Neutron Activation Analysis - Detectors

Detectors

There are a number of detector types and configurations used in NAA. Most are designed to detect the emitted gamma radiation. The most common types of gamma detectors encountered in NAA are the gas ionisation type, scintillation type and the semiconductor type. Of these the scintillation and semiconductor type are the most widely employed. There are two detector configurations utilised, they are the planar detector, used for PGNAA and the well detector, used for DGNAA. The planar detector has a flat, large collection surface area and can be placed close to the sample. The well detector ‘surrounds’ the sample with a large collection surface area.

Scintillation-type detectors use a radiation-sensitive crystal, most commonly thallium-doped sodium iodide (NaI(Tl)), which emits light when struck by gamma photons. These detectors have excellent sensitivity and stability, and a reasonable resolution.

Semiconductor detectors utilise the semiconducting element germanium. The germanium is processed to form a p-i-n (positive-intrinsic-negative) diode, and when cooled to ~77 K by liquid nitrogen to reduce dark current and detector noise, produces a signal which is proportional to the photon energy of the incoming radiation. There are two types of germanium detector, the lithium-drifted germanium or Ge(Li) (pronounced ‘jelly’), and the high-purity germanium or HPGe. The semiconducting element silicon may also be used but germanium is preferred, as its higher atomic number makes it more efficient at stopping and detecting high energy gamma rays. Both Ge(Li) and HPGe detectors have excellent sensitivity and resolution, but Ge(Li) detectors are unstable at room temperature, with the lithium drifting into the intrinsic region ruining the detector. The development of undrifted high purity germanium has overcome this problem.

Particle detectors can also be used to detect the emission of alpha (α) and beta (β) particles which often accompany the emission of a gamma photon but are less favourable, as these particles are only emitted from the surface of the sample and are often absorbed or attenuated by atmospheric gases requiring expensive vacuum conditions to be effectively detected. Gamma rays, however, are not absorbed or attenuated by atmospheric gases, and can also escape from deep within the sample with minimal absorption.