2.3 Neutron spectroscopy based on the time-of-flight measurements

Time-of-flight (TOF) methodology was applied for the neutron spectra investigations with and without the above-mentioned simulator of a NFC. For this goal, two mobile measuring stands for experiments prepared to work in a harsh electromagnetic environment are used [1, 22]. The base for each measuring stand is the cabinet, which represent a Faraday cage featuring outstanding electromagnetic compatibility (EMC) shielding (it is possessed of 80 dB in the range from 30 to 300 MHz, of 60 dB in the range from 300 MHz to 1 GHz and up to 40 dB for the frequency band centered around 3 GHz).

At the data acquisition procedure, the stand is linked with the detectors and triggers only by the fiber optic connections (i.e., for triggering, time marking, and data transmission) with the devices positioned out of the stand, thus having no any galvanic contacts with the main lines and the experimental facilities. The stands are equipped with the converters for two-way signal conversion (Figure 4) and they are battery-powered.

The hybrid module is equipped with the photomultiplier tube (PMT) having 12 focusing dynodes. The conversion of ionizing radiation into light occurs in the fast organic scintillators S (so on the whole the system is named as a PMT + S—Figure 5).

Usually, the scintillators used in the device have 45 mm in a diameter and 50 mm of their length. This length was chosen due to its closeness to the mean free path of 2.5-MeV neutrons in it.

All PMT + Ss (these detectors will be named subsequently as TOF-1 or TOF-2) are inserted into the cylinders made of paraffin (served as collimators) and have a 2-mm lead foil blocking front part of the PMT + Ss to prevent them from scattered neutrons and soft X-rays. The oscilloscopes are triggered through the fiber optic cables. In Figure 6, one may see two stands near the PF-6 with the open door showing PMT + S, an oscilloscope and SACs on their roofs.

#### Figure 4.

Elements of mobile measuring stands used for powering management and optical communication [19].

#### Figure 5.

Key components of the fast neutron scintillation probes (FNSP-1) [19]).

Figure 6.

Experimental complex for measuring the neutrons field produced around the PF-6-based neutron source; it consists of two sets of detectors (silver activation counters and PMT + Ss) placed inside two cabinets.

In Figure 7, one may see an oscillogram of a cosmic radiation, demonstrating the temporal resolution of the PMT + S used in the experiments. It appears to be equal to 2.6 ns of its full width at half maximum (FWHM).

In Figure 8, the oscilloscope traces (OTs) for low (a), medium (b), and high (c) intensity of hard X-rays and neutrons are presented. It is seen that at a highintensity of the X-ray and neutron radiations, the photomultipliers are working in a "current mode of operation" (Figure 8c) rather than in a "single-pulse" recording (as it is in a and b pictures) regime.

Our plan for the first step of experiments on the taxonomy of a simulator is to investigate the angular characteristics of neutron radiation of the PF-6 device itself in empty room (Figure 9a).

It is provided in the most "clean" hall by two stands. In this test, one stand (No. 1) has an immobile position in a direction perpendicular (90°) to the Z-axis of the PF-6 chamber, whereas the other one (stand No. 2) is moved around the PF-6 device.

After these measurements, giving information on the spatial distribution of neutron intensity around the PF-6 device itself, the second step of this procedure starts. In these tests, the same measurements must be done but already with the simulator (in its capacity a chamber of the PF-1000U facility is used). That gives an

Figure 7.

Oscilloscope trace (OT) of cosmic radiation measured by PMT + S (2.6 ns of its FWHM).

#### Figure 8.

Oscillograms of three shots with low, medium, and high intensity of hard X-rays (1st pulses) and neutrons (2nd pulses); in the last oscilloscope trace all flashes produced inside the scintillator by individual X-ray photons and neutrons (a, b) are merged (c) and the PMT begins to work in the "current" mode of operation.

Taxonomy of Big Nuclear Fusion Chambers Provided by Means of Nanosecond Neutron Pulses DOI: http://dx.doi.org/10.5772/intechopen.89364

Figure 9.

Steady-state position of the stand No. 1 (SAC-1 and TOF-1) and a movable stand No. 2 (SAC-2 and TOF-2) in its position No. I (see later) in a "clean-room" condition (a) as well as the same stands placed at their 90° positions to the Z-axis of both chambers of PF-6 (neutron source) and PF-1000U (simulator) devices (b).

opportunity to disclosure imperfections produced in the 3-D neutron field by scattering and absorption of 2.5-MeV neutrons in structures of the PF-1000U chamber as well as in the hall parts (Figure 9b).
