Monday, December 3, 2012

Basic Principles of Radiation Measurement : type of detector

Radiation entering the detector will ionize the gas and produce positive ions and negative ions (electrons). The number of ions to be generated is proportional to the energy of the radiation and inversely proportional to the gas ionization power. Gas ionization power range from 25 eV s.d. 40 eV. The ions produced inside the detector will contribute to the formation of an electrical pulse or electric current.

Primary ions produced by radiation will move towards the appropriate electrodes. The movement of ions will cause pulses or electric current. The movement of these ions above can take place between two electrodes when there are enough electric field. When the electric field the higher the kinetic energy of the ions will be even greater to be able to hold another ionization.

The ions produced by the primary ion is referred to as secondary ions. If the electric field between two electrodes the higher the number of ions produced by a radiation would be very much and called the 'avalanche'.

There are three types of detector gas field working on different areas which rooms ionization detector, proportional detectors, and detector Geiger Mueller (GM).

Room Ionization detectors (Ionization chamber)

As shown in the above gas characteristic curve, the number of ions produced in this area is relatively small so that a high pulse, when applying pulse measurement models, very low. Therefore, typically, measurements using ionization detectors applying current way. When you will use this detector pulse by pulse amplifier is needed is excellent. The advantage this detector is able to distinguish between the energy entering and working voltage required is not too high.

Proportional Detector

Compared with the above ionization region, the number of ions produced in the region is more so proportionately higher pulse will be higher. The detector is more often used for measurements by the pulse.

Seen on the characteristic curves above that the number of ions produced is proportional to the energy of the radiation, so that the detector is able to differentiate radiation energy. However, that is a loss, or a high number of ion pulses produced is strongly influenced by the working voltage and power voltage for this detector must be very stable.

Detector Geiger Mueller (GM)

The number of ions produced in this area very much, reaching a value of saturated, so the pulse is relatively high and require no amplifier pulse again. The main disadvantage of this detector is unable to distinguish the radiation energy into it, because regardless of the amount of energy it produces ions with saturated values. These detectors are the most commonly used detectors, because of the electronic terms is very simple, do not need to use the amplifier. Most of the equipment measuring radiation protection, which should be portable, made of Geiger Mueller detector.


Scintillation detectors

Scintillation detectors always consists of two parts, namely the scintillator and photomultiplier materials. Scintillator material is a solid substance, liquid or gas that will produce sparks of light when subject to ionizing radiation. Photomultiplier is used to change the spark of light generated scintillator material into electrical pulses. Radiation detection mechanisms scintillation detectors can be divided into two stages:

* The process of changing the radiation detector to spark a light in the scintillator material and

* The process of changing spark of light into electrical pulses in the photomultiplier tube

Scintillator material

Scintillation process in this material can be explained by Figure 4. In the crystal scintillator materials are ribbons or area named as the valence band and the conduction band are separated by a certain energy level. In the ground state, the ground state, all electrons in the valence band while the conduction band is empty. When there is the radiation that enters the crystal, there is a possibility that some of the energy will be absorbed by the electrons in the valence band, so it can jump into the conduction band. A few moments later the electrons return to the valence band with energy band activator materials while emitting sparks of light.

Spark of light is proportional to the amount of radiation energy absorbed and influenced by the type of material sintilatornya. The bigger the more spark energy is light. Sparks of light are then 'captured' by the photomultiplier.

 Here are some examples of scintillator materials are often used as a radiation detector.

Crystal NaI (Tl)

Crystalline ZnS (Ag)

Crystal Lii (Eu)

Organic scintillator

Liquid scintillator (liquid scintillation)

The detector is very special compared to the other types of detectors for liquid. Radioactive sample to be measured first dissolved into liquid scintillator so that the sample and detector into a single unit of a homogeneous solution. In this measurement geometry can achieve 100% efficiency because all the radiation emitted by the source will be "captured" by the detector. This method is needed to measure samples b low-energy radiation such as tritium and C14.

Issues that must be considered in this method is the reduced quenching the transparent nature of the solution (liquid scintillator) as it gets mixed samples. The more concentrated the sample concentration will deteriorate the level of transparency so that the spark produced light can not reach the photomultiplier.

Photomultiplier tubes

As discussed earlier, each scintillation detector consists of two parts, namely scintillator materials and photomultiplier tubes. If the scintillator material serves to convert radiant energy into light the spark photomultiplier tube is used to change the spark of light into a beam of electrons, which can be further processed as a credit / electric current.

Photomultiplier tube is made of a hollow tube with a light-proof photokatoda which serves as input on one end and there are several such electrons to double dinode contained in Figure 5. Photokatoda attached to the scintillator material, will emit electrons when it is light with a suitable wavelength. The resulting electrons are directed, with a potential difference, towards dinode first. Dinode it will radiate some secondary electrons when the electrons are.

Secondary electrons generated will go dinode dinode first second and then multiplied to dinode third and so that electrons are collected at the last dinode amount to very much. With a collection of electrons capacitor will be converted into electrical pulses.

Semiconductor Detectors

Semiconductor materials, which were found relatively more recent than the above two types of detectors, made from group IV elements in the periodic table, namely silicon or germanium. This detector has several advantages, namely more efficient than gas field detector, because it is made from solid, and has a better resolution than scintillation detectors.

Basically, insulating materials and semiconductor materials can not forward an electrical current. This is due to all the electrons in the valence band while the conduction band is empty. The difference in energy levels between the valence band and the conduction band in the insulator material is very large so it does not allow electrons to move to the conduction band (> 5 eV) as shown above. Instead, the difference is relatively small in the semiconductor material (<3 eV) to allow electrons to jump into the conduction band where to get additional energy.

Radiant energy entering the semiconductor material is absorbed by the material so that some electrons can move from the valence band to the conduction band. When in between the two ends of the semiconductor materials are a potential difference, there will be an electric current flow. So in this detector, the radiation energy is converted into electrical energy.

The connection is made by connecting the semiconductor N-type semiconductors of the type P (PN junction). The positive pole of the external voltage is connected to the negative pole, while N-type to P type as shown in Figure 7. This causes the charge carriers are attracted to the positive (negative pole) while the negative charge carriers are attracted to the lower (positive pole), forming (depletion layer) layer charge on the connection PN empty. With the blank layer charge this then there will be no electric current. If there is ionizing radiation that enters the empty layer this charge will be formed new ions, electrons and holes, which will move to the poles of positive and negative. Additional electrons and holes is what will lead to the formation of pulses or electric current.

Because of the power or the energy required to produce these ions is lower than the ionization processes in the gas, then the number of ions produced by the same energy will be more. This is why semiconductor detectors are very meticulous in distinguishing the radiation energy about him or known to have high resolution. As an illustration, scintillation detectors for gamma radiation typically has a resolution of 50 keV, that is, the detector is able to distinguish the energy of the radiation that enters the two when both are having different radiation energies greater than 50 keV. Medium semiconductor detectors for gamma radiation typically has a resolution of 2 keV. So it looks that much more thoroughly semiconductor detectors to distinguish radiation energy.

In fact, the ability to distinguish the less energy required in use in the field, such as radiation surveys. However, for other purposes, for example to determine the type of radionuclides or to determine the type and grade of material, this capability is absolutely necessary.

The weakness of the semiconductor detector is more expensive, its use should be very careful because it is easily damaged and some types of semiconductor detectors must be cooled to the temperature of liquid nitrogen dewar necessitating large enough.


Excellence - Weakness Detector

From the discussion above shows that each of the radiation is converted into an electrical pulse with a height proportional to the energy of radiation. It is a phenomenon that is ideal because it is in fact not the case. There are several characteristics that distinguish one type of detector with other detectors are efficiency, speed and resolution.

The efficiency of the detector is a value that indicates the ratio between the number of pulses of electricity generated to the amount of radiation detector receives. Detector efficiency value is determined by the geometry and density of the detector material. The geometry will determine the amount of radiation that can be 'captured' so that the surface area of ​​the detector, the higher the efficiency. While the density of the material affects the amount of radiation detectors that can interact to produce an electrical signal. Materials that have a density detector closer will have a higher efficiency as more radiation interacts with the material.

Speed ​​detector indicates the time interval between the arrival of the radiation and the formation of an electrical pulse. Interact with the radiation detector speed also affect the measurement because if the detector response is not fast enough, while the intensity of the radiation is so high it will be a lot of radiation that are not measurable despite the detector.

Detector resolution is the ability of the detector to distinguish between adjacent radiation energy. A detector is expected to have a very small resolution (high resolution) so as to distinguish accurately the radiation energy. Resolution of the detector caused by the statistics of events that occur in the process of conversion of radiation energy, noise from electronic circuits, as well as the instability of the measurement conditions.

Another aspect to be considered is the construction of the detector because of the complicated construction or design of the detector will be more easily damaged and usually also more expensive.

The following table shows the characteristics of several types of detectors are generally based on several considerations above.

Selection of detector should consider the advantages and disadvantages as well as the specifications table above. For example, the detector used in portable measurement tool (easy to carry) is a best gas field detector, the detector used in measuring instruments for natural radiation (very low intensity) is preferably scintillation detector, while the detectors in spectroscopy systems for materials should analyze semiconductor detector .

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