Amorphous Selenium (a-Se)

Many of the detector systems being developed in our lab use a photoconductor called amorphous Selenium (a-Se) as the X-ray detector material, i.e. the material which 'catches the X-rays', and converts them to either charge or lower energy photons. On this page we describe photoconductors and compare them with phosphor screens, and discuss why we choose to use a-Se in the detector systems we develop.

Photoconductors

Photoconductors are a subset of materials known as semiconductors. In the dark, these materials are insulators but effectively become conductors under illumination.

As light or X-ray photons are absorbed, the energy of the incoming photon excites electrons in the photoconductor to a state known as the conduction band. In the absence of an electric field, the excited electrons return to their ground state, the valence band. However, in the presence of an electric field, the electrons in the conduction band move along the electric field lines. Thus, charges released due to absorption of light or X-rays can be collected by applying a potential across a piece of photoconductive material.

The light sensitive electrical properties of photoconductors make them useful in various devices such as infrared detectors, videocameras, and photocopiers.

Photoconductors vs. Phosphor Screens

Most current and many proposed X-ray imaging detectors are based on a phosphor screen made of material such as gadolinium oxysulfide or cesium iodide. For digital imaging applications the light signal given off by the phosphor screen has to be converted to an electronic signal that can be digitized. We believe that photoconductive X-ray detectors are inherently better suited for digital imaging applications for reasons explained with the following cross-sectional views:

Phosphor Screens

Light is emitted when an X-ray is absorbed within the phosphor screen. This light scatters multiple times off the phosphor grains before it escapes the screen. The scattering causes image blur or resolution loss. Furthermore, there is a trade-off between X-ray quantum efficiency or X-ray absorption and the blurring:Thicker phosphor screens absorb X-rays better but cause more blurring.

Photoconductors

When X-rays are absorbed in a photoconductor charge carriers called electron-hole pairs are produced. (Electrons are negative charge, holes are the absence of an electron, which in effect move like a positive charge in opposite direction from electrons as neighbouring electrons move in to compensate for a missing electron at the "hole".)

An electric field can be applied across a sample of photoconductive material by for example charging the sample with a corotron, a device which deposits positive charge on the photoconductor surface. In the presence of an electric field, the released electrons and holes move to opposite surfaces. The negative electrons cancel the positive surface charge and thus produce variations in the surface charge that correspond to the incident pattern of the X-rays or an X-ray image.

The charge collection was guided by the electric field, so the produced charge pattern faithfully reproduces the X-ray image. The resulting high resolution image is not strongly dependent on the Selenium thickness.

Amorphous Selenium

A number of photoconductors (e.g. silicon, germanium, thallium bromide and most semiconductors) could be used for X-ray imaging detectors but amorphous Selenium (a-Se) has many features that make it well suited for this task.

A-Se is well developed technologically as it has been used as a photoconductor in photocopiers and also in an X-ray imaging technique known as xeroradiography for decades. It is used in its amorphous form, so amorphous selenium plates can be made by evaporation. Thus, in contrast to many crystalline photoconductors, a-Se based detectors can be made large in area relatively easily and inexpensively.

The electric properties, namely the low dark or leakage current, of a-Se also render it suitable for X-ray imaging use. Its other X-ray properties are as follows:

• ~ 1000 electron-hole pairs/50 keV X-ray at an electric field of 10V/um. In other words W +/- = 50 eV at this field strength.
• ~ 50% attenuation of a 50 keV beam with 365 um of Selenium; 50% attenuation of a 20 keV beam with a 30 um of Selenium.