Standard Test Method for Measuring Fast-Neutron Reaction Rates by Radioactivation of Niobium

Importancia y uso:

5.1 Refer to Practice E261 for a general discussion of the determination of decay rates, reaction rates, and neutron fluence rates with threshold detectors (1-29). Refer to Practice E1006, Practice E185, and Guide E1018 for the use and application of results obtained by this test method (30-32).

5.2 The half-life of ^93mNb is 16.1 (2) years (35) and has a K X-ray emission probability of 0.11546 ± 1.516 % per decay (33). The K_α and K_β X-rays of niobium are at 16.5213 to 16.6152 and 18.67 to 18.967 keV, respectively (36). The recommended ⁹³Nb(n,n′)^93mNb cross section comes from the International Reactor Dosimetry and Fusion File (IRDFF-II) cross section compendium (33) and is shown in Fig. 1. The nuclear decay data referenced here are not taken from the latest dosimetry recommended database (34) but are selected to be consistent with the nuclear data used in the recommended IRDFF evaluation.

5.3 Since ^93mNb emits low-energy photons compared to many activated materials used for metrology, some considerations must be taken into account during measurement:

5.3.1 Chemical dissolution of the irradiated niobium to produce very low mass-per-unit area sources is an effective way to obtain consistent results. The direct counting of foils or wires can produce satisfactory results provided appropriate methods and interpretations are employed. It is possible to use liquid scintillation methods to measure the niobium activity provided the radioactive material can be kept uniformly in solution and appropriate corrections can be made for interfering activities.

5.4 The measured reaction rates can be used to correlate neutron exposures, provide comparison with calculated reaction rates, and determine neutron fluences. Reaction rates can be determined with greater accuracy than fluence rates because of the current uncertainty in the cross section versus energy shape.

5.5 The ⁹³Nb(n,n′)^93mNb reaction has the desirable properties of monitoring neutron exposures related to neutron damage of nuclear facility structural components. It has an energy response range corresponding to the damage function of steel and has a half-life sufficiently long to allow its use in very long exposures (up to about 48 years). Monitoring long exposures is useful in determining the long-term integrity of nuclear facility components.

Subcomité:

E10.05

Referida por:

E0720-23, E0721-22, E2005-21, E1854-26

Volúmen:

12.02

Número ICS:

27.120.01 (Nuclear energy in general)

Palabras clave:

activation; activation reaction; cross section; dosimetry; fast neutron monitor; neutron dosimeter; neutron metrology; niobium; pressure vessel surveillance; reaction rate; reaction rate monitor;