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).³ Refer to Practice E1006, Practice E185 and Guide E1018 for the use and application of results obtained by this test method.(34-36)

5.2 The half-life of ^93mNb is 5730 ± 220 days (30) and has a K X-ray emission probability of 0.1099 ± 0.0025 per decay (30). The K_α and K_β X-rays of niobium are at 16.5213–16.152 and 18.618–18.953 keV, respectively. The recommended ⁹³Nb (n,n′)^93mNb cross section comes from the IRDF-90 cross section compendium (31), was drawn from the RRDF-98 cross section evaluations (37) and is shown in Fig. 1.

FIG. 1 IRDF-90 Cross Section Versus Energy for the ⁹³Nb(n,n′) ^93mNb Reaction

5.3 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 40 years). Monitoring long exposures is useful in determining the long-term integrity of nuclear facility components.

1.1 This test method describes procedures for measuring reaction rates by the activation reaction ⁹³Nb(n,n′) ^93mNb.

1.2 This activation reaction is useful for monitoring neutrons with energies above approximately 0.5 MeV and for irradiation times up to about 30 years.

1.3 With suitable techniques, fast-neutron reaction rates for neutrons with energy distribution similar to fission neutrons can be determined in fast-neutron fluences above about 10¹⁶ cm⁻². In the presence of high thermal-neutron fluence rates (>10¹²cm⁻²·s⁻¹), the transmutation of ^93mNb due to neutron capture should be investigated. In the presence of high-energy neutron spectra such as are associated with fusion and spallation sources, the transmutation of ^93mNb by reactions such as (n,2n) may occur and should be investigated.

1.4 Procedures for other fast-neutron monitors are referenced in Practice E261.

1.5 Fast-neutron fluence rates can be determined from the reaction rates provided that the appropriate cross section information is available to meet the accuracy requirements.

1.6 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.7 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.