Radiation measuring instruments/ radiation monitoring instruments
Why is monitoring essential?
As ionizing radiations used in most of the imaging modalities are harmful to human being, monitoring is essential to give safe radiation exposure to the people, radiation monitoring instruments are the devices used to measure the exposure rate of ionizing radiation
it is of two types
- Personal monitors
- Area monitors
Personal monitors – it measure the amount of radiation exposure the individuals have received
Area monitors – which measures the radiation level at different location in the area of radiation installation
Dosimeter measures the absorbed dose of the individual . absorbed dose is the amount of energy absorbed by the individual during the exposure. If the dosimeter absorb an amount of energy equal to the every observed by the medium, (Similar mass, example soft tissue) then it is said to be the tissue equivalent
2) Pocket dosimeter
pocket dosimeter fountain pen size and can be kept conveniently in the pocket.
Principal: Basically that dosimeter is an ionization chamber of 2 ml volume which is charged to a suitable voltage obtained from a separate charger ( 150- 200 volt).
There is an anode wire which is charged positively runs along the center of the pocket dosimeter.Attached to the anode wire is the quartz-fibre electrometer,having a reticle calibrated in terms of Roentgen.The electrometer needle deflects as a whole when it is positively charged.
The dosimeter is now ready for use and when it is exposed to radiation,ionization takes place and the liberated electrons are attracted towards the positively charged anode wire.
And thus there will be a reduction of positive charges and the needle deflects back.
The direction of movement of the fibre is directly proportional to the amount of radiation exposed.
There are two types of pocket dosimeters,
- Self-reading type pocket dosimeter
- Non self reading type pocket dosimeter
In self reading type pocket dosimeter, the chamber is coupled to a built in electrometer and microscope to view the electrometer having a reticle calibrated in terms of Roentgen.
In non self reading type dosimeter,the measurements are made with a reading device which has a built in charger.The dosimeter is initially charged and the deflection of the quartz fiber is at zero on the scale when viewed in the charger or in charger reader.The dosimeter is then ready for use.
After use, the self reading type is viewed against the light and the non self reading type is fitted back to the charger and viewed for the dose measurement.
Film dosimeters are extensively used for cumulative dose measurement in routine personnel monitoring.
The film badge is a simple plastic (with stainless steel lining) holder made to hold a conventional dental film with a number of suitable filters and an open window.
The paper wraped film is placed outside the badge.
The emulsion of a photographic film contains crystals of a silver halide embedded in a gelatin matrix when the emulsion is developed ,metallic silver is deposited in regions that were exposed to radiations.Unaffected crystals of silver halide are removed during fixation.
The amount of silver halide deposited at any location depends upon the amount of energy absorbed by the emulsion at that location.
The transmission of light through a small region of film varies inversely with the amount of deposited silver.The transmission may be measured with an optical densitometer.
Filters are selected to make the sensitivity of the film ie,type of radiation can be determined .Computation of densities under different filters gives the amount of dose recieved.
Generally ,the computation is done in conjunction with standard calibration curves having density pattern of different types of radiation at different energies.
For pure beta-the difference in densities under plastic and open window is compared with standard calibration curve for dose evaluation.
For pure X-rays , gamma rays and thermal neutrons: The doses are evaluated from the densities under copper,lead and cadmium respectively.
For X-ray energy and determination:Computation of densities under copper 1 and copper 2.
For mixed field radiation:the computation of densities under various filters can be used.
THERMOLUMINESCENCE DOSIMETERS (TLD):
A typical TLD badge based on CaSO4 dye in Tezlon matrix consists of a set of discs under different filters .Different filters are for computing different ionizing radiations with different energies .The open window region is provided for beta dose estimation.The TLD system can cover a wide range of dose from 10 mrem to 10,000 rem.
TLD’s exposed to below 10 power 4 rad,can be reused after proper annealing.
Why it is called as Thermoluminescence dosimeter?
As this dosimeter works on the principle of thermoluminescence,ie. By applying heat on the thermoluminescent material (LiF),light energy is released and the amount of light energy released is proportional to the energy absorbed in the LiF ,during irradiation ,it is called as TLD dosimeter.
The thermoluminescent material used here is LiF (LITHIUM FLOURIDE).It may be purchased as loose crystals ,a solid extruded rod,solid chips,pressed pellets or crystals embedded in a Tezlon matrix.Electrons within the crystals of LiF jump from the ‘valence bond’ to the conduction band by absorbing energy from ionizing radiation impinging upon the crystals.
Some of the electrons return immediately to the valence band, others are “trapped” in intermediate energy levels supplied by impurities in the crystals
No. of electrons trapped in intermediate level directly proportional to energy absorbed by the LiF phosphor during irradiation.
If the crystals are heated ,energy is supplied to release the trapped electrons.
Released electrons return to the conduction band ,where they fall to the valence band .
Light is released as the electrons return to the valence band.
This light is directed on to the photocathode of a photomultiplier tube because the amount of light striking the photocathode is directly proportional to the energy absorbed in the LiF during irradiation
The signal from the PMT increases with the radiation dose absorbed in the phosphor.
Advantages of TLD:
- Superior to other dosimeters
Drawbacks of TLD:
- Dose information is lost in a single read out.
- TL films are highly sensitive to disturbing agents such as light, pressure and certain chemicals.
- Pronounced fading of films at higher temparatures and humidities.
- Limited self life
- Need for complex dark room procedure
- Involves many potential sources of error
Electronic dosimeter gives a visual as well as an aural signal to alert the personnel about the exposure rate while carrying out the work.
Radiation levels are been pre-adjusted before the monitoring.When radiation is exposed on GM counter ,which is a tube filled with gases like helium,neon or argon causes ionization .And it produces an avalanche current when it reaches the preset level of radiation.And charges the capacitor to a voltage at which the neon lamp fires and gives a signal to audio stage.Preset levels can be adjusted.Radiation alarm is used extensively by radiation workers carrying out job in and around radiation installations.