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Determination of the anisotropic emission of a Am-241-Be source

23.12.2020

The anisotropic neutron emission of a 241Am‑Be calibration source was measured. The associated correction factor, which was last determined in 1993, was successfully confirmed.

For the calibration of neutron measuring instruments used in the field of radiation protection, various radionuclide sources are recommended in accordance with the international standard ISO 8529 “Reference neutron radiation fields”, Part 1 to 3. The most frequently used sources are 252Cf sources, whose neutron emission is due to spontaneous fission, and alpha‑beryllium sources (e.g., 241Am‑Be), which make use of the high (α,n) reaction cross section of beryllium. The advantage of the 241Am‑Be source is its long half‑life of 432.6 years; its disadvantage is the great amount of active material needed due to its lower specific activity, and thus its relatively large volume.

The 241Am‑Be source used at PTB for calibrations and irradiations is cylindrical with a height and diameter of 25.2 mm. Its current neutron emission rate is (2.94 ± 0.07)·106 s-1. The emission rate is traceable to the primary standard of the National Physical Laboratory (NPL) – the British metrology institute – and describes the neutron emission in the total solid angle.

Due to the distribution of the active material in neutron sources and due to the material and shape of the encapsulation, neutrons are usually not emitted isotropically. This anisotropy is characteristic for each individual source and can reach values up to 20 % – depending on the size and type of the source – and up to 5 % in the calibration direction (polar, 90°). The anisotropy has to be included in the calculation of the neutron fluence rate from the neutron emission rate. Thus, it is an important contribution to the determination of reference values for calibrations and irradiations. The correction factor for the 241Am‑Be source at PTB was last determined in 1993. Within the scope of the revision of the ISO 8529 standard, the anisotropy measurement at this source was repeated.

For the determination of the anisotropy, the ratio of the emission rate at a certain angle to the emission rate of a corresponding point source at that angle is computed. For this purpose, the source is positioned on an angle table in the center of the irradiation room and is moved in 5° steps. A Precision Long Counter (PLC) is used for the measurement. This detector consists of a BF3 counter tube in a paraffin cylinder surrounded with boron plastics and is only sensitive to neutrons coming through the entrance window from the front. The detector is designed in such a way that it is characterized by an almost constant response with regard to the energy of the incident neutrons. The measurements were performed at two different distances between the center of the source and the reference point of the PLC. The count rates were corrected for dead‑time and a shadow object measurement was conducted to correct for inscattered neutrons.

For cylindrical 241Am‑Be sources, which are produced, for example, by pressing AmO2 mixed with Be powder, an isotropic azimuthal emission is to be expected. The current measurements have confirmed this.

In the case of the polar measurement (as a function of the angle between the symmetry axis of the source and the line connecting the center of the source and the reference point of the PLC), the count rates are additionally corrected for the solid angle. The result for the polar anisotropy is shown in Figure 1. The comparison of the new results, which will be included in the revised international standard, with the values determined in 1993 is shown in Table 1. The values agree. For calibrations, the correction factor for a polar angle of 90° is used. This factor was determined with a higher precision here due to a smaller statistical uncertainty.

Figure 1: Results for the correction factor for the anisotropic emission F(θ) – the ratio of the emission rate to the emission rate of a corresponding point source depending on the angle – at a distance of 130 cm and of 205 cm between the center of the source and the reference point of the detector (PLC).

Table 1: The correction factor for the anisotropic emission F(θ) at 0°, 90°, and 180°, determined in 1993 and in 2020.

Contact:

Opens local program for sending emailD. Radeck, Department 6.4, Working Group 6.44