Appl. Radial. Isor. Vol. 40, No. 1. pp. 33-36, 1989 hr. J. Radial. Appl. Instrum. Parr A Printed in Great Britain. All rights reserved

0883-2889189 163.00 + 0.00 Copyright Q 1989. Pergamon Press plc

L ifetime Measurement of the 2083. I-keV Level

in 14’Ce and the 1289.1%keV Level in ls2W

RAVINDER KAUR’* and P. N. TREHAN’

‘Department of Physics, Punjab Agricultural University, Ludhiana-141004, India and 2Department of Physics, Panjab University, Chandigarh-160 014, India

(Received 29 September 1987; in revised form 10 May 1988)

The half life of the 2083. I-keV level in ‘@Ce has been measured using plasticcplastic and plasticcNaI(T1) scintillation coincidence systems. An average of the two measurements shows that r,:, = 3.40 k 0.09 ns for this level. The half life of the 1289.15keV level in ls2W has been found to be 1.09 & 0.04 ns using a plasticcplastic coincidence system.

Introduction

The knowledge of exact lifetimes of various nuclear- energy levels is very important for checking the validity of different theoretical models (England, 1974; Fossan and Warburton, 1974). The lifetime of the 2083.1-keV level in 14’Ce has been measured by many research workers (Curie, 1962; Schmorak et al.,

1964; Chandra, 1964; Bond et al., 1971). All these workers except Chandra (1964) have measured the mean lifetime of this level. Only Chandra (1964) has measured the half life of the 2083.1-keV level directly using a delayed coincidence method. However, the uncertainty quoted in his value is very large (Table 1).

The lifetime of the 1289.15keV level in “*W has been measured by Meiling and Stary (1966), Abou Leila et al. (1970) Hoglund er al. (1971) and by El-Dagmah and Stewart (1983). Surprisingly the value reported by different authors is showing an increasing trend with time. The latest value reported by El-Dagmah and Stewart is considerably larger than the previous values (Table 1).

In view of the above discrepancies it was thought worthwhile to remeasure the half life of the 2083.1- keV level in ““Ce and the 1289.15keV level in ‘**W using a very good timing system, in which time walk and time jitter effects have been considerably reduced relative to other reported measurements.

Experimental

The radioisotopes 140La and ‘**Ta were obtained from the Bhabha Atomic Research Centre, Trombay, Bombay in liquid form. The lmLa source was allowed

*Author for correspondence.

to decay for 2 days before using it for the actual measurements so as to minimise any short-lived activity, if present in the source The impurity checks were made by taking a number of single spectra of 14’La and ‘**Ta using a 64.1-cm’ Ge(Li) detector having energy resolution of -2.1-keV at 1.33-MeV of 6oC0. The half life of the source under study was followed while taking these spectra.

For life-time measurements the sources were pre- pared by depositing a few drops of the liquid at the centre of a cellophane tape fixed on an aluminium ring.

An experimental system of two plastic p-particle detectors (NE 102, 1 x 1 in.), constant fraction discriminators and a time-to-pulse-height converter shown in Fig. 1, was used to measure the half life of the 2083.1 -keV level in i4’Ce and the 1289.15keV level in 18*W. The half life of the 2083.1-keV level was also measured using a plastic-NaI(T1) coincidence system, and the value obtained was combined with the previous value to give an average half life for the 2083.1-keV level.

Table I. Half lives of the 2083. I -keV level in ‘“Ce and 1289. I5-keV level in “‘W

Level

(keV) Ref. ri20-N

2083. I

Curie (1962) Schmork et al. (1964) Chandra (I 964) Bond er al. (1971) Present work

Meiling and Stary (1966) Abou-Leila er al. (1970)

1289.15 HGglund et al. (1971) El-Daghmah and Stewart (I 983) Present work

3.44 i 0.06 3.44 * 0.09

3.3 f 0.4 3.45 + 0.09 3.40 * 0.09

I .oo i 0.05 I .06 + 0.02 I. I2 i 0.02 I. I7 k 0.08 I .09 + 0.04

33

34 RAVINDER KALJR and P. N. TREHAN

SOURCE I

L_._I MCA

Fig. 1. Block diagram of the lifetime measuring system involving plastic detectors.

Measurements and Results taking time spectra with a 22Na source and adding fixed delays in the start or stop channels.

Harf lives of the 2083. I -keV level in 14’Ce and the

1289.15-keV level in ‘82W using a plastic-plastic

scintilIation coincidence system

The source to be studied was placed between the two plastic detectors in a sandwiched geometry. The time calibration of the system was performed using an air-line delay (1.0036 + 0.0002)ns manufactured by General Radio, U.S.A. The linearity of the time scale on the multichannel analyser was checked by

The delayed coincidence technique was used for the life-time measurements. The half life of the 2083.1-keV level in IaCe was obtained by measuring the elapsed time between the detection of the 329-keV feeding the 2083.1-keV level and the associated 487-keV transition draining it. The start channel of the time-to-pulse-height converter was gated with the upper 30% of the Compton continuum due to the 329-keV transition and the stop channel with the

:

: . . TIME CALIBRATION=292 ps /CHANNEL

TIME RESOLUTION

OF PROMPT CURVE = 570 ps

IO3 :

‘:.. ‘k.,,

“..

i - ..’ :.,.,

..:.

z - .:...: ,, .

9 - .. ‘, .*,

::’

Y - ‘: ,.(’

k “,..

T,,2=359 fO.OBns

dl 02,

” ‘,.:. . . ._ ‘.:.;,

_._.’ ‘.‘. .._

5 1 . . . . 1 :_

:..: :. ,,

z -

‘. .‘., ,. ., ., .

., . .’ ., ,’ ‘:.

.,‘.. :...., ;.::

:. . . .‘. .,

10’ -

1 t I I I I

250 500 750 1000 1250 1500

CHANNEL NUMBER

Fig. 2. The 329/487-keV delayed coincidence curve and the prompt curve for the lifetime measurement of the 2083.1-keV level using a plastic-plastic system.

Nuclear-energy levels in lmCe and ‘**W 35

,.,$ : . . ‘, : .: :‘:

: .: : : TIME WaLIBRATION=29.2 ps / CHANNEL

i’ TIME RESOLUTION OF .: : : ‘, :.. :, PROMPT CURVE=672ps

., %’

ti

:_ ‘,‘, i

5

: :

2 102r ( .’ ‘L. ‘;‘{. ,.

:;.,

Y : ::. . T,,; 1.09 t 0.04”s

” ‘:‘.;..

f . ...:

2 ; ‘.‘...,‘;.,

: :. .’ :-+.,

. :.. c .,. .

2

..,.: .;. .: :’ . ...‘::

10’ - ‘..

‘_ .,, . .

u .: .“.

‘Z. “.‘. . . .._. ._ .., . . .._ .<.

. . .,. .. ,.e; . . . . ._ i. . .

: .,...

1 . 1 I I 1 I 300 400 500 600 700 800

CHANNEL NUMBER

Fig. 3. The 264/l 189-keV delayed coincidence curve and the prompt curve for the lifetime measurement of the 1289.15keV level in ‘**W using a plastic-plastic system.

upper 20% of the Compton continuum due to the 487-keV y-ray transition to obtain the 329/487-keV delayed coincidence curve. Prompt spectra were obtained at the same gate settings using a 22Na source and the time resolution of the system was found to be 570 ps. The delayed-time spectrum along with the prompt spectrum as obtained in the present measure-

ments is shown in Fig. 2. The half life of the 1289.15keV level in la2W was

similarly obtained using the 264 and 1189-keV tran- sitions feeding and draining that level, respectively. The 264/l 189-keV delayed coincidence curve was recorded on the multichannel analyser using approxi- mately the upper 70% of the Compton spectrum due to the 264-keV transition to gate the start channel, and approximately the upper 20% of the Compton spectrum due to the 1189-keV transition to gate the stop channel of the time-to-pulse-height converter. The prompt curve was also studied at the same Compton settings using a 6oC0 source. The time resolution of the system at these Compton settings has been found to be 672 ps. The delayed and the prompt-coincidence curves as obtained in the present measurements are shown in Fig. 3.

The delayed parts of the coincidence curves so ob- tained were analysed by the slope method (Newton, 1950) after subtracting the respective background counts and prompt coincidences. The slope of the exponential decay curve was then analysed in segments by the least square method of Neal and Kraner (1965). A computer programme written in FORTRAN IV was run on an IBM 1620 computer at Panjab University, Chandigarh to analyse the data.

From the above analysis, the half life of the 2083. I-keV level in 14’Ce and the 1289-I 5-keV level

in is2W have been found to be 3.39 rf: 0.08 and 1.09 + 0.04 ns, respectively. The uncertainty in the half life comprises random and non-random com- ponents. The random component of the uncertainty arising from the fitting procedure was estimated by the computer programme. This was combined in quadrature with the non-random component result- ing from the uncertainty in the time calibration, instability and non-linearity of the coincidence system.

Half life of the 2083. I -keV level in 14’Ce using a plastic-Na (Tl) coincidence system

The half life of the 2083.1-keV level has also been found using a plastic-Nal(T1) detectors system. The time calibration of the system was performed using an air-line delay (1.0036 rf: 0.0002) ns. The 329/487- keV delayed coincidence curve was obtained by gating the stop channel with the 487-keV photopeak and the start channel with 30% of the Compton continuum due to the 329-keV y radiation. The prompt curve at these energy settings was obtained with a 22Na source. The delayed and prompt spectra as obtained in the present measurements are shown in Fig. 4. The delayed part of the coincidence curve was analysed by the slope method (Newton, 1950) after subtracting background and prompt coincidence counts. The analysis results in tllZ = 3.41 k 0.09 ns for the 2083.1-keV level.

An average of the two measurements giving t ,,* = 3.40 + 0.09 ns has been finally adopted and is shown in Table 1 for comparison with other results. The present value shows a considerably lower un- certainty than does the value of Chandra (1964) the

36 RAVINDER KAUR and P. N. TREHAN

TIME CALIBRATION=28~7ps / CHANNEL

TIME RESOLUTION OF

PROMPT CURVE = 1.09 ns

“.,,.; .-_.,

” . ...‘. y”,..

‘.;.: ,. . . .’ ,.

.‘.’ : T,,2 =3.&l+- 0.09ns

:‘ ., . . . . . . . . ,.. .::-. ,.. :. .“,. ,. .,.’

_‘:._. ::.: ., ‘.. .,... ..

11 1 1 I I 1 250 500 750 1000 1250 1500

CHANNEL NUMBER

Fig. 4. The 329/487-keV delayed coincidence curve and the prompt curve for lifetime measurement of the 2083. I-keV level using a plastic-NaI(T1) system.

only other direct measurement of the half life of the 2083.1-keV level.

The present value of t,,, = 1.09 f 0.04 ns for the

1289.15-keV level in ‘**W is closer to the value reported by Abou-Leila et al. (1970) and Hoglund et al. (1971), than that of El-Dagmah and Stewart (1983). El-Dagmah and Stewart (1983) have actually measured the sum of the lifetimes of the 1289.15 and 1553-keV levels using plastic-Ge(Li) detectors. They have corrected for the half life of the 1553-keV level to obtain the half life of the 1289.15-keV level. This may be the reason for getting a large value of t,,* for the 1289.15-keV level as compared to the present result and the previous measurements.

References

Abou-Leila H., Darwish S. M., Abd-El-Haleim A. and Awward Z. (1970) Lifetimes of some excited states in ‘s2W. Nucl. Phys. A 158, 568.

Bond P. D., McGervey J. D. and Jha S. (1971) Measure- ments of some nuclear lifetimes in the nanosecond region. Nucl. Phys. A 163, 571.

Chandra G. (1964) Measurement of half lives of the 570-keV level of 207Pb and 2083-keV and 2412-keV levels of lUCe. Nuovo Cimento 31, 297.

Curie W. M. (1962) Lifetime of the 2.083-MeV state in ‘“Ce. Nucl. Phys. 32, 574.

El-Dagmah M. S. S. and Stewart N. M. (1983) Levels and transitions in ls2W following the decay of ‘s*Ta. Z. Phys. A 309, 219.

England J. B. A. (1974) Techniques in nuclear structure Physics, Part 2, p. 555. McMillan. London.

Fossan D. B. and Warburton E. ‘K. (1974) In Nuclear Spectroscopy and Reactions (Ed. Cerny J.), Vol. C, p. 307. Academic Press, New York.

Hoglund A., Malmskog S. G., Marcluis A., Valivaara K. G. and Kozyczkowski J. (1971) Absolute transition rates in “‘W. Nucl. Phys. A 169, 49.

Meiling W. and Stary F. (1966) Nanosekunden-Lebens dauermessungen an hochenergetischen Niveaus in lg2W. Nucl. Phys. 84, 534.

Neal W. R. and Kramer H. W. (1965) Mean lives of excited rotational states in heavy even-even nuclei. Phys. Reu. 137, 1164.

Newton T. D. (1950) Decay constants from coincidence experiments. Phys. Rev. 78, 490.

Schmorak M., Wilson H., Gatti P. and Grodzins L. (1964) Gyromagnetic ratio of the 4 + state of 14’Ce. Phys. Rev. B 134, 718.