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Description

The CANBERRA ACT-II Ge Detector was designed specifically for the detection of internally deposited actinides, particularly uranium, plutonium and americium. Because of the low gamma-ray abundance from uranium, and the low energy of the x rays from plutonium, which emits few gammas, this application demands a very specialized detector system. To achieve desired sensitivities, four detectors are placed in virtual contact with the subject in close proximity to the lungs. The measurement must be carried out in a shielded room. For optimum results, the detectors must be closely spaced, the detector background must be low, the resolution must be good, and the sensitivity of the detector must be high over the energy range of interest (13-20 keV for Pu, 60 keV for Am, and 140-190 keV for U). The ACT-II Ge Detector from CANBERRA provides all this performance and more.

Detector Element

CANBERRA pioneered the LEGe (Low Energy Ge) detector element which is used in the ACT detectors. The LEGe detector is a large area detector with a thin outer contact for high sensitivity at low energies. Because of the device geometry, the capacitance is low, and this results in excellent resolution at low to moderate energies. The LEGe has virtually no dead layers or regions which often contribute to background because of charge carrier diffusion from photon interactions therein.

Multi-Attitude Cryostats

In order to provide the flexibility to allow optimum placement of detectors on the chest of the subject under study, the ACT-II detector is able to operate in all attitudes from vertical upright to vertical downlooking.

The ACT-II, with 3 day holding time, does not require an automatic LN₂ refill system. Such systems are available, however, if desired.

Low Background Materials

The detector chamber and the internal cryostat hardware are made from stainless steel and high purity copper respectively. The window is made from a space-age carbon filament composite, which provides strength, safety and lower background than is possible with Be windows (see curves below). Be windows are available optionally.

Additional Features

Additional shielding may be used around ACT-II detectors to reduce 40 K and scattered background contribution from the subject and surrounding materials.

Window Transmission Characteristics