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X-ray and Gamma-Ray Detectors

Introduction

X-rays and gamma-rays are high-energy electromagnetic waves with wavelengths less than 1 nm. X-rays usually originate from inner-electron transitions, and gamma-rays (which are of higher energy than X-rays) originate from nuclear decay processes. X-ray detectors are found in X-ray spectroscopy instruments and in X-ray diffractometers. Detection of gamma rays is necessary for characterization of radioactive samples and in elemental analysis by neutron activation analysis (NAA).

There are three main designs for X-ray and gamma-ray detectors: gas-filled detectors, scintillation counters, and semiconductor detectors. (These detectors can also be used to detect and quantify charged particles such as alpha and beta particles.) In all of these designs, an incoming X-ray or gamma-ray collides with atoms in the detector material to produce a photoelectrons. The photoelectrons collide within the detector to create more electrons. The number of electrons depends on the initial energy of the incident X-ray or gamma-ray. The output of the detector can therefore be analyzed based on pulse height to obtain a spectrum of the incident radiation.


Gas-Filled Detectors

Gas-filled detectors include proportional counters and Geiger counters. They consist of a metal container filled with a gas such as Ar, a window that can transmit X-rays and gamma-rays, such as Be or mylar, and a center wire that serves as an anode. A high voltage is maintained between the metal container and the anode. When high-energy rays or particles that pass into the detector collide with a gas atom, they ionize the atom to create a photoelectron. The photoelectron has a high energy and ionizes other gas atoms with which it collides. The result is a cascade of electrons that are accelerated and collected by the anode and detected as an electrical pulse.


Scintillators

A scintillator is a material that emits light when it absorbs radiation. The light pulse is then converted to an electrical pulse by a photomultiplier tube. Common scintillators are thallium-doped NaI, some plastics, anthracene and other organic solids, and liquid scintillation "cocktails," which are mixed with the sample and are often used in biochemical applications.


Semiconductor Detectors

Semiconductors also produce photoelectrons when high-energy rays or particles strike the detector material. The most common X-ray and gamma-ray detectors use lithium-drifted silicon Si(Li) or lithium-drifted germanium Ge(Li). In these detectors, Li is incorporated into the semiconductor lattice by annealing the semiconductor with Li at a high temperature (~500O). A voltage of approximately 1000 V is placed across the semiconductor material with two electrodes, and the electron cascade produced by a photoelectron is detected as an electrical pulse at the anode.

In addition to being more robust than gas-filled or scintillator detectors, these semiconductor detectors also provide a much higher resolution. Their only disadvantage is the need for cooling, usually with liquid nitrogen, to decrease the dark noise of the detector and current-to-voltage preamplifier.


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