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Calibration of radiation measuring instruments with thermal neutrons

PTB operated for almost 10 years an installation at the research reactor of the GKSS in Geesthacht/Germany for irradiations with thermal neutrons. After its shutdown in 2008, no thermal neutron calibration facility was available in Germany. Now, a new thermal neutron calibration facility based on a moderator assembly has been set up at PTB. It consists of sixteen 241Am-Be radionuclide sources mounted into a graphite block, 1.5 m wide, 1.5 m high and 1.8 m deep. The sources are distributed at eight different positions, at a mean distance of about 1.25 m from the front face of the moderator. Shielding with borated plastic (see Fig. 1 and Fig. 2) reduces the background of scattered neutrons. The neutron field at the reference position, 30 cm in front of the moderator assembly and 75 cm above the floor, has been characterized using calculations, Bonner sphere measurements and gold foil activation. The field is highly thermalized: 99 % in terms of fluence. Using irradiations with and without a cadmium plate at the moderator exit window, a pure thermal neutron field with good collimation is achieved. It is quite homogenous within a 20 cm x 20 cm area, yielding a neutron fluence rate of about 75 cm-2 s-1 and an ambient dose equivalent rate of 3 µSv/h.

PTB thermal neutron facility as built up (Fig. 1) and a horizontal cut as used for calculations (Fig. 2)

Further information

  • Irradiation facility with thermal neutrons operational
    A new facility for the calibration of radiation measuring instruments with thermal neutrons has been set up at PTB. For this purpose, sixteen 241AmBe radionuclide sources were inserted into a graphite block. The entire facility with additional shields has been optimized by means of calculations, and these have been verified via first measurements.

References

  • M. Luszik-Bhadra, M. Reginatto, H. Wershofen, B. Wiegel and A. Zimbal:
    New PTB thermal neutron calibration facility: First results
    Radiat. Prot. Dosim. (2014), Vol. 161, No. 1 – 4, 352 – 356
    doi: Opens external link in new window10.1093/rpd/nct354