Gamma rays following the decay of Nd147 and Sm153

Nuclear Physics 15 (1960) 6 5 3 - - 6 5 6 . (~) North-Holland Pubhsh,ng Co, Amsterdam Not to be reproduced by photoprmt or nncroflhn w~thout written pernnsston from the pubhsher

GAMMA T

J

RAYS

F O L L O W I N G T H E D E C A Y OF N d '47 A N D S m z~

W A L T E R S t, j

H

WEBBERt,

N

C RASMUSSEN and HANS MARK

Department o/Nuclear Eng,neering and Department of Physws, Massachusetts Inst, tute o/Technology, Cambmdge 39, Massachusetts t t R e c e i v e d 28 D e c e m b e r 1959 A t w o m e t e r r a d m s b e n t q u a r t z c r y s t a l s p e c t r o g r a p h h a s b e e n u s e d to s t u d y g a m m a r a y s following t h e f l - - d e c a y of N d l ~ a n d S m 16s A g a m m a r a y of 91 0 5 4 - 0 04 k e V w a s o b s e r v e d following t h e d e c a y of N d x'7. T h i s g a m m a r a y c o r r e s p o n d s to t h e t r a n s i t i o n b e t w e e n t h e first e x m t e d level a n d t h e g r o u n d s t a t e of t h e isotope P m l ' L T h r e e g a m m a r a y s a r e o b s e r v e d in t h e d e c a y of S m 15a T w o s t r o n g lines a t 103 174-0 04 k e V a n d 69 6 6 4 - 0 02 k e V c o r r e s p o n d to s h o r t - h v e d i s o m e r i c levels in E u lss T h e s e h a v e b e e n o b s e r v e d p r e v l o u s l y . I n addxtlon, a w e a k line a t 97.424- 0 04 k e V as p r e s e n t . A line a t a p p r o x i m a t e l y t h i s e n e r g y h a s b e e n o b s e r v e d m t h e electron c a p t u r e d e c a y of G d 15a to E u 15s T h e i n t e n s i t y r a t i o of t h e 103 17 k e V t r a n s i t i o n to t h e 97 42 k e V line is g r e a t e r t h a n 20 to 1

Abstract:

1. I n t r o d u c t i o n and E x p e r i m e n t a l M e t h o d s

The availability of very strong radioactive sources has revived interest in using bent quartz crystal spectrometry to measure nuclear gamma ray energies. Several groups have been pursuing studies of this type in recent years. There are two methods for using the bent quartz crystal to measure gamma ray wave lengths. One is the technique developed b y DuMond 1) and his co-workers with the Mark I spectrometer. In this instrument, a line source is placed on the focal circle of the crystal and a counter on the convex side of the crystal is used to observe the gamma rays. The most accurate wave length measurements of m a n y nuclear gamma ray lines to date have been made with the Mark I instrument. Another w a y of using the bent crystal is to place an extended source on the convex side of the crystal and to use a photographic emulsion on the focal circle of the spectrograph to record the gamma ray lines. This method was introduced b y Y. Cauchois 3) and has been developed for the study of nuclear gamma rays b y the Livermore group s). In the present serms of experiments, a two meter radius spectrograph is used in the Cauchois geometry to measure gamma rays following the/~--decay of various rare earth isotopes. Since it is very difficult to make accurate absolute measurements of gamma ray wave lengths using the Cauchois geometry, it is necessary to cahbrate the photographic plates with gamma rays of accurately t S u b m i t t e d m P a r t i a l F u l f i l l m e n t of t h e R e q m r e m e n t s for t h e degree of M a s t e r of Science a t MIT t t T h i s w o r k w a s s u p p o r t e d b y a g r a n t f r o m t h e N a t i o n a l Scmnce F o u n d a t i o n 653

65z~

T

]', W A L T E R S , J

H. W E B B E R ,

N . C. R A S M U S S E N A N D H A N S MARK

known wave lengths. As in the past 8), the wave lengths of gamma rays and X-rays observed in the decay of Ta ls~ are used for this purpose. The values of the wave lengths used are shown in table 1. TABLE 1 Wave lengths of cahbratmn hnes Element

Line

Wave length m Smgbahn X-umts

Tantalum

Ksh X-ray

215 0 5 0 ± 0 010 s)

Tungsten

Kc¢s X-ray K~ 1 X-ray

213 3 8 3 ± 0 010 s) 208 5 7 1 ± 0 010 a)

68 keV 100 keV

182 6 3 8 ± 0 018 b) 123 5 9 9 ± 0 014 b)

Tungsten gamma rays from W i n

a) E Inglestam, Nova Acta Reglae, Soc. SCL Upahenms 4, No 5 (1936) b) Murray, Boehm, Marmmr and DuMond, P h y s Rev 97 (1955) 1007

The methods used to determine the unknown wave lengths are precisely the same as those described in ref. 3). The wave length scale of the plate is determined by making a least squares fit using the calibration lines of known wave length. The instrument parameter determined in this manner is then used to compute the unknown wave lengths. The errors in the wave lengths are determined using only the statistical theory of errors since systematic errors in the system are small. Wave length in the 100 1T~ region can be determined with a precision of about one part in two thousand. TABLE 2 Gamma rays following the decay of Yb i n (from energy levels m Tm is°) Energy m keV (Present work) 63 109 130 177 197

11±0 77±0 53±0 27±0 99±0

02 04 06 10 13

Energy m keV (DuMond group) 63.12±0 109 7 8 ± 0 130 5 3 ± 0 177.24±0 197 9 7 ± 0

01 02 03 05 06

The wave lengths of the DuMond group gamma rays are taken from. Hatch, Boehm, Marmmr DuMond, Phys. Rev. 104 (1956) 745

and

The initial measurements made with the two meter crystal spectrograph were conducted to check the reliability of the instrument. A Yb le9 source was made at the MIT Reactor and the low energy gamma rays emitted by the source

GAMMA RAYS FOLLOWING THE DECAY OF Nd 1¢? AND S m 16a

6~

were studied. The energms obtained are shown in table 2 and are compared to the energies obtained b y DuMond and his co-workers using the Mark I spectrometer 2. R e s u l t s 2 1. N d 1'~

A sample of Nd~O a was irradiated in the MIT Reactor. The only isotope made in appreciable quantities is 11.1 d a y Nd 14~which results from neutron capture in 17.2 ~ abundant Nd 14e. The source strength obtmned was roughly 0.3 curie. This isotope decays b y fl- emission to P m la~. Many gamma rays have been observed ~) following this decay which correspond to the energy levels in the daughter nucleus. Seventy-nine percent of the fl-decay goes to the first excited level of Pill 14~. A gamma ray of 91.05~-0.04 keV was observed after a source exposure of approximately 20 curie-hours. This energy corresponds to the energy of the lowest observed level. Unfortunately, none of the other gamma rays emitted b y this isotope were intense enough to be visible on the plate. 2 2. S m 168

A sample of Sm~.O a was irradiated in the reactor. The isotope Sm lss is made b y neutron capture on 26.6 ~/o abundant Sm ls~. Sm 15a decays b y fl- emission to Eu 153 with a half life of 47 hours. The fl--decay is accompanied b y g a m m a rays corresponding to energy levels in Eu 15a. The level structure of Eu 1~3 is quite complex. It has been studied b y observing the fl--decay ~) of Sm 15~, the electron capture decay s) of Gd 15a and b y coulomb excitation 9) of stable Eu 1~. Neither ~6-decay apparently populates the rotational levels observed in the coulomb excitation experiments. Two short-lived isomeric states at 173 keV and 103 keV are populated in the fl-decay of Sm 16a. Two gamma rays at 103.17±0 04 keV and 69.66~0.02 keV are observed on the plate. The former corresponds to the 103 keV level to the ground state and the latter is the cascade transition between the 173 and the 103 keV levels. In addition to the two strong gamma rays mentioned above, a very weak line at 97.42~0.04 keV was also observed. A gamma ray of thls energy is observed in the electron capture decay s) of Gd ls3. It corresponds to a transition between a level at 97.42 keV and the ground state in Eu laa. This level has also been observed b y A. W/. Sunyar 10) using an electron-conversion spectrometer. The possibility that the observed gamma ray could arise from a gadolinium impurity in the Stag O a sample can be ruled out on several grounds. The principal one is that the electron capture has a half life of 236 days whereas the activity of the sample used in these experiments was substantially gone after a few 47-hour half-periods. The source strength of the isotope was estimated at one curie and a total exposure of the order of 100 curie-hours was made From the lines on the

656

T J WALTERS, J H WEBBER, N C RASMUSSEN AND HANS MARK

plate it is estimated that the 103 keV gamma ray is at least 20 times as intense as the 97 keV gamma ray. TABLE 3 Experimental results

e,) b) e) a)

j. O E B

Isotope

Gamma ray wave length in X units

P m 1'7

135 88-1-0 05

91 054-0 04

Eu 1is

117 60-1-0 05 127 0 0 ± 0 05 119 92-~0 05

69 66~0.02 97 42-4-0 04 103 1 7 ± 0 04

Gamma ray energy In keV

Previous measurements of gamma ray 91 3 a) 69 97 103 103

66 b) 3 c) 1 8 i 0 04 b) 2 7 ± 0 02d)

M Cork M K. Brlce, R G Helmet and R M Woods, J r , Bull Am. Phys Soc. 3 (1958)64 Beckman, Nuclear Instruments 3 (1958) 27 L. Church and M Goldhaber, Phys Rev. 95 (1954) 626A Anderssen, Proc Phys. Soc. A 69 (1956) 415

The wave lengths and energies of all the gamma rays measured in the present work are shown in table 3. The authors would like to thank Dr. John Garfield of Brookhaven National Laboratory for processing the emulsions. Thanks are also due to the MIT reactor staff and to Professor T. J. Thompson for their help and encouragement. References 1) J V~ M. DuMond, m Beta and Gamma Ray Spectroscopy, Ed. K Smgbahn (North-Holland Publmtnng Co, Amsterdam, 1955) Ch IV Y. Cauchom, Comptes Rendus 199 (1934) 857 Chupp, DuMond, Gordon, Jopson and Mark, Phys Rev. 112 (1958) 518 Murray, Boehm, Marmmr and DuMond, Phys Rev 97 (1955) 1007 T S Walters and H J. Webber, Measurements of Some Rare E a r t h Gamma Rays Using a Two Meter Bent-Crystal Spectrograph, S. M Thesm, MIT, 1959 (unpubhshed) 6 Strommger, Hollander and Seaborg, Revs Mod. P h y s 30 (1958) 585 7 Dubey, MandevlHe and Rothman, Phys. Rev 103 (1957) 1430 8 E. L Church, Private commumcatlon quoted m ref. 6) 9 H. Mark and G T Pauhssen, Phys Rev. 100 (1955) 813 10 A. W. Sunyar, private commumcatlon