-capture with no ?'-rays to 5-0 hr ~e°Ho, for which ?'-rays of energies 87, 194, 650, 730, 890 and 970 keV were observed. This is consistent with our data for mass number 160, and we can thus assign the 28.7 hr half-life to le°Er and the ?,-spectrum to l~°Ho. 161. No Tin-activity was observed in the B-sample, whereas the A-sample showed B. H. KETELLE and A. R. BRosl, Phys. Rev. 116, 98 (1959). (lt) W. E. NERVIK and G. T. SEABORG,Phys. Rev. 97, 1092 (1955).

(to)

Some neutron-deficient isotopes o f thulium and erbium

199

a 3.2 - 0.1 hr half-life with indications of a 1.9 -- 0.5 hr daughter growing in. These two activities can be ascribed to ~61Er and 18~Ho, respectively. For this pair, half-lives of 183 rain (3.1 hr) m) and 2.5 hr (12) have been reported. The ),-spectrum of mass 161 is shown in Fig. 2 and Table 2. The assignment of the ;,-lines can be made on the basis of literature only.(Z,]z, TM One line at 295 keV seems to be new.

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FIG. 4.--Decay of mass number 165 or the thulium fraction recorded with a windowless

flow-type proportional counter. The high production yield of 30.1 hr a~Tm makes the background effects relatively less pronounced. The growth of the 9-8 :- 0.4 hr ]~SEr daughter is shown in the small insert.

162. The activity of this mass number was due solely to background from neighbouring mass numbers and " s m e a r - o u r ' . l~2Tm is probably shortlived. In this case, an upper limit o f a b o u t 45 min can be put on the half-life, assuming 16ZTm to have the same production rate in the cyclotron bombardment, and the same counting efficiency in the flow-counter, as l°aTm. 163. The half-life of leaTm was found to be 1-8 :'-: 0-I hr (see Fig. 3). Indications of a growth in activity with half-life of the order of one hour could be observed. A ~t2) T. H. HANDLEV, Phys. Rec. 94, 945 (1954). ,z31T. H. HANDLEYand E. L. OLSON, Phys. Rer. 93,524 ('1954).

200

S. BJI3RNHOLM et al.

75 rain half-life has been assigned to 16aEr by HANDLEYand OLSON.(l'l) X6aErdecays in turn to XaaHo. This nuclide is believed to possess a very long half-life. We are able to set a lower limit on the 16~Ho half-life of roughly 500 years on the basis of the decay curves. All the ?-rays listed in Table 2 could be identified with the conversion lines reported for the l~Tm decay. (~) Several of the ),-rays appear to be double. 164. No X64Tm activity was detected. Using analogous arguments as in the case of l°~l'm, we set an upper limit of approximately 45 rain for the l~Tm half-life. 165. The decay curve of the B-sample is shown in Fig. 4. 30.1 -- 0.1 hr ~65Tm, with a 9-5 ± 0"5 hr daughter growing in, was observed. The daughter activity is ascribed to l~Er. These two half-lives have previously been measured to be 29 hr ct4) and I0.0 ± 0.1 hr, (m respectively. Two ?-spectra of mass number 165, corresponding to different mixtures of 16STmand 165Er, were recorded. All ?-rays maintain the same relative intensities, whereas the X-ray line in the sample most abundant in ~65Er was more intense. We therefore assign all the ),-rays to the 165Tin-decay, although this procedure is subject to some uncertainty in the case of the weaker lines. The energies of the ?-rays agree well with the previously measured conversion line spectrum. 166. ~66Tm decayed with an 8.0 ± 0.2 hr half-life. The decay was studied, using a six-gap fl-spectrometer in connexion with coincidence measurements. This study will be published separately. 167 and 168. Half-lives of 9.3 + 0.1 days and 95 ± 10 days were found for 167Tm and 168Tm, respectively. 169. The half-life of X69Er, present in the A-sample, was measured to be 9.6 ± 0.1 days. ix4) T. H. HANDLEYand E. L. OL.SON, Phys. Rev. 92, 1260 (1953). (xs) F. D. S. Bo'rEMENT, Proc. Phys. Soc. Lond. 63 A 775 (1950).