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Two-Body Photodisintegration of 4He

(Feldman, Berman)

The problem of the two-body photodisintegration of 4He, i.e., the comparison of the p+3H and n+3He breakup channels, has plagued the nuclear-physics community for over 30 years [Ber70, Ber72]. A careful review of data as of 1983 determined a set of "recommended" cross sections for (g,p) and (g,n) that differed by a factor of 1.7 at Eg = 25 MeV [Cal83], which is not consistent with the expectations of charge symmetry. Subsequent measurements of the (g,p) channel via photodisintegration [Ber88] and radiative capture [Fel90] seemed to be more in agreement with the (g,n) results [Ber80], although further measurements of the proton channel as well as a very recent measurement of the neutron channel [Nil03] have continued to fuel the controversy of the absolute scale of these reaction channels.

Other measurements have endeavored to address the charge-symmetry issue directly by measuring a ratio of relative cross sections (independent of the absolute scale of the individual channels) [Phi79]. The most recent case, a direct measurement of the ratio of these cross sections that extended below Eg = 30 MeV, was shown to be consistent with charge-symmetry calculations, although due to limited statistics the ratio in the primary region of controversy (around 25-26 MeV) has a rather large (~20%) uncertainty [Flo94].

The Trento group has carried out extensive calculations of the total photoabsorption cross section in 4He [Bar01] as shown in Fig. 7, and finds a broad peak at about 26 MeV with a value of 3.3 mb. This is in disagreement with the current sum of (g,p) and (g,n) cross sections (total of 2.4 mb) obtained from individual measurements. An independent estimate of the total photoabsorption cross section inferred from Compton scattering [Wel92] via dispersion relations and the optical theorem is about 2.9 mb, between the two values above.

In view of the state-of-the-art calculations for the A = 4 system, it is an opportune moment to revisit the "charge-symmetry problem" in 4He photodisintegration. With the high flux available at HIGS, it will be possible to obtain data with excellent precision in order to address these questions. Initially, we propose to perform a simultaneous measurement of the absolute cross section of the (g,3H) and (g,3He) channels at q = 90° at a single energy, 26 MeV, which is just below the threshold for three-body breakup of 4He. A low-pressure gas target (P ~ 0.2 atm) would be used to enable passage of both 3H and 3He recoils through the gas. A solid-state telescope with a thin (15-mm) DE detector would allow penetration of the 3H recoils into the stopping detector while the 3He recoils would actually stop in the DE detector. Thus, a clean separation of the two particle species could be obtained. Moreover, since the experiment is conducted below the three-body breakup threshold, protons passing into the back detector could also serve to identify the p+3H channel unambiguously.

Figure 7. Total photoabsorption cross section for 4He. The dotted and dashed curves were calculated by Barnea et al. [Bar01] using the Malfliet-Tjon (MT) and TN nuclear potentials. The data points represent the sum of (g,p) and (g,n) cross sections and the shaded box indicates the total cross section inferred from Compton scattering [Wel92].

An alternate detection scheme would involve a single position-sensitive solid-state detector, without the necessity of a thin DE detector in front. Locating the detector inside the gas volume at a distance of ~4 cm from the beam axis, the energy loss of 3He recoils traversing the gas would be sufficiently greater than that of the 3H recoils to enable a clean separation of the two particles in the stopping detector alone. Since the HIGS beam is monoenergetic to 1-2%, the recoils have a narrow energy spread at the reaction vertex, and they end up with different final energies at the detector mainly due to energy-loss considerations.

The simultaneous measurement of both channels would also serve as a direct ratio measurement at this energy. The ratio is much less sensitive to systematic uncertainties than the absolute cross section. In a ratio measurement, knowledge of the absolute flux of photons is irrelevant, since both reaction channels are measured simultaneously using the same beam. Moreover, other factors such as target thickness and detector acceptance cancel out in the ratio, thus minimizing possible systematic errors.

Ultimately, it will be necessary to address the question of the absolute cross section over a wider range of energies and angles. Using the spherical array of solid-state detectors described in the previous section, this would be possible for future experiments on 4He covering a range of energies from threshold (at about 20 MeV) up to 50 MeV or so. By requiring the detection and specific identification of 3H and 3He recoils, the reaction is unambiguously restricted to the two-body breakup channel. At higher energies, when the 3He recoils pass through the DE detector, standard particle-identification techniques via DE-E energy loss will give clean separation of 3He, tritons, deuterons, and protons.

Below 26.1 MeV (the three-body threshold), the sum of the measured (g,p) and (g,n) data can be compared to the total photoabsorption cross-section calculations of the Trento group [Bar01]. Above that energy, the undetected three- and four-body breakup channels make such a comparison impossible. The Trento group is in the process of generating calculations for the individual reaction channels [Qua03] and so a direct comparison with the (g,n) and (g,p) channels (measured by detecting the 3He and 3H recoils) can be made, instead of comparing only with their sum. New high-precision data from HIGS for these two reaction channels measured simultaneously will help greatly in addressing the questions raised by these recent calculations.


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