IL NUOV0 CIMENT0 VOL. X X X I , N. 2 16 Gennaio 1964

Measurement oI Half-Lives of the 570 keV Level of 2°7pb

and 2 083 keV and 2 412 k e ¥ Levels or ~4°Ce.

GIRISH CHA]NDRA

Tata I~stitute o/ Fte,ida,ttetttal Research - Bo,tttbay

(ricevuto il 19 Agosto 1963)

S u m m a r y . Delayed 7-Y and ~-y coincidence technique has been employed to measure the half-lives of the 570 keV level of 2o7pb and 2083 keV and 2412 keV levels of ~40Ce nuclei, which are found to be (1.8 0.~).+°'2 10_~0 s, (3.3:L0.4)-10-gs and (7:L3)'10-11s, respectively. A bridge type coin- cidence circuit has been used as t ime-to-ampli tude converter for these measurements. I ts performance is described.

1. - I n t r o d u c t i o n .

The s ing le -pa r t i c l e mode l should be e x p e c t e d to be a p p l i c a b l e to l ow- ly ing

s t a tes of 2°Tpb nuc leus s ince i t has on ly one n e u t r o n hole in a d o u b l y closed

shell conf igura t ion . I n f ac t t i le shell m o d e l co r r ec t l y p r e d i c t s t he spin a n d

p a r i t y a s s i g n m e n t s (~). H o w e v e r , t he r e d u c e d E2 t r a n s i t i o n p r o b a b i l i t y B (E2)

for t he 570 k e V level of ~°TPb o b t ~ i n e d f rom the Cou lomb e x c i t a t i o n work (2.3)

is m u c h l a rge r t h a n is e x p e c t e d for t r a n s i t i o n b e t w e e n n e u t r o n s t a tes a n d is

of t he s ame o rde r of m a g n i t u d e as for t r a n s i t i o n b e t w e e n p r o t o n s t a tes (based

on shell model ) . The l i f e t ime of t h e 570 k e V level has been m e a s u r e d b y

GERHOLM (~)~ SU~'YAR (3.5) a n d GORODETZKY (3,~) b u t t h e i r va lues do no t ag ree

(1) D. E. ALBURGER and A. W. SU1NYAR: Phys. Rev., 99, 695 (1955). (2) p . H. STELSON and F. K. McGOwAN: Phys. Rev., 99, l l 2 (1955). (3) 0. NATHAN: Nucl. Phys., 30, 332 (1962). (a) T. 1~. GERHOLM: Ark. ]. Fys., 10, 523 (1956). (5) A. W. SUNYAR: Proc. Peace]ul Ueses At. Energy {Ge~teva), 14, 347 (1958). (6) S. GOROD]]TZKY, R. MANQUENOUILLE, R. RICIIERT and A. KNIPPER: Jour~.

Phys. Rad., 22, 699 (1961).

20~ GIRISH CHANDRX

well with each other. The l ifetime obtained by SUBIYAR (3) is in excellent agreement with STELSON and McGowAN'S results ('~) bu t in very margina l

ag reement with ~ATHAN (3). Such anomaly has also been recent ly (7) observed in semi-magic 140Ce nu-

cleus. I t is a spherically symmet r ic nucleus with a closed neutron shell and 8 protons filling a subshell. The l ifet ime of the first excited s ta te of l~°Ce

at 1596 keV (s) is found to be about 17 t imes shorter than the single p ro ton

es t imate and in contrast with this the l i fet ime of the 2 083 keV level as meas- ured by CVRRIE is longer than the single-proton es t imate by about the same

factor . Section 2 describes the performance of the bridge type t ime- to-ampl i tude

conver ter and the results of l i fet ime measurements are described in Section 3.

2. - T i m e - t o - a m p l i t u d e converter.

A bridge type coincidence circuit using 1~34 diodes has been described by LUNDB¥ (9) who also describes the theory and performance of the circuit

for fas t coincidence work. Bridge coincidence circuits are well known for thei r

high sensi t ivi ty (---0.1 V) and high singles rejection rat io (9.1o). We have found this bridge circuit (using OA73 diodes in place of 12¢34)

suitable for use as a t ime- to-ampl i tude conver ter for measur ing half-lives of

nuclear exci ted states in the nanosecond region. Wi th a pulse width of 10 -s s in bo th channels a resolution of 6.3.10 -l° s has been obta ined using (2 X3) in . plast ic scintillators coupled to RCA6810 photomult ipl iers . A typica l p r o m p t resolution curve, obta ined with IqaI(T]) crystals and ~°Co y-rays (the energy channels being fixed in the 1 MeV region with a window width of abou t 15 °/o of the ampli tude) , is shown in Fig. 1. This has a full width resolution of 1 .4 .10-9s and a slope with apparen t half-life of 2.5.10 -1° s. Considering the

large size (1.75 in. in diam. ×2 in. high) of the lqaI(T1) crystal used a consi-

derable reduct ion of resolution should be possible by using a smaller crysta l

and fast plast ic scintil lators of small size. Wi th pulse width of 10 -s s a l inear

range of 7-10 -9 s has been obtained. I t should be possible to increase the range

of l inear i ty by increasing the width of the pulse to the coincidence circuit (*). No tail has been observed in all the t ime spectra t aken with this conver ter and

(7) W. H. CURRI~: NucL Phys., 32, 574 1062). (s) S. 0FER and A. SCHWA.RZSCHILD: Phys. Rev., 116, 725 (1959). (~) A. LUNDBY: Rev. Sci. Instr., 22, 324 (1951).

(lo) Z. BAY: Nucleonics, 14, 56 (1956). (') Note added in the proo]. - Wi~h pulse width of 48"1C-9s a linear range of

35.10-9s has ben obtained.

MEASURJ0~3fENT OF H A L F - L I V E S OF T I t E 5" /0 keV L E V E L ]gTC. 299

t h e r e f o r e no a n t i - p i l e - u p c i rcu i t , as de sc r ibed b y SCHWARZSCHILD (11), WaS con-

s i de r ed necessa ry . Thus a b r i d z e - t y p e t.ime-to-a, m p l i t u d e c o n v e r t e r is qu i t e

s u i t a b l e for ha l f - l i fe nma.surements as

d e m o n s t r a t e d b y m e a s u r i n ~ sonic half- 103

l ives d e s c r i b e d below. 8

The t i m e s p e c t r a were r eco rded b y 5

N u c l e a r D a t a 512 channe l pu l se -he igh t

a n a l y s e r . Ti le c a l i b r a t i o n of t he con- 4 r

v e r t e r was m a d e b y obse rv in~ the een- L

t r o i d sh i f t s of p r o m p t curves for dif- i

e r e n t l eng th s of R G I 14 A / U cable in one -~ 2

channe l .

I n m e a s u r e m e n t s where een t ro id -

sh i f t m e t h o d was used to ca l cu la t e half- ~ lO ~

l ives care was t a k e n to m i n i m i z e the

e e n t r o i d sh i f t due to o t h e r causes , for g 6 ° L e x a m p l e , change in c o u n t i n g ra t e , t h a n - 4

ge in ene rgy r ange a c c e p t e d b y single-

channe l aua ly se r s a n d smal l l ong - t ime

d r i f t s in t he e lec t ron ics e q u i p m e n t . 2

The s t r e n g t h s of t he sources and the

d i s t a n c e b e t w e e n the c rys t a l s a n d source

were so a d j u s t e d t h a t t he t o t a l eoun t - 10 L

ing ra t e s d u r i n g the (< p r o m p t ~ and << de-

l a y e d ~ runs were a p p r o x i m a t e l y equal . Fig.

The b iases a n d w i n d o w wid th s were

k e p t s ame in b o t h t he runs . The long-

t i m e d r i f t s were m i n i m i z e d b y t a k i n g the

* p r o m p t ,~ a n d (( d e l a y e d ~) co inc idence

runs a l t e r n a t e l y t i l l suff icient counts

.4x10"9 s

I ~-Fsl 2 ~- - :

60 7O 8O 90 chanhel number"

1 . - A prompt resolution curve obtained with 6oCo 7-rays and NaI(T1) crystal 1 t i n . in diameter and 2 in . height. The energy channels were fixed in the 1 MeV region with a window

width of about 15°,o .

were co l lec ted for each source. Tile ha l f - l ives g iven here a re t he ave rages of

fou r s e p a r a t e runs for each e x c i t e d s t a t e measu red .

3 . - R e s u l t s .

3"1. The 2 083 k e V level o/14°Ce. - The hal f - l i fe of t h i s leve l has been m e a s -

u r e d b y Cvnm]~ (7) who f inds a v a l u e of 3 .44 .10 -9 s. H e t o o k co inc idences

b e t w e e n p - r ays s t o p p i n g in a p l a s t i c s c i n t i l l a t o r a n d 487 k e V T-rays s t o p p i n g

in a NaI(T1) c r y s t a l . W e h a v e m e a s u r e d th i s ha l f - l i fe b y t a k i n g co inc idences

(11) A. SCIIWARZSCI-IILD: 2V, UC~. Instr. and Meth., 21, 1 (1963).

3 0 0 G I R I S H C H A N D R A

2

ol

8

2

lo z 30

22Na between 329 and 487 keV T-rays prompt both stopping in NaI(T1) crys-

o f f ~ . ~ i tals. (~-rays were absorbed by x3 ½ in. thiek perspex discs.) By

means of single-channel pttlse- height analysers only the cen- tral portion of the photo-peaks

° ° ° / It o, k e Y , o 0 o/ of the amplitude were ac- ,/O

3" ~ 2412 / ~ I cepted. A prompt resolution 2os3 ,/ II curve also taken

2 ~ 9~ / i I keeping the same biases and

window widths in the single- 0+ '--z--- 0 L i l channel anMysers as in the case

of delayed coincidences. Both these curves are shown in Fig. 2.

40 50 60 70 80 90 channel number

Fig. 2. - Delayed curve for 2083keV level of t4oCe by taking coincidence between 329 and 487 keV ~(-rays. It also shows prompt curve ob- tained with 22Na y-rays for the same energies.

The hMf-life as read from the slope of the delayed curve is found to be (3.3 -L 0 . 4 ) . 1 0 -g s.

This curve will have the ad- mixture of prompt curve due to the ('ompton distribution of

(815--'.1596) keV cascade which will affect the delayed curve only in the vicinity of the maximum but will not affect the linear part of the curve as is evident from the prompt curve obtained with 2~Na. This measurement 4 ( a / ~ . ~ ~ has also been repeated by taking coincidences between ~-rays stopping in anthracene crystal "° and 487 keV y-ray stopping in a NaI(T1) crys- "~ 2 tal. Same value for half-life is obtained.

3"2. The 2412keV level o] 14°Ce. - This g 10~ was measured by taking coincidences between g 8

{_ !

~-rays stopping in an anthracene crystal and ~ 6

the 815keV T-rays stopping in the N a I ( T 1 ) ~ i i ~ ! erystM. The window width in T-channel was about 15 % of the amplitude and about 40 %

Fig. 3. - It, shows the prompt and delayed curves o due to 6oCo and 14°La respectively due to the coin- cidences between ~-rays and 815 keV y-ray in order 10 50 50 70 to measure the half-life of 2412 keV level of 14°Ce. channel number

M E A S U R E M E N T OF t t A L F - L I V E S OF T H E 5 7 0 keV L E V E L E T C . 301

i n the ~-ehannel . The p r o m p t reso lu t ion curve was t a k e n wi th 6°Co wi th the

same biases and window widths . The resul t is shown in Fig. 3. The hMf-life,

as e s t ima ted b y the cent ro id shift me thod , is found to be (7 ~ : 3 ) . 1 0 - ~ s.

3"3. The 570 keV leoel oj 2°7Pb. - GElCrIOLSt (~) has measu red the half-life

of th is level by se l f -compar ison me thod ,

with an a u t o m a t i c coincidence equip-

m e n t a n d found a va lue of (9 ~ :3) -

• 10 - ~ s. La t e r Sv~wAa (5) measu red this

half-l ife us ing a biased diode coincidence

c i rcui t as a t i m e - t o - a m p l i t u d e con-

ve r t e r and found a va lue of (1.1=i=0.15).

-10 -~° s. A more recent lnea~urelll(~llt

of m e a n l i fet ime by St~NYAR (~) is

(1.44 ~ 0.15)" 10 -~° S, G0~t0DETZK¥ et

al (~.6) repor t a, va lue of (1.6 ~ 0.16)-

• 10 ~°s for the l i fe t ime of this level.

We have used the eent ro id shift

m e t h o d to (,ah,ulate the half life of the

570 keV level of ~'°TPb. P las t ic scint i l -

la tors were used in bo th the channels .

The bia.ses of the sinffle-eh~:mel ana ly -

sers were kep t at the Compton edge of

the 1060 nnd 570 keV y-rays respecti-

vely ,~nd the window width were abou t

(10--12) ° o of the amp l i t ude . 6°('o y-rays

were used to record the p r o m p t curve.

The resul t is shown ia Fig. 4. A value

of (t .8~: '~)-10-~°s for the half-life is

ob ta ined . This i:s higher t lm:l the

ear l ier wdues.

103~ 8

6 , L

F L 4 , i

£g

~102 t 8

4

~x2

'l' 1 i / x I

2 i I ~ ×

10 50 60 70 80 channel number

Fig. 4. Measurement of half-life of the 570 keV level of ~°7pb by taking eoinci- den(,e~ between 1060 and 570 keV ,,-rays. Prompt curve due to 6°Co is also sh.wn.

The au thor is t h a n k f u l to Professor B. V. THOSAR for his in te res t in this

work and also to Mr. A. T. RANE for prep~r ing the sources.

R I A S S U N T ( ) (*)

Si b usata la tecnica delle coincidenze y-,; e [~--; r i tardate per misurare le vile medie del livello di 570 keV del ~°TPb e dei livelti di 2083 keV e 2412 keV dei nuclei di 14°Ce, e si b trovato che sono ( +,.2 10_10 1.8 o.~)- s, ( 3 . 3 ~ : 0 . 4 ) ' 1 0 - 9 s e ( 7 ~ : 3 ) . 1 0 - X l s , rispettiva- mente. Per queste misure si 6 us~to un circuito di coincideuza del tipo a ponte, come convertitore tempo-ampiezza. Se ne descrive il funzionamento.

(*) T r a d u z i o ~ e a e~o'a d e l l a R e d a z i o n e .