backThe thermal neutron calibration facility at PTB is based on a moderator assembly that consists of sixteen 241Am-Be(α,n) 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. The size of the moderator exit window is 50 cm x 50 cm and it is equipped with a lead shield in order to reduce low-energy photons. Shielding with borated plastic around the window and on the wall opposite of the window 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 gold foil activation, Bonner sphere measurements, and Monte Carlo calculations. The field is highly thermalized: thermal neutrons contribute with 98.4 % to the total neutron fluence and 83 % to the neutron dose equivalent. The field is homogenous within an area of about 20 cm x 20 cm. The thermal neutron fluence rate at the reference position is about 68 cm-2 s-1, the thermal neutron ambient dose equivalent rate 2.8 µSv h-1, and the thermal neutron personal dose equivalent rate 2.15 µSv h-1. Optionally, a cadmium plate can be mounted in front of the moderator window that eliminates thermal neutrons from the neutron field. Thus, performing an irradiation without and with this cadmium plate and calculating the difference allows the determination of the response of an instrument in an almost pure thermal neutron field.
The fees for irradiations in the thermal neutron reference radiation field of PTB are fixed in accordance with the regulations governing charges for services rendered by the PTB.
In case of questions or interest in irradiations please contact the Working Group (
neutronendosimetrie(at)ptb.de).
An irradiation of personal neutron dosemeters on the ISO water slab phantom at the point of test at the thermal neutron reference field at PTB.