Nuclear lifetimes in 37Ar

Nuclear Physics A202 (1973) 530-534;

f-iq

@

North-Holland

Publishing

Co., Amsterdam

Not to be reproduced by photoprint or microfilm without written permission from the publisher

NUCLEAR R. G. KRUZEK Department

LIFETIMES

7, G. M. CHENEVERT

of Physics and Astronomy,

IN 37Ar

tt, H. G. LEIGHTON

University

of Kentucky,

Lexington,

ttt and B. D. KERN Kentucky

40506, USA :

Received 7 August 1972 (Revised 2 October 1972) Abstract: The nuclear lifetimes of ten levels of 37Ar with energies from 1.41 to 3.61 MeV have been measured by the attenuated Doppler-shift method using the 37Cl(p, ny)37Ar reaction. E

NUCLEAR

REACTfONS

37Cl(p, ny); E = 3.61-5.80 MeV; measured levels deduced T+.

DSA. 37Ar

1. Introduction

The present paper reports the results of measurements of the mean nuclear lifetimes of states in 37Ar, using the Dopp 1e r-shift attenuation method (DSAM) through the 37Cl(p ny)37Ar reaction in which the recoil nuclei were stopped in CdCl,. At the time ‘i this work was initiated little information was available on the lifetimes of the low-lying states of 3‘Ar, and data on the energies, spins, and parities were incomplete. Since then the results of five Doppler-shift measurements using the 34S(a, ny)37Ar reaction ‘), the 35C1(3He, pY)37Ar reaction 3), and the 37Cl(p, ny)3 7Ar reaction 4-6 13 respectively ‘); polarizations and angular distributions using the 37Cl(p, ny)37Ar reaction; the results of two studies on the 36Ar(d, p)’ ‘Ar reaction *- ‘); and measurements on the 1.61 MeV level lo) have been reported. However, where comparison between the five sets of lifetimes is possible, agreement is not satisfactory. Since the interpretation of shell-model calculations is difficult and uncertain when experimental data are not available or not in agreement, it is important to make additional measurements. Also, in view of the uncertainties of the theory describing the slowingdown of ions in matter, it is valuable to have data derived from a different range of recoil velocities and stopping materials. 2. Experimental

procedure

A proton beam from the University of Kentucky 6 MV Van de Graaff accelerator was used to bombard CdCl, targets enriched to 99.28 % in 37C1. Proton energies + Present address: Air Force Armament Laboratory, Eglin AFB, Florida 32542. tt Present address: Medical Physics Department, University of Wisconson, Madison, Wisconsin 53706. ttt Present address: Department of Meteorology, McGill University, Montreal 101, Canada. x Supported in part by the National Science Foundation. 530

-Ar

LIFETIMES

531

ranged from 3.61 MeV to 5.80 MeV, and beam currents were typically 50 nA. Targets of two thicknesses were used during this work. One was a slurry of approximately 10 mg/cm’ surface density on a tantalum plate and the other was a film evaporated to an area1 density of 100 fig/cm2 on a thin gold foil. The targets were mounted in a cylindrical thin-wall aluminium vacuum chamber 5.7 cm in diameter. Gamma-rays were detected with a 35 cm3 Ge(Li) detector which was positioned 4.5 cm from the target and was free to rotate about the chamber in the horizontal plane. The resolution of this detector was 2.6 keV for 1332 keV y-rays. The multichannel analyzer was calibrated with the use of standard y-ray sources, and a source was placed near the target during data collection to provide a reference line in each spectrum as a check on possible amplifier gain shifts. The energy dispersion was typically 0.35 keV/channel. Gamma-ray spectra were collected at angles 0” and 90”, using a 4096-channel ADC in conjunction with a PDP-8/I on-line computer. The peaks of interest in the y-ray spectra were well separated and the centroids were readily determined after the subtraction of a background which was calculated by linear interpolation from the yield in portions of the spectrum adjacent to each peak. Neutron-y coincidence spectra were obtained using conventional fast-slow coincidence techniques, with resolving time 27 = 50 ns. The neutrons were detected with an NE-102 encapsulated liquid scintillator optically coupled to an RCA 6810-A photomultiplier tube. The neutron detector was positioned at an angle of 60” relative to the beam axis and the y-ray detector was positioned at angles of 0” and 90” relative to the 37Ar recoil axis. Pulse-shape discrimination was used to reduce the number of accidental coincidences due to y-rays in this detector. In the coincidence work, the output of a precision pulser was gated into the spectra to check the electronic stability. 3. Acquisition

of data

The proton bombarding energies were selected to be 400 to 600 keV above threshold for excitation of the 37Ar level being studied. Thus, the recoil nuclei were confined to cones of half-angle less than 24”, centered on the beam axis. The recoil ion velocities varied from 0.001~ to 0.004~; most of the data were taken with velocities of 0.001~ to 0.002~. Under these conditions, y-ray spectra taken at several angles exhibited measurable energy shifts due to the Doppler effect. Energy shifts of up to 8 keV were observed. Levels from 1.41 MeV up to 3.61 MeV were populated with the subsequent emission of fourteen observed y-rays. In table 1 in column 1 are the energies of the levels which were studied, taken from ref. ‘). The Doppler shift in the energy of one y-ray from each level was obtained; in the cases of two or more y-rays being emitted the most intense one was used. The y-rays had energies of 1.41, 2.22, 2.49, 2.80, 3.17, 1.58, 3.27, 2.11, 1.92 and 3.61 MeV, respectively, for the levels of column 1. The 879 keV y-ray due to the 2.49 + 1.61 MeV transition was observed in the singles spectrum and in the n-y coincidence spectrum, but the yield was not sufficient for use in a lifetime determination. In column 3 of table 1 appear the observed energy shifts between the 0” and 90” observations.

0.56f0.12

1.1 1.2 7.0 6.4 1.8

3.61

4.61 5.10 5.20 5.50 5.50

5.50 5.50

5.50 5.80

1409.8&0.1 2217.2*0.3 2490.3 +0.3 2796.1 kO.4 3169.8rtO.7 3185.8kl.O

3272.0+0.8 3516.150.8

3525.9h1.5 3605.1*2.0

“) “) ‘) h, ‘)

‘> 1120 82* 25

6OOf300 < 80

300& 60 60& 20 < 50

285f 501t 62 18

310& 38* 80 15 93* 35

< 40 1005 20

22& 14

8 15

122Okl50 910&180 104Ofl90

50*

15

950&-450 45* 30 220*120

30& 15

uncertainties.

< 25

38Ort 50 775+ 97

< 25

1080&150

650f350 600f180 400*130

et al. d,

815

943f135 510+ 67 660 &205

620f185 540*145 755f170

Ragan et al. ‘)

Ivascu et al. ‘)

Luketina

23*

Wong er al. ‘)

present results “)

Mean lifetime t, (fs)

proton energy. The omitted 1.61 MeV level is known to have a lifetime of several ns.

0.11 &to.04 0.70*0.05

0.85 AO.05 0.66&0.05

0.19f0.03 “) 0.18f0.03 0.91 *0.06 ‘) 0.71*0.03 0.41 kO.07

0.17*0.04

F

Fractional shift

of ten levels of 37Ar

Average values from ref. ‘). An uncertainty of 15% has been included quadratically to account for stopping-power ‘) Ref. 3). ‘) Ref. 2). ‘) Ref. 4). d, Ref. 5). Ref. 6). Deduced from n-y coincidence data and also from non-coincidence data. Deduced from n-y coincidence data.

The energy Ep is the bombarding

0.62kO.19 7.2 rtO.5

7.9 *0.5 4.0 +0.3

i-o.2 kO.2 kO.4 *0.3 kO.3

Doppler shift (keV)

(hZV)

(keV)

E. (37Ar) “)

Mean lifetimes, t,,

TABLE1

Caraca

<8

88& 24

370*150

et al. *)

E z ?

z

e

a 0

37Ar LIFETIMES

533

Neutron-y coincidence spectra were obtained for the 2218 keV level, taken at 0 and 90” relative to the 37Ar recoil axis. A counting time of 9 h was required to accumulate these data. The y-ray from the 2796 keV level was accompanied by a line at 2793 keV which was identified as the one-escape line due to the 3304 keV radiation from 34S which was produced by j7Cl(p, ~1)~~s.The interference was removed by the acquiring of neutron-y coincidence spectra. 4. Analysis The nuclear mean lifetimes were extracted from the measured Doppler energy shifts using the DSAM techniques which are described in detail elsewhere ‘i-i3).

37Ar*

STOPPED

IN

CdCI,

2217 keV LEVEL

60

-

60 -

40-

QOOI

QOI

1.0

IO

Fig. 1. A typical F(r,) versus r’mcurve which was calculated for the 2.22 MeV level with incident proton energy of 4.61 MeV, according to the method outlined in the text. The importance of the nuclear collisions is illustrated by the F = 16% at t, = 01= 0.88 ps, where Q is the electronic stopping characteristic time for which F = 50% for purely electronic energy losses.

The formalism of Blaugrund r4) was used to generate the function F(z,) uer~u.sT,, for each value of the 37Ar recoil ion velocity. In order to account for the fact that the recoil ions possessed a range of velocities and a range of angular divergence from the beam axis, the half-angle which describes the cone within which the recoil ions traveled was divided into 20 intervals, and the F(z,) versus z, function was calculated for the average ion energy for each increment. Then the function was weighted by the fraction of the ions which traveled within the associated solid angle, and a sum was taken over the 20 intervals. The angular distribution of the ions was calculated on the assumption that the distribution in the c.m. system was isotropic. This was a reason-

534

R. G. KRUZEK

et al.

able assumption since the incident proton energies were just above the threshold for the (p, n) reaction. The recoil ions were assumed to be stopped completely in the CdCl, target. Actually, the 100 pg/cm’ target stopped approximately 85 % of the recoils; however, the results obtained with this target were in excellent agreement with the results obtained using the thicker slurry target. The experimental F, which is the ratio of the observed mean Doppler shift to the computed mean maximum Doppler shift, was determined for each observation; averages over the several observations at each level are listed in column 4 of table 1. For each experimental F the corresponding mean lifetime z, and the uncertainty in T, were read from a computed list of numerical values of t;(z,) versus z,, as is illustrated in fig. 1. The mean lifetimes are listed in column 5 of table 1. The uncertainties in the F-values were derived from the calculation which was performed in deducing the centroid of each y-ray peak. The uncer~inty of each mean lifetime has been increased by a quadratic contribution of 15 *A to account for estimated uncertainties in the stopping powers r’). 5. Discussion The significant experimental data and the lifetimes are listed in table 1 along with available lifetimes from the work of others. A comparison of results shows that the present lifetimes are in good agreement, in almost every case, with the previously published results of refs. ‘* 3P5V 6), but not with those of ref. “). The causes of the lack of agreement at several of the levels are not well understood. It appears that a critical weighting of these independent measurements can now produce reliable averages for comparison with theoretical calculations.

References 1) R. G. Kruzek, B. D. Kern and G. M. Chenevert, Bull. Am. Phys. Sot. 15 (1970) 1345 2) C. E. Ragan, C. R. Gould, N. R. Roberson, G. E. Mitchell and D. R. Tilley, Phys. Rev. C3 (1971) 1152 3) M. Ivascu, D. Popescu and G. van Middelkoop, Nucl. Phys. Al63 (1971) 418 4) J’. M. G. Caraca, R. D. Gill, P. B. Johnson and H. J. Rose, Nucl. Phys. A176 (1971) 273 5) I. A. Luketina, J. E. Brock and A. R. Poletti, Phys. Rev. C6 (1972) 196 6) E. Wong, B. C. Robertson, K. V. K. lyengar, D. M. Sheppard and W. C. Olsen, Nucf. Phys. A192 (1972) 279 7) P. Taras, A. Turcotte, R, VaiIl~coLlrt and J. Matas, Can. J. Phys, 49 (1971) 1215; P. Taras, A. Turcotte and R. Vailfancourt, Can. J. Phys. 50 (1972) 1182 8) J. W. Champlin, A. J. Howard and J. W. Oh-ress, Nucl. Phys. Al64 (1971) 307 9) S. Sen, C. L. Hollas and P. J. Riley, Phys. Rev. C3 (1971) 2314 10) C. E. Ragan, G. E. Mitchell, D. R. Tilley, C. R. Gould and N. R. Roberson, Phys. Rev. C3 (1971) 2076 II) E. K. Warburton, J. W. Olness and A. R. Poletti, Phys. Rev. 160 (1967) 938 12) J. Lindhard, M. Scharff and H. E. Schiott, Mat. Fys. Medd. Dan. Vid. Selsk. 33, no. 14 (1963) 13) E. K. Warburton, D. E. Alburger and D. H. Wilkinson, Phys. Rev. 129 (1963) 2180 14) A, E. Blaugrund, Nucl. Phys. 88 (1966) 501