ASTM E 263 : 2009

DESIG: E 263 09 ^TITLE: Standard Test Method for Measuring Fast-Neutron Reaction Rates by Radioactivation of Iron ^SIGNUSE:

Refer to Guide E 844 for guidance on the selection, irradiation, and quality control of neutron dosimeters.

Refer to Practice E 261 for a general discussion of the determination of fast-neutron fluence rate with threshold detectors.

Pure iron in the form of foil or wire is readily available and easily handled.

Fig. 1 shows a plot of cross section as a function of neutron energy for the fast-neutron reaction ⁵⁴Fe(n,p)⁵⁴Mn (1). This figure is for illustrative purposes only to indicate the range of response of the ⁵⁴Fe(n,p)⁵⁴Mn reaction. Refer to Guide E 1018 for descriptions of recommended tabulated dosimetry cross sections.

⁵⁴Mn has a half-life of 312.13 days (3) (2) and emits a gamma ray with an energy of 834.845 keV (5). (2)

Interfering activities generated by neutron activation arising from thermal or fast neutron interactions are 2.57878 (46)-h ⁵⁶Mn, 44.95-d (8) ⁵⁹Fe, and 5.2710-y (8) ⁶⁰Co (2,3). (Consult Ref (2) for more precise values currently accepted for the half-lives.) Interference from ⁵⁶Mn can be eliminated by waiting 48 h before counting. Although chemical separation of ⁵⁴Mn from the irradiated iron is the most effective method for eliminating ⁵⁹Fe and ⁶⁰Co, direct counting of iron for ⁵⁴Mn is possible using high-resolution detector systems or unfolding or stripping techniques, especially if the dosimeter was covered with cadmium or boron during irradiation. Altering the isotopic composition of the iron dosimeter is another useful technique for eliminating interference from extraneous activities when direct sample counting is to be employed.

The vapor pressures of manganese and iron are such that manganese diffusion losses from iron can become significant at temperatures above about 700°C. Therefore, precautions must be taken to avoid the diffusion loss of ⁵⁴Mn from iron dosimeters at high temperature. Encapsulating the iron dosimeter in quartz or vanadium will contain the manganese at temperatures up to about 900°C.

Sections 6, 7 and 8 that follow were specifically written to describe the method of chemical separation and subsequent counting of the ⁵⁴Mn activity. When one elects to count the iron dosimeters directly, those portions of Sections 6, 7 and 8 that pertain to radiochemical separation should be disregarded.

Note 1—The following portions of this test method apply also to direct sample-counting methods: 6.1-6.3, 7.4, 7.9, 7.10, 8.1-8.5, 8.18, 8.19, and 10-13.

FIG. 1 ⁵⁴Fe(n,p)⁵⁴Mn Cross Section

1.1 This test method describes procedures for measuring reaction rates by the activation reaction ⁵⁴Fe(n,p)⁵⁴Mn.

1.2 This activation reaction is useful for measuring neutrons with energies above approximately 2.2 MeV and for irradiation times up to about 3 years (for longer irradiations, see Practice E 261).

1.3 With suitable techniques, fission-neutron fluence rates above 10⁸ cm⁻²·s⁻¹ can be determined. However, in the presence of a high thermal-neutron fluence rate (for example, >2 × 10¹⁴ cm⁻²·s⁻¹) ⁵⁴Mn depletion should be investigated.

1.4 Detailed procedures describing the use of other fast-neutron detectors are referenced in Practice E 261.

1.5 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. ^REFERENCE:

ASTM Standards:

D 1193 Specification for Reagent Water

E 170 Terminology Relating to Radiation Measurements and Dosimetry

E 181 Test Methods for Detector Calibration and Analysis of Radionuclides

E 261 Practice for Determining Neutron Fluence, Fluence Rate, and Spectra by Radioactivation Techniques

E 844 Guide for Sensor Set Design and Irradiation for Reactor Surveillance, E 706(IIC)

E 944 Guide for Application of Neutron Spectrum Adjustment Methods in Reactor Surveillance, E 706 (IIA)

E 1005 Test Method for Application and Analysis of Radiometric Monitors for Reactor Vessel Surveillance, E 706(IIIA)

E 1018 Guide for Application of ASTM Evaluated Cross Section Data File, Matrix E 706 (IIB)