FAST NEUTRON SPECTROMETER

Most methods of fast-neutron spectroscopy are based on np scattering and the subsequent measurement of recoil protons. This method underlies the use of thick-layer nuclear emulsions, organic scintillation counters, and ionization chambers filled with hydrogen-containing gases for studying neutron spectra. However, in order to pass from the observed spectrum of recoil protons to the spectrum of the initial neutrons, in all the methods mentioned it is necessary to carry out rather

![Figure 1 schematic]

Fig. 1. Diagram of the apparatus for investigating the spectrum of fast neutrons: FEU (A and B)—photomultipliers; Lim. (A and B)—pulse-amplitude limiters; CF—cathode follower; A—amplifiers; D—discriminators; CU—coincidence unit; K—unit triggered by a pulse from the coincidence unit.

complicated calculations that take into account the angular dependence of the neutron–proton scattering cross sections, which differs for different neutron energies. As a result, the interpretation of the results becomes considerably more complicated, and the accuracy of the data obtained decreases.

Recently a new fast-neutron spectrometer has been proposed, the use of which is free from the shortcomings indicated above.^1

In a spectrometer of this type, the energy of the neutrons is also determined from the energy of the recoil protons, but in this case only protons knocked out in the direction of the initial neutron beam (at an angle of \(0^\circ\)) are registered; their energy is practically equal to the energy of the primary neutrons. The only dependence that in this case must be known in order to pass

from the proton to the neutron spectrum—this is the dependence of the differential forward-scattering cross section (at an angle of \(0^\circ\)) on energy.

In order to register only the protons scattered forward, the authors employed a coincidence scheme (shown in Fig. 1) between the pulses from photomultipliers placed at \(A\) and \(B\), caused respectively by the recoil proton (stilbene crystal in front of PMT-A) and by the neutron scattered at an angle close to \(90^\circ\). To register such neutrons, a NaJ crystal surrounded by a 5-mm layer of silver was placed in front of PMT-B. Capture of neutrons by silver or iodine nuclei was accompanied by the emission of \(\gamma\)-rays, registered by the crystal and photomultiplier. Fourteen-cascade EM-1-6262 tubes were used as photomultipliers. The crystal dimensions were: stilbene—diameter 1.5 cm, thickness 0.5 cm; sodium iodide—a cube with an edge of 2.5 cm.

Fig. 2. Spectrum of neutrons formed in the reaction \(\mathrm{H}^2(dn)\mathrm{He}^3\) (target—thin gas).

Pulses from the photomultipliers were fed into a coincidence block with a resolving time of \(3 \cdot 10^{-8}\) sec. In this, the pulses from PMT-A, caused by the recoil proton, passed through a delay line and were delayed by \((3.5 \pm 1.5) \times 10^{-8}\) sec. The indicated delay time corresponds to the flight time of neutrons scattered at an angle of about \(90^\circ\) over the distance between the stilbene and NaJ crystals, equal to 6 cm. The delay-time interval thus determines the energy interval of the registered neutrons (5–30 kev). The efficiency of registration of neutrons with energy 30 kev in the NaJ counter used was about 6%. The rise time of the pulse from scintillation in NaJ is relatively large—about 0.25 \(\mu\)sec; since in these experiments it was not required that the pulse amplitudes in NaJ be proportional to the neutron energy, it was possible to differentiate the pulses and operate with the “edge” of the leading front of pulses of duration less than \(10^{-8}\) sec. Thus, the pulses from PMT-B were not proportional to the neutron energy; their maximum amplitude was set by limiter \(B\). Similarly, the pulses arriving at the delay line—and further to the coincidence block—from the output of PMT-A were not proportional to the proton energy, since they were limited by limiter \(A\).

In order to investigate the amplitude distribution of PMT-A pulses proportional to the spectrum of protons knocked out at an angle of \(0^\circ\), the authors therefore simultaneously recorded a pulse not only from the output but also from the eleventh dynode of PMT-A. This pulse, proportional to the proton energy, was fed through a cathode follower and amplifier into a special block \(K\), which was opened only upon arrival at it of a pulse from the coincidence block. Thus, the device made it possible to analyze only those pulses from protons that were accompanied by coincidence pulses opening block \(K\). Consequently, the energy distribution was registered only for protons knocked out at an angle of \(0^\circ\), without interference from

of random coincidences. The resolving time of block $K$ was chosen in accordance with the time characteristics of the coincidence circuit and the delay line.

Analysis of the pulses from the 11th FEU-A dynode that had passed through block $K$ was carried out with the aid of a special 25-channel analyzer.

Fig. 2 shows the neutron spectrum obtained by the method described, from the reaction $\mathrm{H}^3(d,n)\mathrm{He}^3$ with an energy of about $2.8$ MeV. The accuracy of the data obtained is entirely satisfactory and exceeds the accuracy of other methods. An additional convenience is the low sensitivity of the instrument to the $\gamma$ background, owing to the introduction of delayed coincidences.

G. I.

References

1. L. Beghian, R. Allen, J. Calvert and H. Halban, Phys. Rev., 86, 1044 (1952).