Radio active measuements Marie Curie

FISICA AMBIENTALE RADIATION DETECTION AND MEASUREMENT Antonio Ballarin Denti

Radiation 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.

Radiation 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.

Radiation 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 Types of radiation sensors: Ionization sensors Scintillation sensors Semiconductor radiation sensors Solid state sensors These sensors are either: Detectors – detection without quantification or: Sensor - both detection and quantification

RADIATION DEVICES ARE BASED ON THE PHYSICAL OR CHEMICAL EFFECTS OF RADIATION IONIZATION IN GASES Mainly used as health physics monitoring instruments IONIZATION AND EXCITATION IN CERTAIN SOLIDS Certain crystalline solids exhibit increases in electrical conductivity and effects attributable to excitation as scintillation, termoluminescence and photographic effect. ACTIVATION BY NEUTRONS Used for neutron detection CHANGES IN CHEMICAL SYSTEMS ..rather insensitive

IONIZATION CHAMBER The most widely used radiation detectors are devices that respond to ionizing radiation by producing electrical pulses

IONIZATION CHAMBERS: to measure exposure rates In health physics instruments the chamber is usually filled with air and is constructed using low atomic number materials

The pulses are generated by the imparting of energy to electrons by the ionizing particles in the sensitive volume of the counter

There are 2 major modes of signal production: CURRENT COUNTING MODE: the magnitude of the output pulse is proportional to the amount of energy deposited in the detector PULSE MODE: the deposited energy serves to trigger an output pulse of constant form every time the interaction occurs.

PROPORTIONAL COUNTERS As the electric field in an ion chamber system is increased the freed e- are accelerated and achieve sufficient kinetic energy to cause additional ionizations within the detector

BASIC ELEMENTS OF A PROPORTIONAL COUNTER The outer cathode must also provide a vacuum-tight enclosure for the fill gas. The output pulse is developed across the load resistance RL.

The different regions of operation of gas filled detectors. The observed amplitude is plotted for events depositing 2 different amounts of energy within the gas

GEIGER-MÜLLER COUNTER If the applied voltage is further increased, gas amplification is so great that a single ionizing particle produces a ionization Avalanche. Each output pulse of current has the same magnitude and no longer reflects any properties of the incident radiation. Mechanism by which additional avalanches are triggered in a Geiger discharge

Geiger-Muller sensor Fig 9.3

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.

W values for gases

A nuclear fabric density sensor Fig 9.1y

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 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.

Scintillation sensor Fig 9.5

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.

Properties of semiconductors

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 sensor

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).

RADIO ISOTOPE GAMMA CAMERA