PROMPT GAMMA ACTIVATION ANALYSIS
Instrument responsible: Zsolt Kasztovszky
The PGAA target chamber is at 1.5 m distance from the end of the guide. The sample chamber can be evacuated or filled up with gases to decrease beam-induced background. To prevent scattering of neutrons to the PGAA sample from the lower beam, a layer of neutron absorber is placed below the sample. The targets are mounted on thin Al frames by Teflon strings. Optionally, an automated sample changer with a capacity of 16 samples can be used. A neutron absorber after the PGAA target chamber stops the upper beam. Although the beam flux is moderate, in the order of 107-108 n cm-2s-1, it is adequate for most sample types.
The detector system of the PGAA facility consists of an n-type high-purity germanium (Canberra HPGe 2720/S) main detector with closed-end coaxial geometry, and a BGO Compton-suppressor surrounded by a 10 cm thick lead shielding. The sample-to-detector distance is adjustable, but it is typically 230 mm. By removing the front detector shielding the HPGe main detector can be placed as close as 12 cm to the target.
The BGO annulus and catchers around the HPGe detect most of the scattered gamma photons. If the events from the HPGe and the BGO are collected in anticoincidence mode, Compton-suppressed spectra can be acquired. An analogue spectroscopy amplifier combined with an ADC and an Ethernet-based multichannel analyzer (Canberra AIM 556A) collects the counts. Thanks to the sophisticated electronics, excellent peak shape and good energy resolution can be achieved, making possible to derive very accurate energy and intensity values for each peak, and therefore to obtain precise element concentrations or nuclear data.
A schematic drawing of the sample chambers and the HPGe-BGO detector assembly is shown in Figure 1.
Figure 1. The PGAA facility, with the standard sample chamber (left) and with the automated sample changer (right)
Table 1. Specifications of the PGAA facility