Measurement of the lifetime of the 1.01 MeV level of 27Al

1 l.E.4

1

Nuclear

Physics

A184 (1972) 574-576;

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MEASUREMENT OF THE LIFETIME OF THE 1.01MeV LEVEL OF 27At H. IMADA and J. A. MCINTYRE Physics

Department

and Cyclotron

Institute,

Texas A & M University, College

Station,

Texas 77843 f

Received 10 December

1971

Abstract: The mean lifetime for the 1.Ol MeV level in 27AI has been measured to be 2.13 10.19 ps. This value agrees with two previous measurements but disagrees by a factor of four from the most recently published lifetime. E

NUCLEAR

REACTIONS

27Al(y, y), E = 1.013 MeV; measured level deduced T,.

n(E-; &,). *‘Al

Several recent reports ’ -“) have been made on the value of the mean lifetime r for the I.01 MeV level in “Al. The most recent publication “) gives a value for 7 of 0.51~$~~ ps while two previous reports give 1.5+0.4ps [ref. ‘)f and 2.2f0.3 ps [ref. “)I. We report here a value for z of 2.13-t-0.19 ps which confirms the earlier two measurements. The technique used has been described in detail in a previous publication “). Briefly, y-rays from a 40000 CI’ 6oCo source are scattered by the Compton effect from a brass scatterer. The scattered y-rays are used to excite the 1.01 MeV level in 27A1 by resonance scattering from an “Al target. The resonance energy for the exciting y-rays is selected by changing the angle of scattering from the brass scatterer. The magnitude of the resonance scattering differential cross section is measured by detecting the resonance-scattered y-rays from the target with a IO cm3 Ge(Li) detector. The total cross section is determined from the angular distribution as calculated from published values for the ground and excited state spins (2 and $) and from the mixing ratio 6 for the electromagnetic transition between the states (S = 0.37kO.03) [ref. ‘)I. The level width and the mean lifetime follow immediately from the total cross section. The pulse-height spectrum of the resonance scattered y-rays is shown in fig. I. There is a peak in the number of counts corresponding to the “Al y-ray energy of 1.01 MeV for the 27A1 target and no peak for the matched silicon target. The number of counts in the peak is proportional to the number of y-rays scattered from the 1.01 MeV level in “AI by the resonance fluorescence process. t Work supported by the US Atomic Energy Commission. 574

1.01 MeV LEVEL OF 27A1

515

The other measurements required for determining the lifetime were the number of y-rays striking the target N and the efficiency E of the Ge(Li) detector. The product NE was determined by placing the Ge(Li) detector in the y-ray beam striking the 27AI target. A check on E was obtained by calibrating the Ge(Li) detector efficiency with standard radioactive sources of several energies. A check on N was obtained by measuring the number of y-rays scattered from the 27A1 target by the Compton effect with its known cross section. Also considered was the 12.2 % polarization of the beam striking the “AI target; the polarization is introduced by the preparation of the beam through Compton scattering from the brass scatterer. Following the usual procedures for analyzing resonance scattering data ‘), the I

I

I

-

lOI,3 keV

.

ALUMINUM

X

SILICON

I

I

1000

1010

I 990

TARGET TARGET

PULSE

HEIGHT

IN

I 1020 keV

Fig. 1. A plot of the number of counts in the Ge(Li) detector with an “Al target and a silicon target. The thickness of the silicon was selected to produce the same amount ofy-ray scattering as the “Al target for non-resonant energies. The lines through the points are for visual purposes only. The area under the 2’Al peak was determined by subtracting point by point using the usual statistical procedure.

576

H. IMADA AND J. A. MCINTYRE TABLE 1 Reported lifetime values for the 1013 keV level in “Al

Mean lifetime (ps)

Ref.

2.1310.19 0.51 $:z”

this work

1.5 hO.4 2.2 50.3

“) *) Y

mean lifetime z for the transition from the 1.01 MeV level to the ground state of * 7Ai was found to be 2.13 f 0.19 ps. The largest contributions to the error are from the measurements of the beam intensity and the detector efficiency. This result is to be compared to the recent measurements listed in table 1. It is seen to agree well with two of the measurements but not with the third, lying over seven standard deviations above that of Hough et al. ‘). Since we used the same y-ray beam and Ge(Li) detector for the measurement of five other nuclear lifetimes and obtained values in agreement (30 % or better) with those published in the literature, it is difficult to explain how the value for the lifetime of the 27Al level could be in error the factor of four required to obtain agreement with ref. ‘). Also, the web-defined peak shown in fig. I excludes the possibility of non-resonant effects being significant. Combining these factors with the confirming measurements of Smulders et al. ‘) and Evers et al. 3), it would appear that the lifetime value is near the value reported here. We thank Mr. E. E. Vezey for constructing the spectrograph used for these measurements. References 1) J. H. Hough, J. W. Keen, P. J. Celliers and W. L. Mouton, Nucl. Phys. A132 (1969) 110 2) 3) 4) 5)

P. D. G. D.

J. M. Smulders, C. Broude and J. F. Sharpey-Schafer, Evers, J. Hertel, 3. W. Retz-Schmidt and S. J. Skorka, K. Tandon and J. A. McIntyre, Nucl. Instr. 59 (1968) M. Sheppard and C. van der Leun, Nucl. Phys. Al00

Can. J. Phys. 46 (1968) 261 Nucl. Phys. A91 (1967) 472 181 (1967) 333