Albedo-Dosimeter

The albedo dosimeter is a dosimeter for the measurement of personal dose by X-ray, gamma, and neutron radiation in personal and local dosimetry.

The dosimeter probe contains a numbered, reusable detector card in an albedo capsule, which carries four thermoluminescence detectors, two of which are arranged behind each "albedo neutron window". A protective case with carrying clip is recommended for mounting the dosimeter.

The measuring system is intended for use in mixed radiation fields (neutrons, photons). For photon radiation, it is type-approved for the measurand H_p(10). Personal dose monitoring with the albedo dosimeter in mixed radiation fields is recommended if the share of the neutron personal dose can reach more than 20% of the photon personal dose.

When exposed to ionizing radiation, free pairs of charge carriers(electrons and holes) are generated in a crystal and become trapped in crystalline vacancies. Heating frees them from these energetically higher and long-lived states and they recombine emitting light (thermoluminescence, TL for short). The light intensity is a direct measure of the irradiated dose. By enrichment with neutron-sensitive isotopes (e.g. Li-6), the crystal can also be sensitized to thermal neutrons.

Das Zweikomponenten-Albedodosimeter enthält zwei TL-Detektorpaare, wovon das erste die vom Körper zurückgestreuten Neutronen (Albedo-Neutronen) und das zweite die auf den Körper einfallenden thermischen Neutronen (Feldneutronen) misst. Die Angabe der Neutronenäquivalentdosis erfolgt aus der Anzeige des Albedodetektorpaares und dem entsprechenden Kalibrierfaktor für den jeweiligen Anwendungsbereich.

Each detector pair consists of a photon sensitive detector and a photon and neutron sensitive detector. The photon-sensitive detector only is used to determine the respective photon dose. The difference between the two detectors is used to determine the respective neutron fraction. The ratio of field to albedo neutrons provides an additional energy correction factor (DIN 6802-4).

The albedo dosimeter can also be used at high-energy accelerators, since the neutron radiation field at these workplaces always has a proportion of scattered neutrons with low energy and thus a signal sufficient for dose determination is generated in the detector. This could be verified by measurements at different high-energy accelerators.

The dosimeter must be worn at a location representative of the radiation field (usually on the upper part of the torso), with the back of the badge (marked "Back") close to the body of the exposed person.

The albedo dosimeter is the property of the monitoring service and is made available to the customer on a rental basis. When used, it must be permanently assigned to a person for a wearing period (usually one month) and sent to the monitoring service for evaluation after the end of the wearing period, stating the carrier and the exposure conditions. Unused dosimeters should be returned to the monitoring service after a quarter of a year so that they can be baked out ("zeroed") there for reuse, since the dosimeters continuously measure the ambient radiation as well.

With the albedo dosimeter, the energy range for neutrons occurring in routine monitoring is adequately covered. Due to the energy dependence of the neutron response, a distinction is made between four typical application ranges (N1 to N4), into which the radiation protection officer must classify the occupationally exposed persons (see DIN 6802-4).

It must be ensured that the dosimeter is only used in the intended area of application. In individual cases, calibration in the respective scattered radiation field may be necessary. Calibrations in the scattered radiation field are carried out in consultation with the measuring point.

The dosimeter is suitable for use at an ambient temperature of +10°C to +40°C and a relative humidity of 10% to 90%. The albedo dosimeter should not be exposed to direct sunlight over 1000 W/m² or mechanical shock over 4900 m/s². If necessary, the dosimeter capsule can be cleaned with a damp cloth. Albedo dosimeters must be protected from moisture and temperatures above +60°C during storage. Care must be taken to ensure that only low levels of ambient radiation and, above all, no thermal neutrons occur at the storage location. The dosimeters must not be opened or mechanically damaged so that the very sensitive detectors are not destroyed or lost.