FAST NEUTRON SCINTILLATION SPECTROMETER

G. Sh. Pekarskii

(Presented by the scientific seminar of the Research Institute of Electronic Introscopy)

The great penetrating power of fast neutrons makes it possible to use them for flaw detection in those cases where $\gamma$-ray flaw detection is practically impossible (the examination of large thicknesses of heavy metals). However, for the practical introduction of neutron flaw detection, there is at present still an insufficient amount of experimental data on the passage of a neutron flux through materials, obtained under barrier-geometry conditions. This, in turn, is explained by a number of technical difficulties associated with such measurements: a considerable $\gamma$ background, both accompanying the neutron source and arising when fast neutrons pass through matter; the low efficiency of spectrometers for fast neutrons; the complexity of deciphering instrumental spectra, and so on.

In order to accumulate experimental data on the spectral and angular distribution of fast neutrons behind various thicknesses of materials under barrier-geometry conditions, we have created and adjusted a fast neutron scintillation spectrometer capable of operating in considerable fields of $\gamma$ radiation.

As the scintillation detector, a solid organic scintillator—stilbene—is used, providing, on the one hand, a sufficiently high efficiency of registration of fast neutrons and, on the other hand, permitting comparatively simple processing of instrumental spectra.

The choice of a single-crystal spectrometer circuit is explained by the fact that with its aid it is possible to model the process of flaw detection of materials in the simplest way.

The discrimination of $\gamma$ radiation is based on the fact that in some organic crystals (in particular, in stilbene) the pulse shape depends on the type of exciting particle [1]. As the separating circuit, a scheme of amplitude-time discrimination proposed in [2] was used. Despite a certain complexity, this circuit has the advantage that the implementation of double—amplitude and time—discrimination makes it possible to reduce significantly the efficiency of registration of $\gamma$ radiation in comparison with circuits using only amplitude discrimination [3, 4, 5].

The complete block diagram of the spectrometer is shown in Fig. 1.
The spectrometer operates as follows.

Pulses from neutrons and gamma quanta are taken from the anode and the penultimate dynode of an FEU-33 photomultiplier, operating in saturation mode on the last dynodes, and are fed to the summing circuit \(CC_l\). The parameters of \(CC_l\) are selected

Fig. 1. Block diagram of the spectrometer.

so that the summed pulses from neutrons have positive polarity, while those from gamma quanta have negative polarity, with a small positive overshoot.

Amplitude discrimination is carried out by means of a highly sensitive low-threshold trigger \(BT\). The pulse from \(BT\), after sharpening, is fed to the coincidence circuit \(CC\).

Time discrimination is based on the fact that the positive overshoots from gamma quanta are delayed relative to the zero time by 12–15 µsec, while the duration of neutron pulses does not exceed 4 µsec. The zero time is set by the positive front of the pulses (from both neutrons and gamma quanta) taken from the penultimate dynode and fed to the highly sensitive trigger \(BT\). This pulse is then sent to the shaping circuit \(CF\) and fed to the coincidence circuit with a duration of 5 µsec.

Thus, in \(CC\), the pulses of the amplitude-discrimination channel and the time-discrimination pulses from neutrons overlap in time, whereas the pulses from hard gamma quanta that have passed the amplitude discrimination do not overlap.

To prevent an overlap pulse from appearing at the output of \(CC\) if one more gamma quantum arrives within 12–15 µsec, the dead time of \(CF\) was chosen to be 20 µsec.

With the aid of \(MGI\)-1, the coincidence pulse is shaped and fed to the control input of the 100-channel analyzer \(AI\)-100. To obtain the linear signal, the 7th dynode of the photomultiplier is used. With the aid of the linear amplifier \(LU\), these pulses are amplified, delayed by means of the delay line \(LZ\) until they coincide in time with the control signal, and fed to the linear input of the analyzer.

When tuning the spectrometer to its optimum operating mode, the influence on the registration efficiency, the gamma-radiation discrimination level, and the spectrometer resolution was investigated for such factors as the total supply voltage of the photomultiplier, the distribution of potentials on the multiplier dynodes, the parameters of the summing circuit, the discrimination threshold in the amplitude and time channels, and others.

The spectrometer was tuned using alpha particles from \(Pu^{238}\) \((E_\alpha = 5.3\ \text{MeV})\) and gamma radiation from \(Co^{60}\) \((E_\gamma = 1.17\) and \(1.33\ \text{MeV})\) and \(Cs^{137}\) \((E_\gamma = 0.661\ \text{MeV})\). Figure 2 presents the tuning results; curve 1 corresponds to operation without discrimination of gamma radiation, and curve 2 to operation with discrimination (the spectra are normalized to the data-acquisition time).

Figure 3 presents the determination of the spectrometric threshold (480 keV) and the discrimination threshold (110 keV); the spectrometer was calibrated using monochromatic \(\gamma\)-radiation from \(\mathrm{Zn}^{65}\) and \(\mathrm{Cs}^{137}\), and the known relation between light output and energy when the crystal is excited by protons and electrons [6].

Fig. 2. Spectra of \(\gamma\)-radiation from \(\mathrm{Co}^{60}\) and \(\mathrm{Cs}^{137}\), and \(\alpha\)-particles from \(\mathrm{Pu}^{238}\). Explanation in the text.

Fig. 3. Determination of the spectrometric threshold and the discrimination threshold.

Experimental determination showed that, with a stilbene crystal \(30 \times 20\) mm, the spectrometer efficiency for neutrons from a Po—Be source is 5.6%, whereas the efficiency for registration of \(\gamma\)-radiation from \(\mathrm{Co}^{60}\) is \(6 \cdot 10^{-4}\%\).

The maximum load of the spectrometer is determined by the dead time of the AI-100 and is 3–5 kHz, whereas the permissible counting rate of the separating device is 30 kHz.

In conclusion, to investigate the possibility of using neutrons to determine defects in wood, spectra of neutrons from a Po—Be source were taken behind layers of wood of various thicknesses.

Figure 4 presents the instrumental spectra: curve 1 is the neutron spectrum of the Po—Be source itself, and curve 2 is after passage through a layer of wood 15 cm thick (the information accumulation time was the same).

Fig. 4. Instrumental spectra of neutrons from a Po—Be source. Curve 1—without absorber; curve 2—behind a 15 cm layer of wood.

REFERENCES

1. F. Brooks. Nucl. Instrum. 4, No. 3, 151, 1959.
2. T. G. Doroshenko, I. V. Filyushkin, V. A. Fedorov. Problems of Dosimetry and Radiation Protection, issue II, p. 179, MEPhI, 1963.
3. R. Owen. Nucleonics. 17, No. 9, 92, 1959.
4. V. G. Brovchenko, G. V. Gorlov. PTE, No. 4, 49, 1961.
5. V. I. Strizhak, G. A. Prokopets. Izvestiya Vuzov, Physics Series, No. 1, 131, 1963.
6. J. B. Birks. Proc. Phys. Soc. A, 164, 10, 1951.