At low applied voltages some of the ion pairs recombine to form neutral atoms. The process of recombination decreases as the applied voltage is increased. Hence, the number of ion pairs collected increases initially with applied bias. (Region-1)

With further increase in applied voltage, all the ion-pairs are collected and no re-combination of ion-pair takes place. In a limited voltage range, the number of ion-pairs produced and collected remains independent of applied voltage but dependent on the energy deposited in the detector volume by the radiation. The detectors operating in this region are called Ionization chambers. (Region-2)

The negative ions (usually electrons) in their path towards the central electrode are acce1erated because of higher electrical field. In the vicinity of the central electrode the electrons gain sufficient energy to produce secondary ionization when they interact with gas molecules. These results in an increase in the number of ion pairs collected which is proportional to the energy dissipated by the incident particle inside the detector. The constant of proportionality (i.e. amplification) increases with increase in applied voltage and may be as high as 10⁴. In this region (at a given voltage) the size of pulses produced by a particle is proportional to the number of primary ions generated by the radiation which in turn depends on the energy of the particle. Hence, by operating the detector in this region one can distinguish radiations of different energies and of different types. The detectors operating in this region are called proportional counters. (Region-3)

In this region the amplification is not constant because the space charge alters the shape of electric field. This effect marks the onset of the region of limited proportionality where the pulse height does not increase in a linear fashion with the energy deposited. This region is not used for radiation detection. (Region-4).

As the voltage is increased further the sensitive region spreads over entire length of the chamber. There is no difference in the pulse height for particles of different energies and types. But, since the number of ion pairs produced are much more, the pulse height is greater compared to that in ionization chamber or proportional counter. The amplification factor may be as high as 10⁸ and the detectors operating in this region are very sensitive. The detectors operating in this region are called GM counters. (Region-5)

At further high voltages, the positive ions bombard the cathode. As a result, ultra-violet radiation and electrons are emitted from the cathode surface, which causes further ionization. At this stage, a continuous discharge occurs and sparking takes place. This region is not suitable for radiation detection. (Region-6)

IONISATION, PROPORTIONAL AND GM regions are commonly used regions of radiation detectors. The detectors used in nucleonic gauges are one of the above detectors.

The ionization chamber is a very versatile device. It can be designed in many shapes and sizes and with different filling gases. The electrode material used in the ionization chamber, the type of gas filled and its pressure and the size of the ionization chamber etc., depend on the radiation intensity to be measured. The gas used in ionization chamber for radiation monitoring is air, filled generally at atmospheric pressure. The wall material have radiation absorption and scattering characteristics identical to that of air i.e. the effective atomic number of the wall material should be close to that of air ( Z= 7.64). An instrument with such an ionization chamber will give energy independent response. Several materials such as graphite, bakelite, and teflon satisfy this requirement of air equivalence in atomic number.

The GM detector is the most widely used detector for detection of nuclear radiations. The great utility of the GM counter is a result of its many characteristics, such as wide variety of shapes, high efficiency, large size output signal, insensitivity to environmental condition, etc.

In this detector, any particle that produces ionization in the gas will produce a discharge, i.e. an avalanche of ion-pairs, even if the primary ionization consists of one ion-pair. The resulting pulse size is independent of primary ionization. As a result, the detector has high sensitivity. However, it cannot be used for the determination of radiation energy.

After an avalanche in the counter, the electrons, because of their large mobility and proximity to the collecting electrode, are immediately collected by the central anode, whereas the heavier positively charged ions in the same duration would have hardly moved. Because of this slow moving positive ion sheath, the electric field near the central wire is lowered and no further pulse formation is possible until the positive ion sheath reaches a certain critical distance. This period is called dead time. This is followed by a recovery time during which all the positive ions reach the cathode and the field near the wire is restored to its original value. During this period, the pulse may not be of sufficient height to actuate the counting equipment. Hence, on account of finite dead-time and recovery time of the order of hundreds of microseconds, all the pulses are not recorded. Hence loss of counts occurs at high count-rates and thus the GM counter fails to register in very high radiation fields.

The probability of a beta particle generating at least one ion-pair along its path through the sensitive volume of a counter is high. Hence, the efficiency of GM counter for beta particles is nearly 100%. In case of gamma radiation, the probability of secondary electron production in the counter gas is very small. All the ionisation produced in a counter by gamma radiation is, substantially, the result of the secondary electrons ejected due to the interaction of gamma radiation with the wall or widow material. Therefore, the counter efficiency for gamma radiation is of the order of 1%.

Proportional counter are gas filled chambers operated in the proportional region, wherein the electrons released in the initial ionization are multiplied as a result of repeated collisions with gas molecules and production of further ion-pairs. The number of electrons collected at the anode and the resultant pulse height are, therefore, proportional to the number of ion-pairs in the initial ionization or in other words proportional to the radiation energy dissipated in the detector medium. This characteristics facilitate the use of proportional counters for identifying different types of radiation and also to study the energy spectrum by the pulse height analysis. Secondly, proportional counters have smaller resolving time compared to GM counter (detector), say, a micro-seconds and hence are capable of responding to high radiation intensities and high exposure rates.

All the detectors described so far register the ionization produced by radiation in a gas. The scintillation detector works on quite a different principle: it measures radiation by detecting tiny flashes of light which radiation produces in certain materials. These light flashes, called scintillation, are converted to electrical pulses and, when fed into suitable electronics, can discriminate between different types of radiation and even between different energies of the same radiation.

There are several types off scintillation counters, but their detector systems always consist of two components which are optically coupled. The first is a scintillator. This is a solid or liquid which emits light pulses when radiation deposits energy in it. This is called the scintillation 'phosphor'. The second component is a photomultiplier tube (PMT) which converts this light pulse into a pulse of electric current.

There are scintillation detectors for alpha, beta, gamma and neutron radiation. Scintillators are made of plastic, organics, or inorganic materials. They can be solid, liquid, and gas. They can be made in all shapes and sizes. Scintillation detectors can be used with portable survey meters or fixed equipment. Incoming radiation interacts with a scintillating material and a portion of/or the total energy is transferred to the scintillating material. The excited scintillating molecules produce light photons during the de-excitation process. A NaI(Tl) detector is commonly used for gamma scintillation detection and gamma analysis. Due to the high sensitivity, NaI(Tl) detectors give high background radiation levels. The detector is shielded to reduce background radiation level before use.

When certain solids are exposed to ionizing radiations, the electrons released in the ionization process are trapped at lattice imperfections in crystalline solids. These electrons remain trapped till they are released by thermal agitation at some elevated temperature. They emit light in the process and the quantity of light emitted as the material is heated may be measured and related to the absorbed dose in the material. The material thus exposed and heated can be reused as such or after an appropriate heating called annealing.

Many thermo-luminescent materials like LiF, Al₂O₃, CaF₂, CaSO₄:Dy are being used to measure the radiation. Of these CaSO₄ : Dy has been found to be very useful for dosimetric purposes, due to its high sensitivity, low fading, indigenous production and many other useful characteristics. These dosimeters are used as personnel monitoring device.

The reaction mainly used is (n,alpha) reaction (i.e. neutrons on interacting with the medium produce alpha particles) e.g. ¹⁰B(n,alpha) ⁷Li. Boron-10 has a high cross section for thermal neutrons for this reaction. The alpha particles released are detected by suitable system. Boron tri-fluoride (BF₃) gas filled or boron coated proportional counters are the most commonly used thermal neutron detectors. Similarly, helium-3 gas, in a proportional counter, also helps in the detection of thermal neutrons by the reaction ³He(n,p) ³H. In personnel monitoring film batch, a cadmium filter is used for the determination of thermal neutron dose. Cadmium filter gives prompt gamma radiation on absorption of thermal neutrons [¹¹³Cd (n,gamma) ¹¹⁴Cd] and the gamma radiation produce blackening in the film.

Fast neutrons are first thermalised (i.e. kinetic energy of fast moving neutrons is reduced to electron volts) using a suitable moderator such as paraffin wax and the thermal neutrons are detected by a BF₃proportional counter.

When fast neutrons collide with hydrogen nuclei in a polythene sheet or similar material, the hydrogen nuclei (protons) acquire a part of the neutron energy and recoil. This recoil proton can be counted using a proportional or a scintillation counter. Scintillators, consisting of ZnS and hydrogenous material like plastic are used as fast neutron detectors.

Elements like sulphur-32, can become radioactive on irradiation with fast neutrons by (n,p) reaction. The radioactivity in resultant ³²P can be measured by suitable detectors and correlated with the incident fast neutrons flux.

Personnel monitoring is the evaluation of radiation doses received by the personnel working with radiation sources.

Fig 3: Personnel Monitoring Badges

Till 1990, the most commonly used personnel monitoring device in our country was the film badge which consists of a photographic film kept in a cassette containing a set of filters [Fig. 3 A]. Using film badges doses from 0.1 mSv to 10 Sv (10 mrem to 1000 rem) of different types of radiation (beta, gamma, x-rays and thermal neutrons) can be evaluated. The film serves as a permanent record and if worn on chest gives the most representative value of radiation dose received by the whole body dose under normal working conditions. Films can be used to assess the radiation dose received from exposure to beta rays, x-rays, gamma rays or thermal neutrons. Thermal neutrons can not affect the film directly. They are detected through emission of gamma radiations by cadmium on absorption of thermal neutrons. For fast neutron monitoring a special NTA film is used. This film records tracks of recoil protons formed due to interaction of fast neutrons with hydrogen atoms. The recoil proton tracks are counted and correlated with fast neutron dose.

Now a days, the radiation dose received by workers is determined by the use of thermo-luminescent dosimeters (TLD) badges. This consists of three CaSO4 :Dy TLD discs embedded in a metallic framework and enclosed in a multi-filter cassette (Fig 3 B). The TLD badge can be used to monitor beta, gamma and x-radiations. The TLD badge can cover a wide range of doses from 0.1 mSv to 10 Sv (10 mrem to 1000 rem).

Radiation dose to personnel can also be assessed by wearing pocket dosimeters. Pocket dosimeters are very useful in certain operations, where the radiation levels vary considerably and may be quite hazardous. They give on the spot information of radiation doses, as the dose recorded can be read directly by the person. Pocket dosimeters are available for determination of exposures up to 2 mGy, 50 mGy and 100 mGy etc., (200 mR, 5 R, 10 R) [Fig. 4]. The pocket dosimeters have a built-in capacitance which can be charged by an external potential and when exposed to radiation the ionization produced in the chamber decreases the voltage across the capacitance. The reduction in the voltage across the capacitance is a measure of the amount of ionization and hence the quantity of radiation exposure. In the self-reading type pocket dosimeters, a fiber electrometer an eye piece raticule is incorporated in the ionization chamber capacitance unit. These can be charged by an external voltage supply and the discharge can be read from the eye piece graticule by looking through it against some light illumination (Fig. 4).

Fig. 4: Pocket Dosimeter

The assessment of radiation levels at different locations in the vicinity of radiation source (Nucleonic Gauges) is generally known as area monitoring. On the basis of the measurements taken, one would be able to determine the adequacy/inadequacy of the existing radiation protection measures.

Normally area monitoring systems should be able to determine radiation levels in the range of 2 µGy/h to 50 mGy/h (0.2 mR/h to 5 R/h) and also have audio indication. Lower range in the instrument is useful in assessing radiation level at occupied areas and for general radiation survey around installations.

The most commonly used area survey monitor has either a miniature GM counter, or ionization chamber or proportional counter, making it useful over a wide range of exposure-rates (Table-1).

Table 1: Area Monitoring Devices

Gamma radiation monitors are periodically tested for its performance by exposing the detector to a known gamma radiation flux. The most commonly method is to place the centre of the detector at a given distance from a cobalt-60 source of known activity and adjust the monitor reading to the calculated radiation level at that distance. While carrying out the performance tests, the detector and the source are supported in air at least 1.5 meters from the floor and walls to reduce scattering effects.

Fast neutron monitors are tested in a manner similar to the gamma radiation monitor, except that cobalt-60 source. A fast neutron source, like polonium/beryllium, is used for test. Thermal neutron monitors are calibrated in a known thermal neutron field. In this case, to obtain thermal neutron field, a fast neutron source is kept in a thermalised medium, like water or paraffin wax and the thermal neutron flux at a required distance is determined by means of activation foils which have been calibrated in known thermal neutron flux.

Radiation monitors should be kept in good working condition and maintain to its performance. They should be periodically checked to confirm that they indicate reliable readings with radiation sources. They should also be checked after any servicing or repair. The simplest method of checking the performance of monitoring instruments is to use the instrument just after it has been calibrated by the manufacturer and record for future reference the exposure-rate at a specific distance from a source of known activity. Performance check can then be made at any time by comparison of the recorded reading with check readings made at the same distance from the source, after making necessary correction for radioactive decay of source. If the check reading after corrections varies considerably the instrument should be sent for servicing and re-calibration. In addition, the operational and handling instruction should be conspicuously observed to ensure prolonged and trouble-free performance of the instrument.

The radiation measuring and monitoring instruments should be re-calibrated. This should be done in authorized laboratories and a certificate of calibration should be obtained. The instruments received after re-calibration should be checked for performance tests mentioned above. The readings so received should be recorded and kept for reference in future.