Threshold Energies of \((\gamma,n)\)-Reactions

A. K. Berzin

(Presented by Prof. A. A. Vorob’ev, Doctor of Physico-Mathematical Sciences)

Measurements of the threshold energies of \((\gamma,n)\)-reactions for the isotopes \(\mathrm{Pr}^{141}\), \(\mathrm{Nd}^{150}\), \(\mathrm{Gd}^{160}\) were carried out using a 25 MeV betatron built by the Tomsk Polytechnic Institute, by the method of direct registration of neutrons produced in the reaction as a function of the maximum energy of the bremsstrahlung radiation. The layout of the experimental apparatus is shown in Fig. 1.

Fig. 1
Layout of the experimental apparatus

The working paraffin block (1), with dimensions \(235 \times 350 \times 235\) mm, surrounded by a layer of cadmium alloyed with lead (80% Cd by weight), 10 mm thick (2), and a layer of boron carbide alloyed with paraffin (60% \(B_4C\) by weight), 8 mm thick (3), was enclosed in a block (4) consisting of an alloy of paraffin with boron oxide (40% \(B_2O_3\) by weight). This entire system was placed in an iron tank (5) with dimensions \(1100 \times 1300 \times 800\) mm, filled with water and closed on top by a paraffin lid 250 mm thick. Layers (2) and (3), block (4), and the layer of water served as reliable protection against external neutrons, i.e., neutrons produced outside the sample under study.

The specimen under investigation (6), enclosed in a thin-walled aluminum ampoule made in the form of a cylinder, was placed in a channel \(d = 12\) mm (passing through the entire setup) at the center of the working block (1).

A beam of betatron X-rays with a half-width of 5 μsec, collimated by a lead collimator (7) 700 mm thick (aperture diameter 12 mm), passed through the channel and struck the specimen under investigation, producing neutrons in it. The neutrons slowed down in the working block were detected by means of a detector (8), consisting of layers of \(\mathrm{B_2O_3}\) and \(\mathrm{ZnS{-}Ag}\) deposited on Plexiglas plates. The light flash produced in the detector was transmitted through a light guide (9) to an FEU-19 photomultiplier (10). The pulse from the FEU-19, amplified by the preamplifier and main amplifier, was fed through an amplitude discriminator to a coincidence circuit (Fig. 2).

Fig. 2

Block diagram of the recording device

1. Preamplifier. 2. Main amplifier. 3. Amplitude discriminator. 4. Coincidence circuit. 5. Energy-control circuit. 6. Timing circuit 1. 7. Timing circuit 2. 8. Scaling device. 9. Mechanical counter.

The second channel is included in the circuit for monitoring the energy of the bremsstrahlung radiation and is triggered by the pulse supplied to the mixing coil. The timing circuit makes it possible to delay the pulse for the time required for dumping the electrons onto the betatron target and for the recovery time of the radio circuit. The selected delay was 30 μsec. After 30 μsec, a starting pulse of duration 350 μsec is fed to the coincidence circuit. The duration of the starting pulse is determined by the rise time of the neutron background.

The energy scale was calibrated according to the known thresholds of the \((\gamma, n)\) reaction of the isotopes: \(\mathrm{Be}^9 = 1.67\) MeV, \(\mathrm{Li}^7 = 7.15\) MeV, \(\mathrm{Cu}^{63}_{29} = 10.85\) MeV. The energy scale was checked before and after taking the main curve.

The average irradiation dose for each energy value was 60–70 roentgens. The intensity was monitored with a thin-walled ionization chamber (11), placed in the X-ray beam and calibrated against a thimble chamber.

As a result of the measurements carried out, the following threshold values of the \((\gamma, n)\) reaction were obtained for isotopes of the rare-earth element group:

\[ \mathrm{Pr}^{141} — (9.6 \pm 0.4)\ \text{MeV}. \]

\[ \mathrm{Nd}^{150} — (7.2 \pm 0.4)\ \text{MeV}. \]

\[ \mathrm{Gd}^{160} — (7.6 \pm 0.4)\ \text{MeV}. \]

In setting up the work, all material available in the literature on this question was used.