Radiation Sensors Chapter 9
Introduction • We have discussed radiation in Chapter 4 when talking about light sensors. • Our particular concern there was the general range occupied by the infrared, visible and ultraviolet radiation. • Here we will concern ourselves with the ranges below and above these. • Range above UV • Range below IR.
Introduction • Range above UV is characterized by ionization – • Frequency is sufficiently high to ionize molecules based on Plank’s equation. • The frequencies are so high (above 750 THz) that many forms of radiation can penetrate through materials and therefore the methods of sensing must rely on different principles than at lower frequencies. • On the other hand, below the infrared region, the electromagnetic radiation can be generated and detected by simple antennas. • We will therefore discuss the idea of an antenna and its use as a sensor.
Introduction • All radiation may be viewed as electromagnetic radiation. • Many of the sensing strategies, including those discussed in chapter 4 may be viewed as radiation sensing. • We will however follow the conventional nomenclature • Will call low frequency radiation “electromagnetic” (electromagnetic waves, electro-magnetic energy, etc.) • Will call high frequency radiation, simply “radiation” (as in X-ray, or cosmic)
Introduction • One important distinction in radiation is based on the Planck equation and uses the photon energy to distinguish between them: • h = 6.6262x10 [joule.second] is Plank’s constant • f is the frequency in Hz • e is called the photon energy.
Introduction • At high frequencies, where particles are concerned, one can view them either as particles or as waves. • The energy in these waves is given by the Planck equation. • Their wavelength is given by de Broglie’s equation (p=mv is the momentum of the particle):
Introduction • The higher the frequency the higher the photon energy. • At high frequencies, the photon energy is sufficient to strip electrons from atoms –ionizing radiation. • At low frequencies, ionization does not happen and hence these waves are called non-ionizing. • The highest frequency in the microwave region is 300 Ghz. The photon energy is 0.02 eV. This is considered non-ionizing. • The lowest frequency in the X-ray region is approximately 3x1016 and the photon energy is 2000 eV. Clearly an ionizing radiation.
Introduction • Some view radioactive radiation as something different than, say X-ray radiation or microwaves • It is often viewed as particle radiation. • One can take this approach based on the duality of electromagnetic radiation, just as we can view light as electromagnetic or as particles – photons. • We will base all our discussion on the photon energy of radiation and not on the particle aspects. • In some cases it will be convenient to talk about particles. (Geiger-Muller counter, for example)
Introduction • Many of the radiation sensors based on ionization are used to sense the radiation itself (detect and quantify radiation from sources such as X-rays and from nuclear sources (and radiation). • There are however exception such as smoke detection and measurement of material thickness through radiation. • In the lower range, the sensing of a variety of parameters through microwaves is the most important • Sensing of the microwave themselves is not
Units • Units for radiation, except for low frequency electromagnetic radiation are divided into three: • Units of activity, • Units of exposure • Units of absorbed dose. • Also - units for dose equivalent. • The basic unit of activity is the Becquerel [Bq] • Defined as one transition (disintegration) per second. • It indicates the rate of decay of a radionuclide.
Units • An older, non-SI unit of activity was the curie (1 curie=3.7x1010 becquerel). • The Becquerel is a small unit so that the [MBq], [GBq] and [TBq] are often used. • The basic unit of exposure is the coulomb per kilogram [C/kg]=[A.s/kg]. • The older unit was the roentgen (1 roentgen=2.58x10 C/kg]. • The [C/kg] is a very large unit and units of [mC/kg], C/kg] and [pC/kg] are often used.
Units • Absorbed dose is measured in grays [Gy] which is [J/kg]. • The Gray is energy per kilogram and 1[Gy]=1[J/kg]. • The old unit of absorbed dose was the rad (1 rad = 100 [Gy]). • The units for dose equivalence is the sievert [Sv] in [J/kg]. • The old unit is the rem (1 rem = 100 [Sv]). • Note that the sievert and the gray are the same. • This is because they measure identical quantities in air. • However the dose equivalent for a body (like the human body) is obtained by multiplying the absorbed dose by a quality factor to obtain the dose equivalent.
Radiation sensors • Will start the discussion with ionization sensors • Then will discuss the much lower frequency methods based on electromagnetic radiation • Three basic types of radiation sensors: • Ionization sensors • Scintillation sensors • Semiconductor radiation sensors • These sensors are either: • Detectors – detection without quantification or: • Sensor - both detection and quantification
Ionization sensors (detectors) • In an ionization sensor, the radiation passing through a medium (gas or solid) creates electron-proton pairs • Their density and energy depends on the energy of the ionizing radiation. • These charges can then be attracted to electrodes and measured or they may be accelerated through the use of magnetic fields for further use. • The simplest and oldest type of sensor is the ionization chamber.
Ionization chamber • The chamber is a gas filled chamber • Usually at low pressure • Has predictable response to radiation. • In most gases, the ionization energy for the outer electrons is fairly small – 10 to 20 eV. • A somewhat higher energy is required since some energy may be absorbed without releasing charged pairs (by moving electrons into higher energy bands within the atom). • For sensing, the important quantity is the W value. • It is an average energy transferred per ion pair generated. Table 9.1 gives the W values for a few gases used in ion chambers.
Ionization chamber • Clearly ion pairs can also recombine. • The current generated is due to an average rate of ion generation. • The principle is shown in Figure 9.1. • When no ionization occurs, there is no current as the gas has negligible resistance. • The voltage across the cell is relatively high and attracts the charges, reducing recombination. • Under these conditions, the steady state current is a good measure of the ionization rate.
Ionization chamber • The chamber operates in the saturation region of the I-V curve. • The higher the radiation frequency and the higher the voltage across the chamber electrodes the higher the current across the chamber. • The chamber in Figure 9.1. is sufficient for high energy radiation • For low energy X-rays, a better approach is needed.
Ionization chamber - applications • The most common use for ionization chambers is in smoke detectors. • The chamber is open to the air and ionization occurs in air. • A small radioactive source (usually Americum 241) ionizes the air at a constant rate • This causes a small, constant ionization current between the anode and cathode of the chamber. • Combustion products such as smoke enter the chamber
Ionization chamber - applications • Smoke particles are much larger and heavier than air • They form centers around which positive and negative charges recombine. • This reduces the ionization current and triggers an alarm. • In most smoke detectors, there are two chambers. • One is as described above. It can be triggered by humidity, dust and even by pressure differences or small insects, a second, reference chamber is provided • In it the openings to air are too small to allow the large smoke particles but will allow humidity. • The trigger is now based on the difference between these two currents.
Ionization chambers - application • Fabric density sensor (see figure). • The lower part contains a low energy radioactive isotope (Krypton 85) • The upper part is an ionization chamber. • The fabric passes between them. • The ionization current is calibrated in terms of density (i.e. weight per unit area). • Similar devices are calibrated in terms of thickness (rubber for example) or other quantities that affect the amount of radiation that passes through such as moisture
Proportional chamber • A proportional chamber is a gas ionization chamber but: • The potential across the electrodes is high enough to produce an electric field in excess of 106 V/m. • The electrons are accelerated, process collide with atoms releasing additional electrons (and protons) in a process called the Townsend avalanche. • These charges are collected by the anode and because of this multiplication effect can be used to detect lower intensity radiation.
Proportional chamber • The device is also called a proportional counter or multiplier. • If the electric field is increased further, the output becomes nonlinear due to protons which cannot move as fast as electrons causing a space charge. • Figure 9.2 shows the region of operation of the various types of gas chambers.
Geiger-Muller counters • An ionization chamber • Voltage across an ionization chamber is very high • The output is not dependent on the ionization energy but rather is a function of the electric field in the chamber. • Because of this, the GM counter can “count” single particles whereas this would be insufficient to trigger a proportional chamber. • This very high voltage can also trigger a false reading immediately after a valid reading.
Geiger-Muller counters • To prevent this, a quenching gas is added to the noble gas that fills the counter chamber. • The G-M counter is made as a tube, up to 10-15cm long and about 3cm in diameter. • A window is provided to allow penetration of radiation. • The tube is filled with argon or helium with about 5-10% alcohol (Ethyl alcohol) to quench triggering. • The operation relies heavily on the avalanche effect • UV radiation is released which, in itself adds to the avalanche process. • The output is about the same no matter what the ionization energy of the input radiation is.
Geiger-Muller counters • Because of the very high voltage, a single particle can release 109 to 1010 ion pairs. • This means that a G-M counter is essentially guaranteed to detect any radiation through it. • The efficiency of all ionization chambers depends on the type of radiation. • The cathodes also influence this efficiency • High atomic number cathodes are used for higher energy radiation ( rays) and lower atomic number cathodes to lower energy radiation.
Scintillation sensors • Takes advantage of the radiation to light conversion (scintillation) that occurs in certain materials. • The light intensity generated is then a measure of the radiation’s kinetic energy. • Some scintillation sensors are used as detectors in which the exact relationship to radiation is not critical. • In others it is important that a linear relation exists and that the light conversion be efficient.
Scintillation sensors • Materials used should exhibit fast light decay following irradiation (photoluminescence) to allow fast response of the detector. • The most common material used for this purpose is Sodium-Iodine (other of the alkali halide crystals may be used and activation materials such as thalium are added) • There are also organic materials and plastics that may be used for this purpose. Many of these have faster responses than the inorganic crystals.
Scintillation sensors • The light conversion is fairly weak because it involves inefficient processes. • Light obtained in these scintillating materials is of light intensity and requires “amplification” to be detectable. • A photomultiplier can be used as the detector mechanism as shown in Figure 9.5 to increase sensitivity. • The large gain of photomultipliers is critical in the success of these devices.
Scintillation sensors • The reading is a function of many parameters. • First, the energy of the particles and the efficiency of conversion (about 10%) defines how many photons are generated. • Part of this number, say k, reaches the cathode of the photomultiplier. • The cathode of the photomultiplier has a quantuum efficiency (about 20-25%). • This number, say k1 is now multiplied by the gain of the photomultiplier G which can be of the order of 106 to 108.
Semiconductor radiation detectors • Light radiation can be detected in semiconductors through release of charges across the band gap • Higher energy radiation can be expected do so at much higher efficiencies. • Any semiconductor light sensor will also be sensitive to higher energy radiation • In practice there are a few issues that have to be resolved.
Semiconductor radiation detectors • First, because the energy is high, the lower bandgap materials are not useful since they would produce currents that are too high. • Second, high energy radiation can easily penetrate through the semiconductor without releasing charges. • Thicker devices and heavier materials are needed. • Also, in detection of low radiation levels, the background noise, due to the “dark” current (current from thermal sources) can seriously interfere with the detector. • Because of this, some semiconducting radiation sensors can only be used at cryogenic temperatures.
Semiconductor radiation detectors • When an energetic particle penetrates into a semiconductor, it initiates a process which releases electrons (and holes) • through direct interaction with the crystal • through secondary emissions by the primary electrons • To produce a hole-electron pair energy is required: • Called ionization energy - 3-5 eV (Table 9.2). • This is only about 1/10 of the energy required to release an ion pair in gases • The basic sensitivity of semiconductor sensors is an order of magnitude higher than in gases.
Semiconductor radiation detectors • Semiconductor radiation sensors are essentially diodes in reverse bias. • This ensures a small (ideally negligible) background (dark) current. • The reverse current produced by radiation is then a measure of the kinetic energy of the radiation. • The diode must be thick to ensure absorption of the energy due to fast particles. • The most common construction is similar to the PIN diode and is shown in Figure 9.6.
Semiconductor radiation detectors • In this construction, a normal diode is built but with a much thicker intrinsic region. • This region is doped with balanced impurities so that it resembles an intrinsic material. • To accomplish that and to avoid the tendency of drift towards either an n or p behavior, an ion-drifting process is employed by diffusing a compensating material throughout the layer. • Lithium is the material of choice for this purpose.
Semiconductor radiation detectors • Additional restrictions must be imposed: • Germanium can be used at cryogenic temperatures • Silicon can be used at room temperature but: • Silicon is a light material (atomic number 14) • It is therefore very inefficient for energetic radiation such as rays. • For this purpose, cadmium telluride (CdTe) is the most often used because it combines heavy materials (atomic numbers 48 and 52) with relatively high bandgap energies.
Semiconductor radiation detectors • Other materials that can be used are the mercuric iodine (HgI2) and gallium arsenide (GaAs). • Higher atomic number materials may also be used as a simple intrinsic material detector (not a diode) because the background current is very small (see chapter 3). • The surface area of these devices can be quite large (some as high as 50mm in diameter) or very small (1mm in diameter) depending on applications. • Resistivity under dark conditions is of the order of 108 to 1010.cm depending on the construction and on doping, if any (intrinsic materials have higher resistivity). • .
Semiconductor radiation detectors - notes • The idea of avalanche can be used to increase sensitivity of semiconductor radiation detectors, especially at lower energy radiation. • These are called avalanche detectors and operate similarly to the proportional detectors • While this can increase the sensitivity by about two orders of magnitude it is important to use these only for low energies or the barrier can be easily breached and the sensor destroyed.
Semiconductor radiation detectors - notes • Semiconducting radiation sensors are the most sensitive and most versatile radiation sensors • They suffer from a number of limitations. • Damage can occur when exposed to radiation over time. • Damage can occur in the semiconductor lattice, in the package or in the metal layers and connectors. • Prolonged radiation may also increase the leakage (dark) current and result in a loss of energy resolution of the sensor. • The temperature limits of the sensor must be taken into account (unless a cooled sensor is used).
Microwave radiation sensors - introduction • Microwaves are often employed in the sensing of other quantities because of the relative ease of generating, manipulating and detecting microwave radiation. • Use in speed sensing, in sensing of the environment (radar, doppler radar, mapping of the earth and planets, etc.) are well known. • All of these applications and sensors are based on the properties – especially the propagation properties of electromagnetic waves.
Electromagnetic waves • Properties of waves were discussed in ch. 6. • Electromagnetic waves differ from acoustic waves in two fundamental ways • The electromagnetic wave is a transverse wave (acoustic waves are longitudinal) • The electromagnetic wave is the variation in space and time of the electric and magnetic field. • The electric field intensity E and the magnetic field intensity H are transverse to the direction of propagation of the wave and to each other.
Electromagnetic waves • The electric and magnetic field can propagate in matter as well as in vacuum. • A visual interpretation of how an electromagnetic wave propagate is shown in Figure 9.7. • The properties of the electromagnetic wave are significantly different than those of the acoustic wave numerically. • The most important is the speed of propagation of the wave (also called phase velocity).