THERMOLUMINESCENCE DOSIMETER AND G.M COUNTER

PRESENTER- DR. VIJAY P RATURIMODERATOR-MR.TEERTHRAJ VERMA

J.R 2, king george medical university, lucknow

- The maximum allowable dose is limited to 20 mSv per year for people working with radiation. This limit is 1 mSv for the normal population.

- The effects of irradiation on an organism may change according to the dose, type of contamination, and the features of the radiation source.

- It is crucial to perform measurements on the radiation generators used for diagnostic or therapeutic purposes.

- The measurement of radiation is called dosimetry, and the equipment used for dosimetric procedures is called a dosimeter or a detector .

- Dosimetry is the monitoring of individuals to accurately determine their radiation dose equivalent.

- When radiation interacts with the human body, there are no perceptible sensations and usually no immediate effects.

- We could therefore receive an amount of radiation that could injure our tissues severely without realizing it at the time.

- To protect ourselves and others, we must use and rely upon instruments to quantify and qualify radiation measurements.

Radiation Controlled area: An area where entry, activities, and exit are controlled to help ensure radiation protection and prevent the spread of contamination.

Radiation Restricted Area:- it includes all "Radiation Areas," "High Radiation Areas," rooms or areas in which there is radioactive materials in such quantities that "Caution: Radioactive Material" signs are required , and certain other areas which may be so defined by the "Health Physicist."

METHODS OF MEASURING ABSORDED DOSE:-

A> Calorimetry:- Basic Method Of Determining absorbed dose. - It Is Based On Principle That The Energy Absorbed In A

Medium From Radiation Appears Ultimately As Heat Energy, thus Resulting In Increase In Temp of Absorbing Medium.

- To measure such change in temp ,thermistor is used (it’s a semiconductor which show change in electrical resistance with small change in temp).

- Most of these apparatus are difficult to construct ,and are considered impractical for clinical dosimetry.

B> Chemical dosimetry:- Energy absorbed from ionizing radiation may produce chemical changes,& if this change can be determined , it can be used as a measure of absorbed dose.

- Ferrous sulphate or fricke’s dosimeter is considered to be most developed system for precision measurement if absorbed dose.

- Dosimeter consist of 1mmol/l of fes04 ,1 mmol/l of nacl & 0.4 mol/l h2so4

- When solution is irradiated fe2+ --- fe3+ ,fe3+ conc is determined by spectrophotometry of dosimeter solution.

C> Solid state methods:-

Integrating type dosimeter

(a) thermoluminescent crystals

(b)radiophotoluminescent glass

(c)films

Electrical conductivitydosimeter

(a)Semiconductor junction detector

- Of this most widely used for measurement of absorbed dose are TLD , diodes and films

- Thermoluminescence:- when a crystal is irradiated ,very minute fraction of absorbed energy is stored in crystal .some of this energy can be recovered later as visible light if material is heated.- Two categories

- Fluorescence - emission of light during or immediately after irradiation( time < 10 power -8 sec)

- Not a useful reaction for TLD use.

- Phosphorescence - emission of light after the irradiation period. Delay can be seconds to months. 

- TLDs use phosphorescence to detect radiation. 

THERMOLUMINESCENE DOSIMTETRY

Theory of thermoluminescent dosimetry:-

- In crystal lattice ,electronic energy level by mutual interaction between atoms give rise to energy bands.

- Impurities in crystals create energy traps , providing metastable states for the electrons.

- When the material is irradiated , some of e- in valence

band(ground state)receive sufficient energy to be raised to the conduction band.

- The vacancy thus created is called positive hole.

- The e- and the hole moves independently ,until they fall fall into a trap(metastable state).

(forbidden band)

BAND THEORY OF SOLIDS

BOHR’S ATOM LEVELS

- Emission of light during these transition is called fluorescence.

- If e- in the trap requires energy to get out of the trap and fall to the valence band, emission of light is called phosphorescence(delayed fluorescence).this process can be speeded up with moderate amount of heating ,the phenomenon is called as thermoluminescence.

Glow Curves- Plot of thermoluminescence against temp is called

glow curve.

- As the temp of TL material exposed to radiation is increased the probability of releasing e- increases.

- Area under this curve is directly proportional to the amount of radiation that was absorbed in the chip.

-The individual glow peaks are numbered and correspond to different trap depths.

- The light emitted (TL) first increases, reaches a maximum value and falls again to zero.

- Most phospor contain a number of traps at various energy levels in the forbidden band.

- TLD must be calibrated before it can be used for measuring an unknown dose, because response of tld is affected by their previous radiation and thermal history.

- The material must be annealed to remove residual effect.

- Standard preirradiation annealing procedure for liF is 1 hour of heating at 400c and then 24hour at 80c

- Slow heating removes peak 1 and 2 of glow curve, making it more stable.

- Tld is available in many forms and sizes, hence can be used for measuring dose in build up region ,around brachytherapy sources, and for personnel dose monitoring.

ANNEALING

- Annealing is used to determine trap of interest.

- Low temperature traps fades away with time at room temperature.

- Basically just want high temperature traps to remain.

- 400 d celsius for 1 hour reset trap structure and eliminates any electron in residual trap.

- 80 d celsius for 24 hour eliminates the trap that result in peak 2 to stabilize glow curve.

TLD READER CONSTRUCTION- Irradiated material is placed in heater cup or

planchet

- Emitted light is measured by photomultiplier tube converts light into electrical energy. current is then amplified and measure by recorder

- TL dosimeters most commonly used in medical applications are LiF:Mg,Ti, LiF:Mg,Cu,P because of their tissue equivalence.

- Other TLDs, used because of their high sensitivity, are CaSO4:Dy, Al2O3:C and CaF2:Mn.

- TLDs are available in various forms (e.g., powder, chips, rods, ribbon, etc.).

- Their range of measurement spans anywhere from <1 mrem up to as much as 100000 rem.

- It should have a single glow curve no interfering glow curve- High TL sensitivity i.e. more light output per unit of dose- Emissive spectra in visible, preferably in the range 400-500 nm- Negligible thermal fading (loss of TL signal due to ambient conditions like temperature etc.)- Glow peak preferably around 200 degree celsius- Should be tissue equivalent- Should be cheap, easy to manufacture and simple annealing procedure - Linearity between dose and light output over wide range

PROPERTIES OF TL MATERIAL

- In India CaSO4:Dy(1:3) embedded Teflon TLD disk are used for personnel monitoring. Characteristics of CaSO4:Dy- Effective Atomic No- 15.- Main Peak -200 degree celsius- Emission Maximum480-570 nm- Fading- less than 5 % per month(at 25 degree Celsius) - Self fading - 0.01 mSv/month- Linearity in Response :linear upto 30 Gy- Colour of emitted light :yellowish -white

TLD BADGE

PERSONNEL NUMBERNAME

RADIATION TYPE

PERIOD OF USE

TRANSPARENT PLASTIC

SEALED POLYTHENE POUCH

METALLIC FILTER

CROCODILE CLIP

Three dosimeter discs clipped on the card

Dosimeter Disc1(Dl)

Regions of TLD CassetteFront: Cu (1.0mm thick) 3.5 cm x 1.5 cm +Al (1.0 mm thick) disc of dia 15.6 mm.

Back: Cu (1.0mm thick) disc of dia 15.6mm +Al (1.0 mm thick) disc of dia 15.6 mm.

Dosimeter Disc2(D2)

: Front: Perspex 180mg.cm-2

Back: Same as front

Dosimeter Disc3(D3)

: Open: 12mg.cm-2 (identification paper + polythene seal)

- A TLD badge based on CaSO4: Dy Teflon discs has been designed and is in regular Personnel Monitoring use since 1975.

- The TLD badge has shown satisfactory performance for monitoring beta and gamma doses of radiation workers.

- At present about 40,000 radiation workers are covered with TLD monitoring service in our nuclear industry, medical and industrial institutions as well as research institutions.

- The complete Personnel Monitoring TLD badge consists of a TLD card and a plastic cassette for holding the TLD card. The badge is affixed to the clothing of a person with the help of a crocodile clip attached to the badge.

-Adults, minors, and declared pregnant women who are likely to exceed of 10% of the below values from external sources of radiation must be monitored. -Also, monitors must be worn by anyone who enters a high radiation area.

- Measurement of output from Co-60 units and accelerators used in medicine and industry.

- Area survey of medical (diagnostic and therapeutic) and industrial radiographic installations.

- Measurement of stray and leakage radiation around X-ray tubes and source containers.

- Monitoring of high levels of contamination from beta sources.

- Estimation of activities of various radionuclides used in brachytherapy and nuclear medicine.

- To measure dose rates in rectum and bladder of patient undergoing treatment with Cobalt on Cesium implants for carcinoma of uterine cervix.

- Personnel Monitoring.

APPLICATION OF TLD

- One badge should be worn at chest level.- Should be worn below the lead apron if used.- If selectively high doses are expected to hands and head- additional wrist and head badges may be used.

POSITIONING OF THE BADGE

INSTRUCTION FOR USERS OF TLD BADGESDO’s

• Load the card properly in the TLD cassette, name and personnel NO. should be in the front, visible from outside

• Use TLD card of the valid service period.• Handle the TLD badge with care• Store the badge in radiation free place when not in use

by you.• Report any unusual radiation incident to your RSO

Don’t

- Don’t share your TLD badge with someone else, your badge is your own.

- Once loaded don’t open the badge till the end of the service period.

- Don’t pierce or open the sealed polythene pouch of the TLD card

- Don’t use a damaged or broken cassette of which filters have come out ask for replacement.

- Don’t leave the badge in radiation area, in washing machines or near the vicinity of hot plates or furnaces.

INSTRUCTIONS FOR USERS OF TLD BADGES

ADVANTAGES (AS COMPARED TO FILM DOSIMETER BADGES) INCLUDES:- Able to measure a greater range of

doses.

- Small in size - point dose measurements possible.

- Available in various forms.

- Some are reasonably tissue equivalent .

- Not expensive.

- Reusable.

- Lack of uniformity – batch calibration needed

- Storage instability

- Fading

- Light sensitivity

- Spurious TL (cracking, contamination)

- No permanent record

TLD DISADVANTAGES

- A Geiger–Müller counter, also called a Geiger counter, is a type of particle detector that measures ionizing radiation.

- It detects the emission of nuclear radiation alpha particles, beta particles, and gamma rays by the ionization produced in a low-pressure gas in a Geiger–Müller tube.

- In wide and prominent use as a hand-held radiation survey instrument, it is perhaps one of the world's best-known radiation instruments.

GEIGER –MULLER COUNTER

- A GM counter consists of a tube filled with “Q-gas” (98% helium and 1.3% butane)

- As in an ion chamber, the detector records every interaction instead of measuring the average current that occurs after several reactions. In other words, one ionizing event will produce a pulse or a count in the GM tube.

- GM counter does not differentiate between types of radiation or their energies. For this reason, most GM counters are calibrated to give counts per minute (CPM).

G-M counters being used as gamma survey monitors, seeking radioactive satellite debris

- GM counters are usually used to simply detect the presence of radioactive material.

- GM counters are used to detect low-energy X and gamma rays.

G.M. COUNTER

APPLICATION OF GM COUNTER:-

- PARTICLE DETECTION

- GAMMA RAY AND XRAY DETECTION

- NEUTRON DETECTION

- GAMMA MEASUREMENT – PERSONNEL PROTECTION( AREA GAMMA ALARM) AND PROCESS CONTROL.

Geiger tube filled with BF3 for detection of thermal neutrons

CaSO4:Dy and LiF:Mg, Cu, P thermoluminescent dosimeters for environmental monitoring in ambient areas of a nuclear power plant.Zeng XS1, Zeng JX, Tan GX, Mai WJ.Author informationAbstractThis paper describes CaSO4:Dy and LiF:Mg, Cu, P which were used for ambient environmental monitoring before the nuclear power plant operation in Guangdong Daya Bay, China, in 1991. Since LiF:Mg, Cu, P was first used as an environmental dosimeter in this laboratory, the intercomparison of both thermoluminescent dosimeters, including laboratory irradiation and environmental exposure in Beijing reference spots, was conducted in cooperation with National Institute of Metrology and Laboratory of Industrial Hygiene, measured values of both thermoluminescent dosimeters were in agreement with the error being less than +or- 2% for the laboratory irradiation. The results of measurement by both thermoluminescent dosimeters were quite in agreement with environmental reference exposure rates measured by a pressurized ionization chamber. The largest error of CaSO4:Dy environmental monitoring results in Daya Bay also showed that the differences of measurement results between two thermoluminescent dosimeters were not significant. The experiment results indicated that LiF:Mg, Cu, P was a good environmental dosimeter.PMID: 8609029 [PubMed - indexed for MEDLINE]

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