Active Standard ASTM E496 | Developed by Subcommittee: E10.07
Book of Standards Volume: 12.02
Historical (view previous versions of standard)
Significance and Use
5.1 Refer to Practice E261 for a general discussion of the measurement of fast-neutron fluence rates with threshold detectors.
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 Ed < 20 MeV) for the 3H(d,n)4He reaction show that:
1.1 This test method covers a general procedure for the measurement of the fast-neutron fluence rate produced by neutron generators utilizing the 3H(d,n) 4He reaction. Neutrons so produced are usually referred to as 14-MeV neutrons, but range in energy depending on a number of factors. This test method does not adequately cover fusion sources where the velocity of the plasma may be an important consideration.
1.2 This test method uses threshold activation reactions to determine the average energy of the neutrons and the neutron fluence at that energy. At least three activities, chosen from an appropriate set of dosimetry reactions, are required to characterize the average energy and fluence. The required activities are typically measured by gamma ray spectroscopy.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.
E170 Terminology Relating to Radiation Measurements and Dosimetry
E181 Test Methods for Detector Calibration and Analysis of Radionuclides
E261 Practice for Determining Neutron Fluence, Fluence Rate, and Spectra by Radioactivation Techniques
E265 Test Method for Measuring Reaction Rates and Fast-Neutron Fluences by Radioactivation of Sulfur-32
E720 Guide for Selection and Use of Neutron Sensors for Determining Neutron Spectra Employed in Radiation-Hardness Testing of Electronics
International Commission on Radiation Units and Measurements (ICRU) ReportsICRUReport26—N Dosimetry for Biology and Medicine
NIST DocumentTechnicalNote1297&md for Evaluating and Expressing the Uncertainty of NIST Measurement Results
ICS Number Code 17.240 (Radiation measurements); 27.120.30 (Fissile materials and nuclear fuel technology)
UNSPSC Code 26142004(Neutron irradiators); 26142108(Nuclear reactor in core neutron flux instrumentation)