PROJECT ON IONIZATION DOSIMETER

8/7/2019 PROJECT ON IONIZATION DOSIMETER

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A

PROJECT

ON

IONIZATION

DOSIMETERPRITISH KOHLI –U09ME700

2nd

YEAR MECHANICAL ENGINEERING

SARDAR VALLABHAI NATIONAL INSTITUTE OF TECHNOLOGY, SURAT



CONTENTS

INTRODUCTION

IONIZATION CHAMBER DOSIMETRY SYSTEMS

GEIGER COUNTER

GEIGER MULLER TUBE

EXAMPLES OF DOSIMETER

UNITS OF RADIATION

RADIATION SAFETY LIMITS

OTHER RADIATION DETECTION TECHNIQUES



1. INTRODUCTION

Dosimeters are electronic devices that are used to evaluate the degree of exposure that an individual experiences when

working in a potentially hazardous setting. Devices of this type are utilized in places where people deal with hazardous

waste or radioactive substances, or where there is regular exposure to high levels of sound. The function of the

dosimeter is to measure the rate of exposure and make sure that no one is subjected to what is considered an unsafe

level.

The most common type of instrument is a gas filled radiation detector (IONIZATION CHAMBER). This instrument

works on the principle that as radiation passes through air or a specific gas, ionization of the molecules in the air

occurs. When a high voltage is placed between two areas of the gas filled space, the positive ions will be attracted to

the negative side of the detector (the cathode) and the free electrons will travel to the positive side (the anode).

These charges are collected by the anode and cathode which then form a very small current in the wires going to the

detector

Ionizing radiation, such as X-rays, alpha rays, beta rays, and gamma rays, remains undetectable by the senses, and the

damage it causes to the body is cumulative, related to the total dose received. Therefore, workers who are exposed to

radiation, such as radiographers, nuclear power plant workers, doctors using radiotherapy, workers in laboratories

using radio nuclides, and some HAZMAT teams are required to wear dosimeters so their employers can keep a record of

their exposure, to verify that it is below legally prescribed limits.



2. IONIZATION CHAMBER DOSIMETRY SYSTEMS

1. CHAMBERS AND ELECTROMETERS

Ionization chambers are used in radiotherapy and in diagnostic radiology for the determination of radiation dose.

Ionization chambers come in various shapes and sizes, depending upon the specific requirements, but generally they all

have the following properties:

● An ionization chamber is basically a gas filled cavity surrounded by a conductive outer wall and having a central

collecting electrode (see Fig). The wall and the collecting electrode are separated with a high quality insulator to reduce

the leakage current when a polarizing voltage is applied to the chamber.

● A guard electrode is usually provided in the chamber to further reduce chamber leakage. The guard electrode

intercepts the leakage current and allows it to flow to ground, bypassing the collecting electrode. It also ensures

improved field uniformity in the active or sensitive volume of the chamber, with resulting advantages in charge

collection.

● Measurements with open air ionization chambers require temperature and pressure correction to account for the

change in the mass of air in the chamber volume, which changes with the ambient temperature and pressure.

2. CYLINDRICAL (THIMBLE TYPE) IONIZATION CHAMBERS

3. PARALLEL PLATE TYPE IONIZATION CHAMBERS 4. BRACH THERAPY CHAMBERS

5. EXTRAPOLATION CHAMBERS

Extrapolation chambers are parallel-plate chambers with a variable sensitive volume.



3. GEIGER COUNTER

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

They are notable for being used to detect if objects emit nuclear radiation.

Description and Operation

Geiger counters are used to detect ionizing radiation (usually beta particles and gamma rays, but certain models can

detect alpha particles). An inert gas-filled tube (usually helium, neon or argon with halogens added) briefly conducts

electricity when a particle or photon of radiation makes the gas conductive. The tube amplifies this conduction by a

cascade effect and outputs a current pulse, which is then often displayed by a needle or lamp and/or audible clicks.

Modern instruments can report radioactivity over several orders of magnitude. Some Geiger counters can be used to

detect gamma radiation, though sensitivity can be lower for high energy gamma radiation than with certain other types

of detectors, because the density of the gas in the device is usually high, allowing most high energy gamma photons to

pass through undetected (lower energy photons are easier to detect, and are better absorbed by the detector. Examples

of this are the X-ray Pancake Geiger Tube).

Schematic of a Geiger Counter A deflection needle type Geiger Counter



4. GEIGER-MULLER TUBE

A Geiger–Muller tube (or GM tube) is the sensing element of a Geiger counter instrument that can detect a single

particle of ionizing radiation, and typically produce an audible click for each. It was named for Hans Geiger who invented

the device in 1908,[1] and Walther Muller who collaborated with Geiger in developing it further in 1928.[2] It is a type of

gaseous ionization detector with an operating voltage in the Geiger plateau.

The Geiger counter is sometimes used as a hardware random number generator.

The usual form of GM tube is an end-window tube. This type is so-named because the tube has a window at one end

through which ionizing radiation can easily penetrate. The other end normally has the electrical connectors. There are

two types of end-window tubes: the glass-mantle type and the mica window type. The glass window type will not detect

alpha radiation since it is unable to penetrate the glass, but is usually cheaper and will usually detect beta radiation and

X-rays. The mica window type will detect alpha radiation but is more fragile.

Most tubes will detect gamma radiation, and usually beta radiation above about 2.5 MeV. Geiger –Müller tubes will not

normally detect neutrons since these do not ionise the gas. However, neutron-sensitive tubes can be produced which

either have the inside of the tube coated with boron or contain boron trifluoride or helium-3 gas. The neutrons interact

with the boron nuclei, producing alpha particles or with the helium-3 nuclei producing hydrogen and tritium ions and

electrons. These charged particles then trigger the normal avalanche process.

Although most tubes will detect gamma radiation, standard tubes are relatively inefficient, as most gamma photons will

pass through the low density gas without interacting. Using the heavier noble gases krypton or xenon for the fill effects a

small improvement, but dedicated gamma detectors use dense cathodes of lead or stainless steel in windowless tubes.

The dense cathode then interacts with the gamma flux, producing high-energy electrons, which are then detected.

Description and Operation

A Geiger–Muller tube consists of a tube filled with a low-pressure (~0.1 Atm) inert gas such as helium, neon or argon

(usually neon), in some cases in a Penning mixture, and an organic vapor or a halogen gas. The tube contains electrodes,

between which there is a potential difference of several hundred volts, but no current flowing. The walls of the tube are

either entirely metal or have their inside surface coated with a conductor to form the cathode while the anode is a wire

passing up the center of the tube.

When ionizing radiation passes through the tube, some of the gas molecules are ionized, creating positively charged

ions, and electrons. The strong electric field created by the tube's electrodes accelerates the ions towards the cathode

and the electrons towards the anode. The ion pairs gain sufficient energy to ionize further gas molecules through

collisions on the way, creating an avalanche of charged particles.

This results in a short, intense pulse of current which passes (or cascades) from the negative electrode to the positive

electrode and is measured or counted.

Most detectors include an audio amplifier that produces an audible click on discharge. The number of pulses per second

measures the intensity of the radiation field. Some Geiger counters display an exposure rate (e.g. mR·h), but this does

not relate easily to a dose rate as the instrument does not discriminate between radiation of different energies.



Quenching

The ideal GM tube must produce a single pulse on entry of a single particle. It must not give any spurious pulses, and

must recover quickly to the passive state. Unfortunately for these requirements, the positive argon ions that eventually

strike the cathode become neutral argon atoms in an excited state by gaining electrons from the cathode. The excited

atoms return to the ground state by emitting photons and these photons cause avalanches and hence spurious pulse

discharge. Quenching of this process is thus important because a single particle entering the tube is counted by a single

discharge, and so the tube is unable to re-set and detect another particle until the discharge has been stopped. Also, the

tube is damaged by prolonged discharges.

External quenching uses external electronics to remove the high voltage between the electrodes. Self-quenching or

internal-quenching tubes stop the discharge without external assistance, by the addition of a small amount of a

polyatomic organic vapor such as butane or ethanol; or alternatively a halogen such as bromine or chlorine.

If a poor diatomic gas quencher is introduced to the tube, the positive argon ions, during their motion toward the

cathode, would have multiple collisions with the quencher gas molecules and transfer their charge and some energy to

them. Thus, neutral argon atoms would be produced and the quencher gas ions in their turn would reach the cathode,

gain electrons therefrom, and move into excited states which would decay by photon emission, producing tube

discharge. However, effective quencher molecules, when excited, lose their energy not by photon emission, but bydissociation into neutral quencher atoms. No spurious pulses are thus produced.

Halogen Tubes

The halogen GM tubes were invented by Sidney H. Liebson in 1947,[3] and are now the most common form, since the

discharge mechanism takes advantage of the metastable state of the inert gas atom to ionize the halogen molecule and

produces a more efficient discharge, which permits it to operate at much lower voltages, typically 400–600 volts instead

of 900–1200 volts. It also has a longer life because the halogen ions can recombine whilst the organic vapor cannot and

is gradually destroyed by the discharge process (giving the latter a life of around 108 events).



5. EXAMPLES OF DOSIMETER

BEAM THERAPY DOSIMETER (By DRDO)Beam Therapy Dosimeter is portable digital instrument for an accurate measurement of dose and dose rate. The

detector is a 0.5 cc. ionization chamber provided with a perspex buildup cap, of wall thickness 4.75 mm, to enable

calibration and use of chamber with Co-60 radiation. The measuring system is an electrometer operational amplifierwith a high megohm resistor for exposure rate measurement and a low leakage capacitor for exposure measurement

connected in the feedback path. The output of the measuring system is displayed on a Digital Panel Meter.

Specifications:-

Detector: 0.5 cc. air equivalent ionization chamber

Wall material : Tufnol. Perspex build-up cap of thickness 4.75 mm provided for Co-60 radiation

Range

Two linear ranges:

Exposure rate : 0-1.999 Gy/min.

0-19.99 Gy/min.Exposure : 0- 1.999 Gy

0- 19.99 Gy

Linearity : � 1% or� 1 digit.

Energy range : 30 KeV- 1.3 MeV high energy . photons and electrons (with appropriate calibration

factors)

Accuracy : + 3%

Reproducibility : + 0.1%

Chamber voltage : + 180 (V) and + 90(V/2) Volts

Power requirement : Mains (23OV AC, 50 Hz) OR 6 Volts DC (4 x1.5 V'D'type cells)

Dimensions : 430 mm x. 300 mm x 120 mm

Weight : 5.5 Kg (without batteries)

Application:-

Useful for measuring beam outputs from accelerators, X-ray and Telecobalt machines and for evaluating HVT of X-ray

beams

.



The Sentry - Personal Alarming Dosimeter and Rate Meter

The Radiation Alert® Sentry is a Personal Alarming Dosimeter / Rate meter designed to protect personnel who are

exposed to x-ray or gamma radiation. The pocket sized unit has built in memory for recording data points for tracking

accumulated exposure. The Sentry Software option enables the user to generate incident reconstruction for analysis.

The vibrating and audio alarm can be easily set to desired levels for dose and dose rate. The Sentry features an audio

switch to choose between audible clicks or a discreet silent mode. A headphone jack is also included.

Specifications

Detector: Uncompensated GM Tube

Switch Functions: Power, Audio Clicks On/Off, Vibrate & Audible, Alert, Silent Vibrating Only Alert

Operating Range: Dose Rate: .1 - 15 R/hr, Accumulated Dose: .1 - 65 R

Dose Rate Linearity: Better than ±15% up to 15 R

Energy Response: Down to 30 KeV

Gamma Sensitivity: 1.5 cps/mR/hr referenced to Co-60

Audible Alarms: 90db @ 1 ft.

• accumulated dose

• dose rate

Alarm Thresholds:

Dose Alarm Default 500 mR

Rate Alarm Default 50 mR/hr

Alarm and warning levels are user selectable with

Optional Sentry Software.

Connectors: Headphone Connector, Internal Serial Header connector (cable comes w/ Optional

Sentry Software).

Power Requirements: 9 Volt Battery. Typical battery life is 1500 hrs at background.

Temperature Range: -20° - + 50°C (-4° - +122°F)

Weight: 88 g (3.1 oz.), 8.3 oz w/ Xtreme Boot Option

Size: 3.8 x 2.4 x 1 in (96.5 x 70 x 25.4 mm)

Includes: Carrying Case



6. UNITS OF RADIATION

Radioactivity is measured in Becquerel (Bq) per second. 1 Bq means one disintegration per second. It is also

measured in Curie (Ci), named for Madam Curie, who shared Nobel Prize with her husband. 1 Curie = 3.7 x

1010

Bq or disintegrations per second. The radiation absorbed dose is measured in Gray, rad, rem and Sievert

(Sv).

In the United States, absorbed dose is commonly given in rad or Gray and other protection quantities, such as

equivalent dose and effective dose, are given in rem. The following table is provided to help avoid confusion

among persons not familiar with these quantities. The use of the newer system of units would be particularly

useful during radiological incidents involving international responders.

Roentgen :A unit for measuring the amount of gamma or X rays in air.

Rad :A unit for measuring absorbed energy from radiation.

Rem :A unit for measuring biological damage from radiation.

Conversions for Effective Dose, Equivalent Dose, Dose Equivalent, and ambient dose equivalent

0.001 rem = 1 mrem = 0.01 mSv

0.01 rem = 10 mrem = 0.1 mSv

0.1 rem = 100 mrem = 1 mSv

1 rem = 1000 mrem = 10 mSv

10 rem = 100mSv = 0.1 Sv

100 rem = 1000 mSv = 1 Sv (Sievert)

1000 rem = 10 Sv

Conversions for Absorbed Dose

0.001 rad = 1 mrad = 0.01 mGy

0.01 rad = 10 mrad = 0.1 mGy

0.1 rad = 100 mrad = 1 mGy

1 rad = 1000 mrad = 10 mGy

10 rad = 100 mGy = 0.1 Gy

100 rad = 1000 mGy = 1 Gy (Gray)

Measured Dose (Temporary Measurements) – gamma radiation or X-rays

1 R (roentgen) = 0.01 Gy = 0.01 Sv



7. RADIATION SAFETY LIMITS

Accepted good practice for the use of radioactive material and radiation-producing machines results in personnel

radiation exposures being reduced to values as low as reasonably achievable (ALARA). International Commission on

Radiological Protection(ICRP) has published guidelines for maintaining radiation doses at ALARA levels. "As exposure

may involve some degree of risk, and thus some detriment, the comprehensive system of dose limitation is aimed at the

following principal objectives:

a) To ensure compliance with the dose limits;

b) To avoid the use of unnecessary sources of exposure;

c) To provide for operational control of specific procedures, both individually and in combination, so that the resulting

doses are as low as is reasonably achievable, economic and social considerations being taken into account; and

d) To provide a general framework to ensure that these doses are justifiable in terms of benefits that would not

otherwise have been received.

DOSE LIMITS

ICRP has recommended dose limits for occupational and public exposures. In setting the dose limits for radiation worker,

ICRP has ensured that, for a continued exposure at that level, the estimated risk is not unacceptable. By considering

total harm arising from somatic(fatal cancer, non -fatal cancer) and hereditary effects, the commission recommends a

limit to the effective dose of 20 mSv per year averaged over 5 years(100 mSv in 5 years) with further provision that the

effective dose should not exceed 50 mSv(30 mSv as per AERB) in any single year. The effective dose limit ensures the

avoidance of deterministic effect in all body tissues and organs. The annual limits are 150 mSv for the lens and 500 mSv

for the skin, averaged over any 1 cm2 regardless of the area exposed. For internal radiation exposure, limits are

prescribed in terms of annual limits on intake (ALI) for different radio nuclides, which are based on a committed

effective dose of 20 mSv. The basis for the control of occupational exposure of women who are not pregnant is the same

as that for men. However, once pregnancy has been declared, the conceptus should be protected by applying a

supplementary equivalent dose limit, to the surface of the woman's abdomen of 2 mSv for the remainder of pregnancy

and by limiting the intakes of radio nuclides to about 1/20 of ALI. Members of the public include children who might be

subjected to an increased risk and may receive no direct benefit from the exposure. Hence, the limit for public exposure

is an effective dose of 1 mSv in a year. However, in special circumstances, a higher value of effective dose could be

allowed in a single year, provided the average over 5 years does not exceed 1 mSv per year. For preventing deterministic

effects in lens of the eye and skin, annual dose limits of 15 mSv and 50 mSv respectively have been recommended for

the public.



The sensitivity of various types of cells is shown below. The dose given in each case is the lowest amount of

radiation that cells in the tissue can absorb without being damaged:

Fetus: 2 Gy

Bone marrow: 2 Gy

Ovaries: 2–3 Gy

Lens of the eye: 5 Gy A child's bone: 20 Gy

An adult's bone: 60 Gy

A child's muscle: 20–30 Gy

An adult's muscle: 100 or more Gy

These children live in a village not far from the Chernobyl nuclear plant. Four years after the 1986 Chernobyl accident,

these children are suffering intestinal problems from exposure to radiation.

Radiation levels being detected in Japan.



8. OTHER RADIATION DETECTION TECHNIQUES

FILM DOSIMETRY The film badge dosimeter, or film badge, is a dosimeter used for monitoring cumulative exposure to ionizing radiation.

The badge consists of two parts: photographic film, and a holder. The film is removed and developed to measure

exposure.

THERMOLUMINESCENCE DOSIMETRY

A thermo luminescent dosimeter, or TLD, is a type of radiation dosimeter. A TLD measures ionizing radiation exposure

by measuring the amount of visible light emitted from a crystal in the detector when the crystal is heated. The amount

of light emitted is dependent upon the radiation exposure. Materials exhibiting thermo luminescence in response to

ionizing radiation include but are not limited to calcium fluoride, lithium fluoride, calcium sulfate, lithium borate,

calcium borate, potassium bromide and feldspar.

SEMICONDUCTOR DOSIMETRY A silicon diode dosimeter is a p–n junction diode. The diodes are produced by taking n type or p type silicon and

counter-doping the surface to produce the opposite type material. These diodes are referred to as n–Si or p–Si

dosimeters, depending upon the base material. Both types of diode are commercially available, but only the p–Si type is

suitable for radiotherapy dosimetry, since it is less affected by radiation damage and has a much smaller dark current.

Radiation produces electron–hole (e–h) pairs in the body of the dosimeter, including the depletion layer. The charges

(minority charge carriers) produced in the body of the dosimeter, within the diffusion length, diffuse into the depletedregion. They are swept across the depletion region under the action of the electric field due to the intrinsic potential. In

this way a current is generated in the reverse direction in the diode.

FILM DOSIMETER TLD DETECTOR HIGH PURITY GERMANIUM DETECTOR

PROJECT ON IONIZATION DOSIMETER

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