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Feasibility Study into the Use of an Aluminum Ionization Chamber as a Gamma Dosimeter in a Mixed Neutron and Gamma-Ray Field

Antoaneta Roca, Yuan-Hao Liu, Ray Moss, Finn Stecher-Rasmussen, Sander Nievaart

Nuclear Technology / Volume 168 / Number 1 / October 2009 / Pages 196-201

Dosimetry / Special Issue on the 11th International Conference on Radiation Shielding and the 15th Topical Meeting of the Radiation Protection and Shielding Division (Part 1) / Radiation Protection /

Generally, the determination of the gamma-ray dose in a mixed neutron-gamma field is obtained by using "neutron-insensitive" detectors. For this purpose, graphite, magnesium, and aluminum ionization chambers are available. It is known that graphite chambers suffer from porosity, and magnesium chambers encounter oxidation and manufacturing problems. So far, the aluminum chamber is mostly applied in fast neutron fields. This study presents the results of an aluminum chamber, flushed with argon gas, when applied in a neutron and gamma mixed field. A computer model of the ionization chamber is developed for an accurate interpretation of the responses. Special interest is given to the charge that can be measured after the irradiation has stopped, which is due to decay of 28Al.

The Monte Carlo code MCNPX is used to simulate the neutrons, gammas, and charged particles in and around the Al-Ar chamber. The detector is modeled in detail, and all possible reactions that can occur in the materials of the chamber are incorporated. The response of the Al-Ar chamber is compared with the results of a Mg-Ar chamber in terms of collected charge.

All individual components contributing to the signal of the detector are identified and calculated. Although the decay charge produced by aluminum is much higher, in comparison to magnesium, a better estimation of the gamma dose is expected when the decay charge in aluminum can be accurately determined. Another advantage is that the higher activation in Al can be used for identifying the neutron contribution. Despite the great detail in the model used, there is an [approximately]25% discrepancy between the experimental and simulated total charges for both the Mg-Ar and Al-Ar chambers, which evidently requires further investigation.

The Al-Ar chamber can be used complementarily to the Mg-Ar chamber as gamma dosimeter in a mixed field of neutrons and gammas.

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