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Study of ²⁷Al and ²⁴Fe(p,xγ) reactions at E_p = 800 MeV

H. Vonach, A. Pavlik. and A. Wallner

Institut fürRadiumforschung und Kernphysik, University of Vienna

A-1090 Vienna. Austria

M. Drosg

Intitut für Experimentalphysik. University of Vienna A-1090 Vienna. Austria

R. C. Haight and D. M. Drake

Los Alamos National Laboratory

Los Alamos, NM 87545. U.S.A.

S. Chiba

Japan Atomic Energy Research Institute Tokai-mura, Ibaraki-ken 319-11. Japan

Spallation of ²⁷Al and ⁵⁴Fe by 800 MeV protons was investigated at the WNR-facility of the Los Alamos National Laboratory. Production cross sections for a considerable number of residual nuclei were determined for both targets by use of three different methods

The mass and charge distributions of the reaction products from Spallation reactions induced by protons in the energy range from several hundred MeV to several GeV had been investigated in a number of experiments (see, e.g , Ref 1) Cross sections for the formation of radioactive residual nuclei with half-lives exceeding some hours have been investigated by conventional y-ray spectroscopy and accelerator mass spectrometry (AMS), the production of stable isotopes of noble gases has been measured by gas production measurements and mass analysis. Much less information, however, exists about the production of stable isotopes from elements other than noble gases, and for short-lived residual nuclei. Only one experiment has been reported, in which the full mass and charge distribution of the spallation products has been measured by detection of the recoiling nuclei in so-called inverse kinematics at E = 600 MeV [2]. Although the results of this experiment are in reasonable overall agreement with the above mentioned activation, AMS and gas production measurements, there are a number of discrepancies exceeding experimental errors [1 ]

The present experiment was performed using in-beam y-ray spectroscopy This method, which so far has not been used in the study of spallation reactions, allows the determination of cross sections for formation of stable and very short-lived isotopes, both of which cannot be measured by conventional activation techniques Two nuclei, ²⁷Al and ⁵⁴Fe, have been selected for this study because they have been studied before in a number of papers [1,2] These nuclei are well suited for the goal of obtaining a rather complete mass and charge distribution of the residual nuclei by combining our results with the existing data base In addition, checks of the results obtained by very different methods become possible for a number of residual nuclei.

Thin foils of ²⁷A1 and ⁵⁴Fe (6.82 ±0.27 mg/cm² and 12.13 ±0.24 mg/cm²) were irradiated with the 800 MeV proton beam of the WNR facility of the Los Alamos National Laboratory [3]. The y-radiation from the proton-induced reactions was observed with a high-purity Ge-detector The foils were irradiated in a scattering chamber located in the beam line from the accelerator to the neutron producing target of WNR.

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The γ-radiation from the targets was measured with a high-purity Ge-detector at a distance of about 30 m from the targets at an angle of 150° relative to the proton beam. The WNR beam consisted of 40 macro-pulses per second separated by either 16 66 or 33 33 ms Each macro-pulse had a length of 600 ms. Within each macro-pulse the beam consisted of narrow micro-pulses of a width of about 1 ns at intervals of 1 8 us Accordingly the prompt Y-radiation originating from the so-called γ-cascade deexciting the final residual nucleus was observed by its time correlation to the micro-pulses The γ-radiation from the decay of the short-lived residual nuclei formed in spallation reactions was measured between the macro-pulses by setting a time window of 15 ms after the end of each «micro-pulse. After the experiment the irradiated Al and Fe foils were transferred to the Institut für Radiumforschung und Kernphysik in Vienna and the γ-radiation from all long-lived activities produced in the samples was measured with a calibrated high-purity Ge-detector. From the measured short-and long-lived activities production cross sections for the respective nuclei were derived In the prompt γ-ray spectrum it was possible to observe the transitions from the first excited 2^- state to the ground state for all even-even nuclei strongly populated in the spallation reactions and to deduce the production cross sections of these mostly stable nuclei from these data In this way production cross sections for 36 nuclides from the proton interactions with ⁵⁴Fe and for 12 nuclides in case of ²⁷Al could be measured; in addition meaningful upper limits were obtained for a number of further nuclides in both cases.

The present data as well as the results of all previous measurements were compared with the predictions of the semi-empirical systematics of Tsao and Silberberg [4] and with calculations according to the quantum molecular dynamics model (QDM) followed by statistical decay (SDM) [5,6].

The quality of the theoretical description by the QMD model is approximately as good as that of the semi-empirical systematics, although the QMD model does not contain any parameters fitted to the existing data on nuclide production cross section. However, there are still deviations up to a factor of two between the experimental data and both calculations even for nuclides in the peaks of the mass distribution. Especially the QMD-model seems to underestimate the production of nuclides with masses very close to the target nucleus e.g ⁵⁵Fc or ⁵⁵Mn from ⁵⁶Fe, which may be caused by some direct reaction mechanisms not included in the QMD-model Unfortunately the overall spread even of the more recent data is still so large that it is difficult to draw general conclusions on possible systematic dependencies of the observed discrepancies on Z and A.

References

1. R. Michel et al., .Nucl.Inst.Meth B 103. 183 (1995)

2. W. R. Webber. J C Kishand D A Schrier. Phys. Rev. C41. 547 (1990)

3. P. W. Lisowski.C. D. Bowman. G J Russell and S A Wender. Nucl.Sci.Eng. 106, 208 (1990)

4. R. Silberberg and C. H. Tsao, Astrophys J 220. 315, 335 (1973); R. Silberberg, C. H. Tsao and M. M.Shapiro. in Spallation Nuclear Reactions and Their Applications. eds. B. S. P. Shen and M. Merker (Reidel. Dordrecht. 1976) p. 49, C. H. Tsao and R. Silberberg. Proc.16th Cosmic Ray Conf (Kyoto. 1979) vol 2. p. 202; R. Silberberg. C. H. Tsao and J. R. Letaw. Ap J Suppl Ser 58 873 (I985), R. Silberberg, C. H. Tsao and J. R. Letaw. 20th Int Cosmic Ray Conf. (Moscow. 1987) vol 2. p 133

5. K Niifa et al. Phys. Rev C 52. 2620 (1995).

6. S Chiba et al. Phys Rev C 54. 285 (1996)