5.1 Experimental results on measurements of anisotropy of neutron emission using PF-6 device positioned in an "empty" hall

One may see the results of anisotropy measurements after their processing in Figure 20. As in Figure 13a, this is the top view. Thus, it is a projection of the plane of SACs (70 cm above Z-axis) to the plane containing PMT + Ss and Z axis. In particular, for example, for the SAC-2, the so-called "flat" angle presented as α = 0° corresponds in the 3-D diagram to the actual angle θ = 12°. Juxtaposition of the experimental figures and the theoretical data (see Figure 11 and the side analysis) one may find a number of dissimilarities. However as it was mentioned before, the beam of deuterons is not a parallel one because of the magnetization of ions in the self-generated magnetic fields [8, 22]). Besides, this beam of fast deuterons is very powerful and its energy spectrum is very broad extending to the MeV range (so this stream is non-monoenergetic one). It is a reason for the observed in the experiment a certain "leveling" of the "8-digit" form of anisotropy shown in Figure 11. In addition, there are two peculiarities of the graph 20 that must be discussed.

1. It is seen that in forward directions (V, VI, and VII), a value of anisotropy is noticeably larger compared with those known from literature [8]. Usually these values are about 1.4–1.7. Such figures can be obtained by means of formula taken from [20, 21] if the most typical deuteron energy will be equal to Ed ≈ 100 keV and at the condition that the basic part of fusion neutrons are generated by magnetized deuterons that are flying out of the pinch within a cone having an angle to Z axis equal to about 20° [22]. But in these experiments, the magnitudes of neutron fluxes are equal to 1.8, 2.1, and 2.4 observed in these directions.

#### Figure 20.

Polar chart of neutron fluxes measured for different angles in relation to Z-axis of the PF-6 device for the case of the "clean-room" environments (here it is a projection from the plane 70-cm above the plane of Z-axis of the PF-6 chamber); in this chart, the radii of successively enlarged rings correspond to anisotropy coefficients equal to 1, 2, and 3, respectively; the coefficients are normalized on the value for the position 4.

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

2.One may notice a bit strange result for the position 2 of Figure 20: just opposite to the previous case, we observed a low value of the neutron flux measured at the angle α = 157°.

It is possible to find an interpretation of these features by comparing these experimental results with the MCNP modeling of the process.

Some parts of the PF-6 device itself as well as the hall environment can influence the ideal theoretical representation. The Monte-Carlo (MCNP-5) computations have been provided especially to describe the PF-6 itself and its surroundings. As it is known, the MCNP technique does not permit undertaking an inverse problem. Thus, we have provided a number of numerical simulation for the following dissimilar experiment's situations:


For identification of an influence of each component of the environment, we have provided six successive sets of the computations: with vacuum everywhere and with separating transformers on their places; vacuum everywhere with capacitors in their locations; vacuum everywhere with floor and ceiling of the hall; vacuum everywhere and hank of cables; all components are presented; only vacuum.

3.All calculations of the neutron flux density have been fulfilled for seven detectors placed in their real positions coinciding with those in the present experiments. Detectors were represented by spheres of 15 cm radius each.

Calculations have been done for two cases—with absence and in presence of the cadmium foils enveloping each SAC—see Tables 5 and 6 correspondingly. The first one contains results of calculations of neutron flux density (cm<sup>2</sup> ) of the whole spectrum of neutrons reaching the detector. The second one comprises data obtained for calculations of flux density of neutrons with energy spectrum above 500 keV.

The values of flux densities in seven directions around the PF-6 chamber (i.e., anisotropy) were calculated for each case taking into account the distance of the detector from the source using an inverse quadratic law for radii. Examination of these tables shows quite clearly:





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

> Table 6.

 Results of calculations of neutron flux density [cm2] of the spectrum of neutrons above 0.5 MeV reaching the detector.

Examination of our geometry of the PF-6 chamber used in these experiments has also shown that too high intensity in the "forward" direction (points 5, 6, and 7 —values up to 2.4) may be explained (additionally to the above-mentioned influence of concrete ceiling and floor) very likely by a specificity of the anode construction in this case. Instead of an aperture usually made in the anode center to prevent evaporation of debris by the electron beam we had in this case a special central insert made of rhenium. In this case, such an insert made of refractory materials helps to produce the most representative group of fast deuterons having higher energy compared with a common case. We observed some years ago this effect with the central anode insert made of tungsten. Because of this fact, the value of the projection of Ed max onto Z axis appears to be here in the range 150–200 keV, whereas the real value of these deuterons taking into consideration their preferential escaping angle [22] of about 25° can be estimated approximately as 300 keV.
