Angular distribution of photoneutrons from Ta181 and Bi209
Abstract Generated abstract
The study investigates the angular distribution of photoneutrons emitted from Ta181 and Bi209 under bremsstrahlung irradiation, a measurement relevant to assessing mechanisms of photonuclear reactions. Using a 24 MeV betatron as the gamma-ray source, the authors irradiated tantalum and bismuth samples and registered recoil protons in nuclear photoemulsions placed at several angles relative to the incident radiation, with shielding and ionization monitoring used in the experimental arrangement. The measured track counts show angular distributions that are well represented, within experimental uncertainty, by functions dominated by a sin squared theta dependence: approximately 0.35 plus 0.65 sin squared theta for Ta181 and 0.45 plus 0.55 sin squared theta for Bi209.
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ANGULAR DISTRIBUTION OF PHOTONEUTRONS FROM Ta$^{181}$ AND Bi$^{209}$
A. K. BERZIN, A. E. KOLCHIN, A. P. MAMONTOV
The angular distribution of photoneutrons, which makes it possible to judge the mechanism of photoneutron reactions and, consequently, the degree of suitability of existing theoretical concepts, has so far been studied so little that it does not allow any general conclusions to be drawn. Therefore every new experiment is of undoubted interest.
In the present work the angular distribution of photoneutrons from Ta$^{181}$ and Bi$^{209}$ was investigated. As the source of $\gamma$ radiation, a betatron with a maximum energy of 24 MeV, built at the Tomsk Polytechnic Institute, was used.
The neutrons produced in the samples under study were registered by recoil protons in nuclear photoemulsions of the NIKFI Ya-2 type, 100 $\mu$ thick.
The experimental arrangement is shown in Fig. 1. The sample under study (6) was placed at the center of the bremsstrahlung $\gamma$ radiation, collimated to a diameter of 30 mm, at a distance of 110 cm from the betatron target. The radiation intensity was measured by a thin-walled aluminum ionization chamber (2), placed in the scattered beam. Photographic plates (7) were set at a distance of 60 mm from the sample. To shield them from scattered neutrons, the photographic plates and the sample were placed inside a block cast from a mixture of paraffin (80% by weight) and B$_2$O$_3$ (20% by weight). The wall thickness of the paraffin block was 130 mm. Protection from scattered $\gamma$ radiation was provided by a lead collimator (3) 150 mm thick and an additional lead block (4) 50 mm thick.
Fig. 1. Experimental arrangement: 1—betatron target; 2—monitor; 3—collimator; 4—lead shielding; 5—paraffin block; 6—sample under study; 7—photographic plates.
The samples were made of pure tantalum and bismuth. The tantalum sample had the form of a hollow cylinder 30 mm high, with outer and inner diameters of 25 mm and 16 mm, respectively. The bis-
The bismuth target was made in the form of a solid cylinder 25 mm in diameter and 30 mm high. In the case of Ta, the photographic plates were set at angles of 30, 60, 90, 120, and 150°; in the case of Bi—at angles of 30, 60, 90, 115, 135, and 150° to the direction of the X-rays. The total radiation dose received by the samples was, for Bi, 120 r, and for Ta, 220 r.
The plates, processed by the usual method, were examined under an MBI-3 microscope with a 90× immersion oil objective and a 10× eyepiece. During examination only those recoil-proton tracks were selected which began inside the emulsion.
In each plate the same area was examined. The results of these measurements are presented for Ta in Table 1, and for Bi in Table 2.
Table 1
| Angle \(\Theta\) (degrees) | 30 | 60 | 90 | 120 | 150 |
|---|---|---|---|---|---|
| Number of tracks | 263 | 422 | 540 | 416 | 357 |
Table 2
| Angle \(\Theta\) (degrees) | 30 | 60 | 90 | 115 | 135 | 150 |
|---|---|---|---|---|---|---|
| Number of tracks | 352 | 470 | 627 | 550 | 384 | 293 |
Fig. 2. Angular distribution of photoneutrons from Ta.
Fig. 3. Angular distribution of photoneutrons from Bi.
Figs. 2 and 3 show the angular distribution obtained for photoneutrons from \( \mathrm{Ta}^{181} \) and \( \mathrm{Bi}^{209} \). The experimental points for \( \mathrm{Ta}^{181} \), within the error limits, fit well the curve \(0.35 + (0.65 \pm 0.1)\sin^2 \Theta\), and for \( \mathrm{Bi}^{209}\)—the curve \(0.45 + 0.55\sin^2 \Theta\).