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Measurement of the energy distribution of prompt photons in PTB's monoenergetic neutron reference fields


Personal dosemeters for neutron radiation are generally designed for the simultaneous measuring of the photon and neutron personal dose. When determining the neutron response of personal dosemeters it thus has to be ensured that the influence of parasitic photons in the neutron fields on the displayed personal neutron dose is negligible.

When generating neutrons by means of ion‑induced nuclear reactions at accelerator facilities, photons will practically always be generated by nuclear reactions of the ions or neutrons in the production target. Due to the generally very low half‑life of nuclear excited states, these "prompt" photons are closely time‑correlated with the impact of the ions on the target. In addition, there is a second and generally less intensive photon component resulting from the activation of target materials or the inelastic scattering of neutrons at the structure materials of the facility. These secondary photons no longer have a pronounced time correlation with the ions.

With the aid of the new Tandetron accelerator of PTB's accelerator facility PIAF, routinely pulsed ion beams can be generated such that the time correlation with the ion beam can be used to separate prompt photons from uncorrelated photons and neutrons. Therefore, inorganic scintillators with a good energy resolution, which have been available for several years now, can be used to measure the energy distribution of prompt photons.

For a systematic characterization of the prompt photon component in the neutron reference fields, a 1.5“ x 1.5“ CeBr3 detector was thoroughly characterized at PTB, and the photon response matrix was calculated by means of the Monte Carlo program MCNP. This detector was used to measure the pulse height distributions of the prompt photons of all monoenergetic ISO neutron reference fields. Based on the measured pulse height distributions, the energy distribution of the photons was calculated by deconvolution with the GRAVEL algorithm. Figure 1 shows the measured pulse height distribution and the deconvoluted photon energy distribution for the 2.5 MeV neutron field. The peaks in the distributions can be explained by gamma radiation of nuclides of the target materials because nuclear states are predominantly excited by inelastic scattering of the accelerator ions in the target.

The ratios of the photon and neutron personal dose can be calculated from the photon energy distributions. This enables a more exhaustive characterization of the fields and an estimation of the maximum influence of parasitic photons on the neutron display of personal dosemeters. The contribution made by photons following the radioactive decay of activation products still has to be examined.

Zwei Diagramme: Impulshöhenverteilung und entfaltete Photonenenergieverteilung

 Figure 1: Measured pulse‑height distribution of the prompt photons in the 2.5 MeV neutron reference field (left) and deconvoluted photon energy distribution (right). The peaks are caused by gamma emissions of excited states in the target materials that are mainly the result of inelastic scattering of accelerator ions in the target.


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