Nuclear Radiation Detectors

1. Ionization Chamber:

An ionization chamber is a device used for two major purposes: detecting particles in air (as in a smoke detector), and for detection or measurement of ionizing radiation.

An ionization chamber is an instrument constructed to measure the number of ions within a medium (which we will consider to be gaseous, but can also be solid or liquid). It consists of a gas filled enclosure between two conducting electrodes. The electrodes may be in the form of parallel plates (parallel plate ionization chambers: PPIC) or coaxial cylinders to form a convenient portable detector in which case, one of the electrodes may be the wall of the vessel itself.

When gas between the electrodes is ionized by any means, such as by alpha particles, beta particles, X-rays, or other radioactive emission, the ions and dissociated electrons move to the electrodes of the opposite polarity, thus creating an ionization current which may be measured by a galvanometer or electrometer.

Each ion essentially deposits or removes a small electric charge to or from an electrode, such that the accumulated charge is proportional to the number of like-charged ions. A voltage potential that can range from a few volts to many kilovolts is applied between the electrodes, and allows the device to work continuously by mopping up electrons and preventing the device from becoming saturated. The current that originates is called a bias current, and prevents the device from reaching a point where no more ions can be collected.

Ionization chambers are widely used in the nuclear industry as they provide an output that is proportional to dose and have a greater operating lifetime than standard Geiger tubes (in Geiger tubes (cf. Geiger-Müller tube) the gas eventually breaks down). Ionization chambers are used in nuclear medicine to determine the exact activity of radioactive therapeutic treatments. Such devices are called 'radioisotope dose calibrators'. Ion chambers are sometimes microphonic as they are very sensitive devices and non ion related charges can be set up inside due to the piezoelectric effect.

2. Geiger–Müller tube

A Geiger-Müller tube (or GM tube) is the sensing element of a Geiger counter instrument that can detect a single particle of ionizing radiation, and typically produce an audible click for each. It was named for Hans Geiger who invented the device in 1908, and Walther Müller who collaborated with Geiger in developing it further in 1928. It is a type of gaseous ionization detector with an operating voltage in the Geiger plateau.The Geiger counter is sometimes used as a hardware random number generator.

Description and operation

A Geiger-Müller tube consists of a tube filled with a low-pressure (~0.1 Atm) inert gas such as helium, neon or argon, (Usually neon) in some cases in a Penning mixture, and an organic vapor or a halogen gas and contains electrodes, between which there is a potential difference of several hundred volts, but no current flowing. The walls of the tube are either metal or the inside coated with metal or graphite to form the cathode while the anode is a wire passing up the center of the tube.

When ionizing radiation passes through the tube, some of the gas molecules are ionized, creating positively charged ions, and electrons. The strong electric field created by the tube's electrodes accelerates the ions towards the cathode and the electrons towards the anode. The ion pairs gain sufficient energy to ionize further gas molecules through collisions on the way, creating an avalanche of charged particles.

This results in a short, intense pulse of current which passes (or cascades) from the negative electrode to the positive electrode and is measured or counted.

Most detectors include an audio amplifier that produce an audible click on discharge. The number of pulses per second measures the intensity of the radiation field. Some Geiger counters display an exposure rate (e.g. mR·h), but this does not relate easily to a dose rate as the instrument does not discriminate between radiation of different energies.

GM tubes

The usual form of tube is an end-window tube. This type is so-named because the tube has a window at one end through which ionizing radiation can easily penetrate. The other end normally has the electrical connectors. There are two types of end-window tubes: the glass-mantle type and the mica window type. The glass window type will not detect alpha radiation since it is unable to penetrate the glass, but is usually cheaper and will usually detect beta radiation and X-rays. The mica window type will detect alpha radiation but is more fragile.

Most tubes will detect gamma radiation, and usually beta radiation above about 2.5 MeV. Geiger-Müller tubes will not normally detect neutrons since these do not ionise the gas. However, neutron-sensitive tubes can be produced which either have the inside of the tube coated with boron or contain boron trifluoride or helium-3 gas. The neutrons interact with the boron nuclei, producing alpha particles or with the helium-3 nuclei producing hydrogen and tritium ions and electrons. These charged particles then trigger the normal avalanche process.

Quenching

The G.M. tube must produce a single pulse on entry of a single particle. It must not give any spurious pulse and recover quickly to the passive state. But unfortunately the positive Ar ions that eventually strike the cathode become neutral Ar atoms in an excited state by gaining electrons from the cathode. The excited atoms return to the ground state by emitting photons and these photons cause avalanches and hence spurious pulse discharge. Quenching is important because a single particle entering the tube is counted by a single discharge, and so it will be unable to detect another particle until the discharge has been stopped, and because the tube is damaged by prolonged discharges.

External quenching uses external electronics to remove the high voltage between the electrodes. Self-quenching or internal-quenching tubes stop the discharge without external assistance, and contain a small amount of a polyatomic organic vapor such as butane or ethanol; or alternatively a halogen such as bromine or chlorine.

If the diatomic gas(quencher) is introduced in the tube, the positive Ar ions, during their slow motion to the cathode, would have multiple collisions with the quencher gas molecules and transfer their charge and some energy to them. Thus neutral Ar atoms would reach the cathode. The quencher gas ions in their turn reach the cathode, gain electrons thereform and move into excited states. But these excited molecules lose their energy not by photon emission but by dissociation into neutral quencher molecules. No spurious pulses are thus produced.

3. Scintillation counter

A scintillation counter measures ionizing radiation. The sensor, called a scintillator, consists of a transparent crystal, usually phosphor, plastic (usually containing anthracene), or organic liquid (see liquid scintillation counting) that fluoresces when struck by ionizing radiation. A sensitive photomultiplier tube (PMT) measures the light from the crystal. The PMT is attached to an electronic amplifier and other electronic equipment to count and possibly quantify the amplitude of the signals produced by the photomultiplier.

The scintillation counter is based on the work of Antoine Henri Becquerel, who discovered the phosphorescence of certain uranium salts. Scintillation counters are widely used because they can be made inexpensively yet with good quantum efficiency. The quantum efficiency of a gamma-ray detector (per unit volume) depends upon the density of electrons in the detector, and certain scintillating materials, such as sodium iodide and bismuth germanate, achieve high electron densities as a result of the high atomic numbers of some of the elements of which they are composed. However, detectors based on semiconductors, notably hyperpure germanium, have better intrinsic energy resolution than scintillators, and are preferred where feasible for gamma-ray spectrometry. In the case of neutron detectors, high efficiency is gained through the use of scintillating materials rich in hydrogen that scatter neutrons efficiently. Liquid scintillation counters are an efficient and practical means of quantifying beta radiation.

Scintillation counter apparatus

When a charged particle strikes the scintillator, a flash of light is produced, which may or may not be in the visible region of the spectrum. Each charged particle produces a flash. If a flash is produced in a visible region, it can be observed through a microscope and counted - an impractical method. The association of a scintillator and photomultipier with the counter circuits forms the basis of the scintillation counter apparatus. When a charged particle passes through the phosphor, some of the phosphor's atoms get excited and emit photons. The intensity of the light flash depends on the energy of the charged particles. Cesium iodide (CsI) in crystalline form is used as the scintillator for the detection of protons and alpha particles; sodium iodide (NaI) containing a small amount of thallium is used as a scintillator for the detection of gamma waves.

The scintillation counter has a layer of phosphor cemented in one of the ends of the photomultiplier. Its inner surface is coated with a photo-emitter with less work potential. This photoelectric emitter is called as photocathode and is connected to the negative terminal of a high tension battery. A number of anodes called dynodes are arranged in the tube at increasing positive potential. When a charged particle strikes the phosphor, a photon is emitted. This photon strikes the photocathode in the photomultipier, releasing an electron. This electron accelerates towards the first dynode and hits it. Multiple secondary electrons are emitted, which accelerate towards the second dynode. More electrons are emitted and the chain continues, multiplying the effect of the first charged particle. By the time the electrons reach the last dynode, enough have been released to send a voltage pulse across the external resistors. This voltage pulse is amplified and recorded by the electronic counter.

Applications for Scintillation counters

The scintillation counters can be used in a variety of applications.

• Medical imaging
• National and homeland security
• Border security
• Nuclear safety

Since 9/11 several security situations have emerged where detection of radioactive material, emitting lethal gamma rays, during transportation has become very important. Several products have been introduced in the market utilising scintillation counters for detection of such materials. These include scinitllation counters designed for freight terminals, border security, ports, weigh bridge applications, scrap metal yards and contamination monitoring of nuclear waste. There are variants of scintillation counters mounted on pick-up trucks and helicopters for rapid response in case of a security situation due to dirty bombs or nuclear waste

A scintillation counter with security applications at ports, weigh bridges and scrap metal yards.

Scintillation counter as a spectrometer

Scintillators often convert a single photon of high energy radiation into high number of lower-energy photons, where the number of photons per megaelectronvolt of input energy is fairly constant. By measuring the intensity of the flash (the number of the photons produced by the x-ray or gamma photon) it is therefore possible to discern the original photon's energy.

The spectrometer consists of a suitable scintillator crystal, a photomultiplier tube, and a circuit for measuring the height of the pulses produced by the photomultiplier. The pulses are counted and sorted by their height, producing a x-y plot of scintillator flash brightness vs number of the flashes, which approximates the energy spectrum of the incident radiation, with some additional artifacts. A monochromatic gamma radiation produces a photopeak at its energy. The detector also shows response at the lower energies, caused by Compton scattering, two smaller escape peaks at energies 0.511 and 1.022 MeV below the photopeak for the creation of electron-positron pairs when one or both annihilation photons escape, and a backscatter peak. Higher energies can be measured when two or more photons strike the detector almost simultaneously (pile-up, within the time resolution of the DAQ chain), appearing as sum peaks with energies up to the value of two or more photopeaks added.


4. Semiconductor Detector

A semiconductor detector is a device that uses a semiconductor (usually Si or Ge ) to detect traversing charged particles or the absorption of photons. In the field of particle physics, these detectors are usually known as silicon detectors.

When their sensitive structures are based on a single diode, they are called semiconductor diode detectors. When they contain many diodes with different functions, the more general term semiconductor detector is used.

Semiconductor detectors have found broad application during recent decades, in particular for gamma and x-ray sprectometry and as detectors.

Semiconductor radiation detector

In these detectors, radiation is measured by means of the number of charge carriers set free in the detector, which is arranged between two electrodes. Ionizing radiation produces free electrons and holes. The number of electron-hole pairs is proportional to the energy transmitted by the radiation to the semiconductor. As a result, a number of electrons are transferred from the valence band to the conduction band, and an equal number of holes are created in the valence band. Under the influence of an electric field, electrons and holes travel to the electrodes, where they result in a pulse that can be measured in an outer circuit. The holes travel in the opposite direction and can also be measured. As the amount of energy required to create an electron-hole pair is known, and is independent of the energy of the incident radiation, measuring the number of electron-hole pairs allows the energy of the incident radiation to be found.

The energy required for production of electron-hole-pairs is very low compared to the energy required for production of paired ions in a gas detector. Consequently, in semiconductor detectors the statistical variation of the pulse height is smaller and the energy resolution is higher. As the electrons travel fast, the time resolution is also very good, and is dependent upon rise time. Compared with gaseous ionization detectors, the density of a semiconductor detector is very high, and charged particles of high energy can give off their energy in a semiconductor of relatively small dimensions.

Semiconductor particle detectors

Silicon detector

Most silicon particle detectors work, in principle, by doping narrow (usually around 100 micrometers wide) strips of silicon to make them into diodes, which are then reverse biased. As charged particles pass through these strips, they cause small ionization currents which can be detected and measured. Arranging thousands of these detectors around a collision point in a particle accelerator can give an accurate picture of what paths particles take. Silicon detectors have a much higher resolution in tracking charged particles than older technologies such as cloud chambers or wire chambers. The drawback is that silicon detectors are much more expensive than these older technologies and require sophisticated cooling to reduce leakage currents (noise source) as well as suffer degradation over time from radiation.

Diamond detector

Diamond detectors have many similarities with silicon detectors, but are expected to offer significant advantages, in particular a high radiation hardness and very low drift currents. At present they are much more expensive and more difficult to manufacture.

Germanium detector

High Purity Germanium detector (disconnected from liquid Nitrogen dewar)

Germanium detectors are mostly used for spectroscopy in nuclear physics. While silicon detectors cannot be thicker than a few millimeters, germanium can have a depleted, sensitive thickness of centimeters, and therefore can be used as a total absorption detector for gamma rays up to few MeV. These detectors are also called High-Purity Germanium detectors (HPGe) or Hyperpure Germanium detectors. Before current purification techniques were refined, Germanium crystals could not be produced with purity sufficient to enable their use as spectroscopy detectors. Impurities in the crystals trapped electrons and holes, ruining the performance of the detectors. Therefore, Germanium crystals were doped with Lithium ions (Ge(Li)), in order to produce an intrinsic region in which the electrons and holes would be able to reach the contacts and produce a signal.

When Germanium detectors were first developed, only very small crystals were available. Low efficiency was the result, and Germanium detector efficiency is still often quoted in relative terms, as discussed above. Crystal growth techniques have improved, allowing detectors to be manufactured that are as large as or larger than commonly available NaI crystals, although such detectors cost more than €100,000.

Present-day HPGe detectors commonly still use lithium diffusion to make an n⁺ ohmic contact, and boron implantation to make a p⁺ contact. Coaxial detectors with a central n⁺ contact are referred to as n-type detectors, while p-type detectors have a p⁺ central contact. The thickness of these contacts represents a dead layer around the surface of the crystal within which energy depositions do not result in detector signals. Typical dead layer thickness are several hundred micrometers for an Li diffusion layer, and a few tenths of a micrometer for a B implantation layer.

The major drawback of Germanium detectors is that they must be cooled to liquid nitrogen temperatures to produce spectroscopic data. At higher temperatures, the electrons can easily cross the Band gap in the crystal and reach the conduction band, where they are free to respond to the electric field. The system therefore produces too much electrical noise to be useful as a spectrometer. Cooling to liquid nitrogen temperatures, 77.36 K, reduces thermal excitations of valence electrons so that only a gamma ray interaction can give an electron the energy necessary to cross the band gap and reach the conduction band. Cooling with liquid nitrogen is inconvenient, as the detector requires hours to cool down to operating temperature before it can be used, and cannot be allowed to warm up during use. Ge(Li) crystals could never be allowed to warm up, as the Lithium would drift out of the crystal, ruining the detector. HPGe detectors can be allowed to warm up to room temperature when not in use. Appropriate care must be taken when working with liquid nitrogen; the major hazards are Cold burn and oxygen depletion as the liquid nitrogen boils, producing a significant volume of nitrogen gas.

Commercial systems are now available that use advanced refrigeration techniques to eliminate the need for liquid nitrogen cooling.