APPLICATION OF Ya-2 TYPE NUCLEAR PHOTOEMULSIONS FOR INVESTIGATING THE NEUTRON SPECTRUM FROM A BETATRON

A. K. BERZIN, B. M. YAKOVLEV, A. A. YATIS

Earlier investigations of the composition of the radiation beam of betatrons with energies up to 25 MeV [1, 2, 3] showed that, in the beam, in addition to high-energy bremsstrahlung $\gamma$-radiation, there are high-energy electrons, slow and fast neutrons. The study of the angular and energy distribution of the scattered $\gamma$-radiation of the accelerator using the induced-activity method (as threshold detectors, isotopes of the elements $\mathrm{Cu}^{63}$ and $\mathrm{C}^{12}$ were used, having thresholds for the $(\gamma,n)$ reaction of 10.85 MeV and 18.77 MeV, respectively) led to the establishment of high-energy components in the spectrum of the scattered $\gamma$-radiation, as well as activation of parts of the installation under the action of these $\gamma$-quanta and the neutron fluxes accompanying them. In connection with these investigations, we set ourselves the task of determining the neutron spectrum, since knowledge of the energy distribution of neutrons proves to be very important in setting up a number of physical and other experiments. The present work gives the results of measuring the energy distribution of neutrons in the beam of bremsstrahlung $\gamma$-radiation of one of the 25 MeV betatrons manufactured at the Tomsk Polytechnic Institute. Nuclear photoemulsions were used as the neutron detector.

It should be noted that, in the present case, recording neutrons by recoil protons with the aid of nuclear photoemulsions is a rather complicated task, since difficulties arise owing to the necessity of carrying out these experiments in the presence of a background of high-energy $\gamma$-quanta, which leads to strong blackening of the plates.

To determine the neutron spectrum in the field of bremsstrahlung $\gamma$-radiation of the betatron, we used domestic NIKFI Ya-2 type photographic plates with a photoemulsion thickness of 200 microns. These nuclear photoemulsions make it possible to detect protons with energies up to 30–35 MeV. The photographic plates were placed in the beam of $\gamma$-quanta at a distance of 1 m from the betatron target, and their irradiation was carried out with a strictly fixed dose of $\gamma$-radiation. Irradiation of the plates was carried out without packaging, which made it possible to eliminate possible errors due to protons formed as a result of nuclear reactions in the packaging material.

The optimal dose of bremsstrahlung $\gamma$-radiation in irradiating the plates was 5 r. Monitoring of the intensity of the $\gamma$-radiation of the installation...

was carried out with the aid of two ionization chambers, one of which was placed at a distance of 1 m from the target, at the location of the plates. After the appropriate processing, the scanning of the photographic plates and the measurement of the recoil-proton tracks were performed on MBI-3 and MBI-8M microscopes using an ocular scale. The ocular scale was calibrated with an object micrometer for different optics according to the instructions supplied with the microscope. Since the track of a recoil proton passes obliquely through the emulsion, it is necessary to measure the horizontal and vertical projections of the track. The horizontal component \(L\) of the track was measured with the microscope ocular scale, while the vertical component \(H\) of the recoil-proton track was measured by focusing the microscope optical system on the ends of the track using the micromechanism. The path length \(R\) of the recoil proton in the emulsion was determined from the relation

\[ R=\left[L^{2}+(H\cdot S)^{2}\right]^{1/2}\ \text{microns}, \]

where \(S\) is the shrinkage coefficient, whose value for the Ya-2 emulsion of 200 microns was 1.97.

Knowing the range of the particle in the emulsion, one can, using the range–energy curves [4] or tables [5], estimate the proton energy. It should be noted that these tables were compiled for photographic emulsions of the Ilford C-2 type, but they may also be used for our emulsions, since the difference in the composition of these and other emulsions is insignificant.

Fig. 1.

From the results of our measurements, the recoil-proton spectrum shown in Fig. 1 was constructed. The proton spectrum extends in energy from 0.25 to 6.25 MeV and has a maximum in the region from 1.25 to 1.5 MeV. It should be noted that these first measurements of the neutron spectrum from recoil protons in nuclear photographic emulsions were carried out in the absence, in the beam of \(\gamma\)-quanta from the betatron, of the filter and of the collimator sleeves (diaphragms); the irradiation field, limited by the collimator aperture, was equal to 10.5 cm at a distance of 1 m from the target.

Results of repeated measurements of the neutron spectrum using photographic emulsions with a filter present in the beam and for various irradiation fields, as well as measurements of the neutron energy distribution in a tissue-equivalent phantom, will be published subsequently.

References

1. B. M. Yakovlev, R. P. Meshcheryakov, A. L. Gryaznov. Report at the Third All-Union Conference on Electron Accelerators, Tomsk, 1959.

2. A. L. Gryaznov, R. P. Meshcheryakov, B. M. Yakovlev. Scientific report No. 0857771, NII TPI, 1959.

3. A. K. Berzin, B. M. Yakovlev, A. A. Yatis. Izv. TPI, vol. 122, 21 (1962).

4. I. Robblatt. Nature, 164, 550, 1951.

5. P. Demers. Ionographie, p. 252, 1958.