3.4 Neutron spectral measurement procedure by time-of-flight technique for the "clean-room"condition and for the experiments with a simulator

The typical fast probe OT of the PMT + S-1, that is, obtained by the immobile stand No. 1 with TOF-1, for the experiment made in a "clean-room" condition is given in Figure 17.

The PMT + S probes No. 1 (immobile) and No. 2 (movable) are placed in the horizontal plane coinciding with a Z-axis of the PF-6 chamber. The abovementioned TOF method was used to obtain information on the angle neutron spectral distribution. Again, the PMT + S-1 position (TOF-1) was preserved from one side of the PF-6 chamber but the PMT + S-2 stand (TOF-2) was moved along the steps 1 through 7 shown in Figure 13a.

As it was mentioned above, the neutrons' energy is 2.5 MeV [8] at the angle 90<sup>0</sup> to Z-axis of the chamber; so for the detectors Nos 1 and 2 placed at 1.05 m we have to move forward the hard X-ray pulse by its time-of-flight equal to 3.5 ns (vhxr = 3 <sup>10</sup><sup>8</sup> m/s) and the neutron pulse—by 48.5 ns (vn = 2.1667 <sup>10</sup><sup>7</sup> m/s) as it is shown in Figure 18a and b.

This correction on TOF of both types of radiation (we have taken a mean value for it calculated for the 33 shots) provides the time interval for the delay of the neutron pulse peak appeared near the anode of the DPF chamber in relation to the front of the hard X-ray pulse. We found that in these experiments, it was equal to

Figure 17.

Typical OT of hard X-ray (1st—ΔtFWHM = 10 ns) and neutron (2nd) pulses of the PF-6 device (neutron pulse: ΔtFWHM = 20 ns).

Figure 18.

Initial (measured—a) and corrected for TOF positions of hard X-ray (1st) pulse and 2.45-MeV neutron (2nd) pulse (b).

Δt = 25 ns for each stand in this collection of experiments. Later, one must recheck this figure in each shot by using fixed stand No. 1. Typically, the rise-time of the hard X-ray pulse is vertical practically, that is, it is equal to the fast probe temporal resolution and, consequently, to the measurements' uncertainty. These readings establish for us a foundation for the following measurements and amendments fitted for all other neutron pulses observed at different angles and at dissimilar remoteness from the PF-6-based pulsed source of neutrons. So in every shot, we begin our temporal calculations from the front of the hard X-ray pulses, subsequently moving the neutron pulses to the time moment delayed in relation to the X-ray front namely by this Δt.

After such a procedure, we obtained an opportunity to calculate TOF of this neutron pulse to the detector No. 2 in its each specific location. Using formula (6), this measured and corrected time-of-flight can easily be recalculated into the energy of this neutron group producing the neutron pulse maximum. Results of calculations at usage of the above procedure and the formula give the angle dependence of the neutron spectral distributions in the space around our PF-6 device in a "clean" room conditions.

The same procedure of neutron spectra distortions has been provided in the experiments with the PF-1000U chamber simulator.
