5.1 Refer to Practice E261 for a general discussion of the measurement of fast-neutron fluence rates with threshold detectors.

FIG. 1 Variation of 0 Degree ³H(d,n)⁴He Differential Cross Section with Incident Deuteron Energy (1)
FIG. 2 Variation of 0 Degree ³H(d,n)⁴He Differential Cross Section with Incident Deuteron Energy (1)

5.2 Refer to Test Method E265 for a general discussion of the measurement of fast-neutron fluence rates by radioactivation of sulfur-32.

5.3 Reactions used for the activity measurements can be chosen to provide a convenient means for determining the absolute fluence rates of 14-MeV neutrons obtained with ³H(d,n)⁴He neutron generators over a range of irradiation times from seconds to approximately 100 days. High purity threshold sensors referenced in this test method are readily available.

5.4 The neutron-energy spectrum must be known in order to measure fast-neutron fluence using a single threshold detector. Neutrons produced by bombarding a tritiated target with deuterons are commonly referred to as 14-MeV neutrons; however, they can have a range of energies depending on: (1) the angle of neutron emission with respect to the deuteron beam, (2) the kinetic energy of the deuterons, and (3) the target thickness. In most available neutron generators of the Cockroft-Walton type, a thick target is used to obtain high-neutron yields. As deuterons penetrate through the surface and move into the bulk of the thick target, they lose energy, and interactions occurring deeper within the target produce neutrons with correspondingly lower energy.

5.5 Wide variations in neutron energy are not generally encountered in commercially available neutron generators of the Cockroft-Walton type. Figs. 1 and 2 (1)⁶ show the variation of the zero degree ³H(d,n)⁴He neutron production cross section with energy, and clearly indicate that maximum neutron yield is obtained with deuterons having energies near the 107 keV resonance. Since most generators are designed for high yield, the deuteron energy is typically about 200 keV, giving a range of neutron energies from approximately 14 to 15 MeV. The differential center-of-mass cross section is typically parameterized as a summation of Legendre polynomials. Figs. 3 and 4 (1,2) show how the neutron yield varies with the emission angle in the laboratory system. The insert in Fig. 4 shows how the magnitude, A₁, of the P₁(θ) term, and hence the asymmetry in the differential cross section grows with increasing energy of the incident deuteron. The nonrelativistic kinematics (valid for E_d < 20 MeV) for the ³H(d,n)⁴He reaction show that:

FIG. 3 Energy and Angle Dependence of the ³H(d,n)⁴He Differential Cross Section (1)
FIG. 4 Change in Neutron Energy from ³H(d,n)⁴He Reaction with Laboratory Emission Angle (2)

5.5.1 Fig. 5 (2) shows how the neutron energy depends upon the angle of scattering in the laboratory coordinate system when the incident deuteron has an energy of 150 keV and is incident on a thick and a thin tritiated target. For thick targets, the incident deuteron loses energy as it penetrates the target and produces neutrons of lower energy. A thick target is defined as a target thick enough to completely stop the incident deuteron. The two curves in Fig. 5, for both thick and thin targets, come from different sources. The dashed line calculations come from Ref (3); the solid curve calculations come from Ref (4); and the measured data come from Ref (5). The dash-dot curve and the right-hand axis gives the difference between the calculated neutron energies for thin and thick targets. Computer codes are available to assist in calculating the expected thick and thin target yield and neutron spectrum for various incident deuteron energies (6).

FIG. 5 Dependence of ³H(d,n)⁴He Neutron Energy on Angle (2)

5.6 The Q-value for the primary ³H(d,n)⁴He reaction is + 17.59 MeV. When the incident deuteron energy exceeds 3.71 MeV and 4.92 MeV, the break-up reactions ³H(d,np) ³H and ³H(d,2n)³He, respectively, become energetically possible. Thus, at high deuteron energies (>3.71 MeV) this reaction is no longer monoenergetic. Monoenergetic neutron beams with energies from about 14.8 to 20.4 MeV can be produced by this reaction at forward laboratory angles (7).

5.7 It is recommended that the dosimetry sensors be fielded in the exact positions where the dosimetry results are wanted. There are a number of factors that can affect the monochromaticity or energy spread of the neutron beam (7,8). These factors include the energy regulation of the incident deuteron energy, energy loss in retaining windows if a gas target is used or energy loss within the target if a solid tritiated target is used, the irradiation geometry, and background neutrons from scattering with the walls and floors within the irradiation chamber.