ASTM E262 : 13

Standard Test Method for Determining Thermal Neutron Reaction Rates and Thermal Neutron Fluence Rates by Radioactivation Techniques

Standard Details

1.1 The purpose of this test method is to define a general procedure for determining an unknown thermal-neutron fluence rate by neutron activation techniques. It is not practicable to describe completely a technique applicable to the large number of experimental situations that require the measurement of a thermal-neutron fluence rate. Therefore, this method is presented so that the user may adapt to his particular situation the fundamental procedures of the following techniques.

1.1.1 Radiometric counting technique using pure cobalt, pure gold, pure indium, cobalt-aluminum, alloy, gold-aluminum alloy, or indium-aluminum alloy.

1.1.2 Standard comparison technique using pure gold, or gold-aluminum alloy, and

1.1.3 Secondary standard comparison techniques using pure indium, indium-aluminum alloy, pure dysprosium, or dysprosium-aluminum alloy.

1.2 The techniques presented are limited to measurements at room temperatures. However, special problems when making thermal-neutron fluence rate measurements in high-temperature environments are discussed in 9.2. For those circumstances where the use of cadmium as a thermal shield is undesirable because of potential spectrum perturbations or of temperatures above the melting point of cadmium, the method described in Test Method E481 can be used in some cases. Alternatively, gadolinium filters may be used instead of cadmium. For high temperature applications in which aluminum alloys are unsuitable, other alloys such as cobalt-nickel or cobalt-vanadium have been used.

1.3 This test method may be used to determine the equivalent 2200 m/s fluence rate. The accurate determination of the actual thermal neutron fluence rate requires knowledge of the neutron temperature, and determination of the neutron temperature is not within the scope of the standard.

1.4 The techniques presented are suitable only for neutron fields having a significant thermal neutron component, in which moderating materials are present, and for which the average scattering cross section is large compared to the average absorption cross section in the thermal neutron energy range.

1.5 Table 1 indicates the useful neutron-fluence ranges for each detector material.

TABLE 1 Useful Neutron Fluence Ranges of Foil Material

Foil Material	Form	≈ Useful Range (neutrons/cm²)
Indium	pure or alloyed with aluminum	10³ to 10¹²
Gold	pure or alloyed with aluminum	10⁷ to 10¹⁴
Dysprosium	pure or alloyed with aluminum	10³ to 10¹⁰
Cobalt	pure or alloyed with aluminum	10¹⁴ to 10²⁰

1.6 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.