lecture radiation detection and protection

8/13/2019 Lecture Radiation Detection and Protection

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The limiting amounts of radiation that can be received by a

worker is established by federal codes and regulations.

Special units that describe radiation exposure for all types ofradiation have been developed for this purpose.

The objective of radiation protection is to ensure that theamount of radiation received by a worker will be below theestablished limit and therefore be harmless.

In this lecture, instrumentation that is used to measureradiation exposure of individuals and to monitor radiationlevels in the plant will be discussed.

Also, means of reducing radiation exposure be described.

RADIATION DETECTION AND PROTECTION


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THREE CONCEPTS OF RADIATION PROTECTION

The best way to avoid becoming a weekend traffic statistic is

to stay off the highways.

With radiation, too, the best policy is avoidance.

But, for those times when avoidance is not possible, the

amount of radiation received can be reduced in three basic

ways:

(1) by controlling the length of time of exposure

(TIME)

(2) by keeping the distance from the radiationsource as large as possible (DISTANCE)

(3) by placing material between you and the source

of radiation.(SHIELD)


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CONCEPT ONE: REDUCE TIME DURATION

If the radiation in a particular area is constant, the exposure

of personnel to radiation can be controlled by limiting the

amount of time an individual stays in the area.

In the Figure, a worker is shown to be exposed to a small amount

of radiation for fifteen minutes and for thirty minutes.

In thirty minutes, he receives twice the radiation dose hereceives in fifteen minutes .


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CONCEPT TWO: INCREASE DISTANCE

Radiation exposure can be reduced by keeping the distance

between the radiation source and the individual as large as

possible, as shown in Figure.

If a small radioactive source gives a dose of 100 millirems tosomeone standing one foot away from it, it will give a dose of

only 25 millirems to someone standing two feet away from it

and only 4 millirems five feet away.

In general, the dose decreases in proportion to the square ofthe distance from the source.


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CONCEPT THREE: INCREASE SHIELDING

Alpha radiation can be stopped or shielded with a sheet of

paper, while a small thickness of aluminum or plastic will be

sufficient shielding for beta radiation.

Gamma and neutron radiation require considerably more material

for shielding.

For gamma radiation, various thicknesses of dense material,

such as steel, lead, concrete, or water, can be used to reduce

radiation to desired levels.


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CONCEPT THREE: INCREASE SHIELDING

A convenient concept to use for a rough gamma shielding estimate

is the tenth value thickness.

The tenth value thickness is the thickness of material that will

reduce the radiation level from a source by a factor of ten.

The tenth value thickness is a measure of the effectiveness of

shielding materials. For example, to shield a relatively high

energy gamma source, the tenth value thickness of lead is 1.5

inch, steel is 3.0 inch and water is 24 inch.


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Since one cannot see, feel or weigh the radiations, the method of radiationdosimetry depends on measuring the effects of radiations on matter.

These effects include ionization in gases , ionization and excitation in certainsolids , changes in chemical systems and activation by neutrons .

Majority of the health physics monitoring instruments use detectors based onionization of gas.

The absorption of radiation in a gas results in the production of ion pairsconsisting of a negative ion (the electron) and a positive ion.

A moderate voltage applied (~200 to 250V) between two plates (electrodes) inclose proximity causes the negative ions to be attracted to the positive electrode

(anode) and positive ion to the negative electrode (cathode)

This flow of ions constitutes an electric current which is a measure of theintensity of radiation In the gas volume.

The current is extremely low (~ 10 -12 amperes) and a sensitive electronic circuit(amplifier) is used to measure it. This arrangement is called ionization chamber .

RADIATION MEASUREMENT


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IONIZATION CHAMBER

The design of the chamber and the filling gas used depends on the particularapplication.

In health physics instruments the ionization chamber is usually filled with airand constructed of material of low atomic number.

If the instrument is required to respond to beta radiation the chamber must havethin walls or a thin entrance window (e.g. PDM).


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IONIZATION CHAMBER(PORTABLE DOSERATE METER PDM)


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If in an ion chamber system, the applied voltage is increased beyond a certainpoint, an effect known as gas amplification occurs.

This is because the electrons produced by ionization are accelerated by theapplied voltage to a sufficiently high energy to cause further ionizationthemselves before reaching the anode, and a cascade of ionization results.

PROPORTIONAL COUNTER

Thus a single ionizing particle or photon can produce a pulse of current which islarge enough to be detected due to a gas amplification factor of typically ~10 4.

Over a certain range of voltage (~ 250 to 650V) the size of pulse is proportionalto the amount of energy deposited by the original particle or photon and so thesystem is known as a proportional counter .

The term counter means that the output is a series of pulses , which may becounted by some means rather than an average current like ion chamber.


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If the voltage in the ionization system is increased still further (> 800V ) the gasamplification is so great that a single ionizing particle produces an avalanche ofionization resulting in a very large pulse of current.

The size of the pulse is the same, regardless of the quantity of energy initiallydeposited.

GIEGER MULLER COUNTER

In a typical avalanche created by a single original electron, many excited gasmolecules are formed by electron collisions in addition to the secondary ion.

The resulting photon due to de-excitation are the key element in the propagationof the chain reaction that makes up the Geiger discharge, in which themultiplication by a single avalanche is ~10 6 to 10 8.

The Geiger discharge therefore grows to envelop the entire anode wire,regardless of the position at which the primary initiating event occurred.


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In practice both proportional and Geiger-Muller counters are usually constructedin the form of a cylinder which forms the cathode, with a central thin wire whichis the anode.

The whole is enclosed in a glass or metal tube which is filled with a special gasmixture.

The Geiger Muller tube is very widely used in monitoring equipment because itis relatively rugged and can directly operate on a simple output circuit.

GIEGER MULLER COUNTER (Contd.)


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Radiation detectors are located in different areas within a nuclear power plant

to monitor radiation levels continuously.

Each monitor is connected to a readout device, usually in the reactor controlroom.

These monitoring stations are equipped to give an audible alarm signal when aradiation level set point is exceeded.

The radiation detector is positioned in the plant and wired to an audible alarmbox and readout module located in the reactor control room.

The alarm is set to go off when radiation reaches a pre-set level.

Tripping of the alarm alerts the operator in the reactor control room who readsthe actual radiation level on the readout module and takes appropriate action.

AREA MONITORS


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AREA MONITORS


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FILM BADGES

Film badge is used to measure beta and

gamma radiation exposure.

It is composed of dental sized

sheets of photographic film

contained in a lightproof wrapper.

The film is darkened by radiation;

the larger the radiation exposure,

the darker the film.

The film package is placed in a

plastic frame containing small pieces of three different metals

that act as a filter by providing

various amounts of shielding to

incident radiations.


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FILM BADGES

By comparing the exposure under

the three different filters, it is possible to determine the energy

of incident gamma rays and the

gamma ray dose.

Since beta particles do not have

enough energy to penetrate the

plastic frame, a window is left in

the holder to determine beta

exposure.

A different type of film is usedif it is desired to measure

neutron exposure.


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FILM BADGES

Another common type of film badgesthat are used have metal foils in

lower section.

These foils can be used to

determine the exposure of an

individual to neutron radiation.

The upper section functions just

like the previous film badge

consisting of a plastic frame with

windows and a special slot for photographic film.


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POCKET DOSIMETERS

A pocket dosimeter is often used

in conjunction with a film badge.

One type shown is a small

electroscope about the size and

shape of a fountain pen.

It consists of a small, air-filled

chamber in which a movable quartz

fiber and a fixed wire element is

suspended.

A stationary graduated scale can

be seen behind the quartz fiber.

The quartz fiber moves to indicate

the amount of radiation exposure.


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POCKET DOSIMETERS

A pocket dosimeter is often used

in conjunction with a film

badge.

A separate charging unit is used

to put a positive charge on both

the fixed wire element and the

movable quartz fiber; as a result,

the quartz fiber is repelled from

the wire.

Radiation penetrating the

dosimeter forms charged

particles. The negative particlescollect on the wire and the quartz

fiber.

As a result, the repulsive force

is reduced and the quartz fiber

moves closer to the wire.


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TLD BADGES

A recent development in measuring radiation is the

thermoluminescent dosimeter, abbreviated as TLD.

A TLD utilizes special materials with atoms that are

excited to higher energy when exposed to radiation. Upon

heating, these excited atoms return to their original

energy levels, releasing light in the process.

By measuring the amount of light released, the amount of

radiation exposure can be determined.


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ELECTRONIC DOSEMETERS

The main application of EDs has

been to provide an on-the-job method of dose measurement and

control.

A new generation of electronic

dosemeters has become available,

using solid state detectors and

taking advantage of developments

in information technology.

By means of in-built micro-

processors and memory they can be programmed to perform a variety of

functions, such as logging dose

for a specific task or for a

shift, or storing information on

the characteristics of the

radiation field.


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ELECTRONIC DOSEMETERS

With gate entry facilities they

can be used as a form of security pass, giving recorded access to

controlled areas.

The devices therefore offer the

advantage of combining the

operational dose control and the

long-term legal dose measurement

functions. An example of a modern

electronic dosemeter is shown in

Figure.

The disadvantages, compared with

film dosemeters or TLD, are that

they are relatively costly

initially, bulky and are

potentially susceptible to

electromagnetic fields.


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SODIUM IODIDE (TI) DETECTOR

Thallium-activated sodium iodide (NaI(Tl)) detector is the first

practical solid detector used for gamma rays and is still the

most popular one for this purpose.

The gamma rays interact in the detector and they produce

electrons and in some cases (E > 1.02 MeV) positrons as well.

When these electrons move through the crystal, they excite the

atoms and while de-exciting, the atoms produce a small flash oflight called scintillation.

When the scintillations fall on the photocathode of the

photomultiplier (PM) the electrons are produced.

The initial pulse from photocathode is very small which is

amplified in 10 stages, in a series of dynodes known as

photomultiplier tube, to get a large enough pulse.

Thallium is added into Nal to shift the scintillation

spectrum and to match it with the photocathode response.


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SODIUM IODIDE (TI) DETECTOR

The main advantage of NaI(Tl) detector is its good detection

efficiency.

However, the energy required to produce one electron from

photocathode is roughly 1 KeV which reduces the number ofinformation carriers and thus results in poor energy resolution

as compared to Ge(Li) or HPGe detectors.

Another disadvantage is that NaI(Tl) is hygroscopic and must be

enclosed in some material to avoid the absorption of moisture.


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PORTAL AND HAND/SHOE MONITORING

Figure (a) on left shows a typical

portal monitor that is used to

check for possible radioactive

contamination on clothing of

personnel.

Figure (b) on right below shows a

typical hand and shoe monitor that

is used to check for possible

radioactive contamination on the

hands and soles of shoes.

These two types of monitors may be

located at the exits of controlled or

restricted areas within a nuclear

power plant.

lecture radiation detection and protection

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