Editing Neutron activation analysis (section)

==Detectors==
[[File:NeutronActivationAnalysis-ATF-WashingtonDC-1966-DrDennisBogdan.jpg|thumb|right|200px|Gamma-Ray Scintillation Detector for Neutron Activation Analysis with ATF Forensic Laboratory Analyst in Washington, D.C. (1966)]]
There are a number of detector types and configurations used in NAA. Most are designed to detect the emitted [[gamma ray|gamma radiation]]. The most common types of gamma detectors encountered in NAA are the [[Proportional counter|gas ionisation]] type, [[Scintillation counter|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 [[Kelvin|K]] by [[liquid nitrogen]] to reduce [[dark current (physics)|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 semiconductor|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 particle|alpha]] (α) and [[Beta particle|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.