DR. TAHA HEWALA DOSE MEASUREMENT MARWA REDA MEKY …

Pocket dosimeter

DR. TAHA HEWALA

DOSE MEASUREMENT

MARWA REDA MEKY

1211-3-103

Abstract

Pocket Dosimeter (PD) is a personnel monitoring Instrument.it

based on ionization of gases in small chamber called free air

ionization chamber or Air Wall. Providing an immediate reading

of any types of Exposure Radiation is considered to be unique

feature for PD.

There are two types of Pocket Dosimeter

Indirect Reading Dosimeter (IRD).

Direct Reading Dosimeter (DRD).

Pocket dosimeters are used to provide the wearer with an

immediate reading of his or her exposure to x-rays and

gamma rays.

As the name implies, they are commonly worn in the

pocket. To measure radiation dose, the response of the

instrument must be proportional to absorbed energy.

A basic instrument for doing this is the free-air ionization

chamber.

the Concept of Free Air Chamber

the exposure unit can be satisfied by the instrument shown in Figure 1. The X-

ray beam enters through the portal and interacts with the cylindrical column

of air defined by the entry port diaphragm.

- All the ions resulting from interactions between the X-rays and the volume of

air (A–B–C–D), which is determined by the intersection of the X-ray beam with

the electric lines of force from the edges of the collector plate C, is collected

by the plates, causing current to flow in the external circuit.

- Most of these collected ions are those produced as the primary ionizing

particles lose their energy by ionizing interactions as they pass through their.

(The primary ionizing particles are the Compton electrons and the

photoelectrons resulting from the interaction of the x-rays ‘photons’ with the

air).

- The guarding G and the guard wires W help to keep these electric field lines straight

and perpendicular to the plates. The electric field intensity between the plates is high

enough to collect the ions before they recombine but not great enough to accelerate

the secondary electrons produced by the primary ionizing particles to ionizing energy.

-The guard wires are connected to a voltage-dividing network to ensure a uniform

potential drop across the plates. The number of ions collected because of X-ray

interactions in the collecting volume is calculated from the current flow and the

exposure rate, in roentgens per unit time, is then computed.

Direct Reading Dosimeter (DRD)

Also called Capacitor \ Self Reading Dosimeter or Quartz Fiber Electroscope which has the advantage of being reusable.

Construction

a direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen.

The dosimeter contains a small ionization chamber with a volume of approximately two milliliters.

Inside the ionization chamber is a central wire anode, and attached to this wire anodes a metal coated quartz fiber.

Mechanism of Action for DRD

When the anode is charged to a positive potential, the charge is distributed between

the wire anode and quartz fiber.

- Electrostatic repulsion deflects the quartz fiber, and the greater the charge, the

greater the deflection of the quartz fiber.

-Radiation incident on the chamber produces ionization inside the active volume of

the chamber.

-The electrons produced by ionization are attracted to, and collected by, the

positively charged central anode.

--This collection of electrons reduces the net positive charge and allows the quartz

fiber to return in the direction of the original position.

-The amount of movement is directly proportional to the amount of ionization

which occurs. Or on another words, amount of Discharge, consequently the

change in position of fiber proportional to radiation exposure.

-By pointing the instrument at a light source, the position of the fiber may be

observed through a system of built-in lenses.

-The fiber is viewed on a translucent scale which is graduated in units of

exposure (mR). Typical industrial radiography pocket dosimeters have a full

scale reading of 200milliroentgens but there are designs that will record higher

amounts.

Advantages and Disadvantages

1-The main advantage of this type is that, Doesn`t have to be re-

charged, so self-Reading allow user to check exposure at any time.

2-Care should be taken against erroneous readings due to electrical

leakage, but, a High degree of accuracy is not important, Reliability is.

Potential for discharging and reading loss due to dropping or bumping

are a few of the main disadvantages of a pocket dosimeter.

Indirect Reading Dosimeter (IRD):

Also called Condenser \ Non-self-Reading Dosimeter the indirect reading

type an auxiliary device is necessary in order to read the measured dose.

This device, which is, in reality, an electrostatic voltmeter that is calibrated

in roentgens, is called a “charger-reader”. And can measure X-ray also

Gamma-ray.

So the main disadvantage of Condenser type is it MUST be charged after

each reading.

-Non-self-Reading Dosimeter are usually used for measuring any level of

Exposure in range 5-200 mR or Up to this range, also these dosimeters can

be used to measure Beta radiation whose energies exceed 1 MeV.

-By coating the inside of the chamber with boron, the pocket dosimeter

can also be made sensitive to thermal neutrons (~ 0.025 eV) and emits

Alpha particles that causes Ionization, or Coating with Plastic, made it

sensitive to fast neutron (

Construction & Mechanism of Action for IRD:

the instrument consists of an outer cylindrical wall, made of electrically

conducting plastic. Coaxial with the outer wall, but separated from it by a

very high-quality insulator, is a center wire.

This center wire, or central anode, is positively charged with respect to the

wall.

When the chamber is exposed to X-radiation or to gamma radiation, the

ionization, which is produced in the measuring cavity as a result of

interactions between photons and the wall, discharges the condenser,

thereby decreasing the potential of the anode.

This decrease in the anode voltage is directly proportional to the ionization

produced in the cavity, which in turn is directly proportional to the radiation

exposure.

APPLICATION

MEDICAL APPLICATION:

Worker IN hospital including fluoroscopy, portable

radiography and angiography.

in the area of radiotherapy, where dosimeters are used

to calibrate the beam profiles of radiation treatment

equipment and to monitor patients exposed to doses of

radiation.

MANUFACTURING RADIOPHARMACEUTICAL PLACE.

IN OTHER FIELDS:

• INDUSTRY.

• AGRICULTURE.

• OUTER SPACE.

• PLACES HAVE HIGH LEVEL OF RADIATION.

• monitoring fire fighters and police who have to work in

or near accidental radiation environments.

Resources

-Attix, F. H. Introduction to Radiological Physics and Radiation Dosimetry.

John Wiley and Sons, New York, 1986.

-Radiation Dosimetry, by John Cameron, Environmental Health

Perspectives Vol. 91, pp. 45-48, 1991

- Cember, Herman, Introduction to Health Physics, Third Edition, McGraw-

Hill Health Professions Division, New York, 1996.

- Burnham, J.U., Radiation Protection, Point Lepreau Generating Station,

New Brunswick Power Corporation, Rev. 4, 2001.

Thank you…