Film Dosimetry

3/2/2011 
Introduction
In the United States, three types of do

can be used to measure the “dose of r

• Film

Introduction

• Thermoluminescent dosimeters• Optically stimulated luminescen

dosimeters
Film dosimeters are most commonly 

employed to evaluate personnel exposx-rays.

Introduction

Hence, film dosimetry is often used in dentistry, veterinary practice, medicine universities.
Over time, an exposure to light or ioniz

radiation can change the color of sommaterials. Silver bromide is very susceptible to th

Introduction

Silver bromide is very susceptible to th

phenomenon. The exposure to light or radiation changes the silver bromide, a white salt, to metallic silver which is blCertain chemicals can speed up (catathis color change. 
Introduction

E li E li

Support Support

Emulsion Emulsion

Emulsion

SINGLE 

EMULSION

DOUBLE 

EMULSION
The emulsion consists of a gelatinous matrix 

which silver bromide (AgBr) crystals are susThe properties of the film depend on the sizum), shape, and number of these crystals (c10

9 AgBr grains per cc). Some silver iodide 

crystals are also present.Introductioncrystals are also present.

Support

Gelatinous Matrix
The silver bromide crystals have defects tha

groups of positively charged silver ions (Ag+will refer to these as “sensitivity specks.”Introduction

SILVER BROMIDE (AgBr) CRYSTAL

Sensitivity Specks
The highest energy electrons in the silver br

crystal occupy a range of energies known aValence Band.Introduction

Valence 

Band

e

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e

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e

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e

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The conduction band is a higher energy leve

normally empty of electrons. If an electron wthe Conduction Band, the electron would be Introduction

Conduction 

Band

Valence 

Band

e

-

e

-

e

-

e

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e

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e

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e

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The range of energies between the Valence 

Conduction bands is the Band Gap. If the cwere pure, electrons could not occupy the BIntroduction

Conduction 

Band

Valence 

Band

Band Gap

e

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e

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e

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e

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e

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e

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e

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e

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e

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A sensitivity speck (a group of Ag+ ions) ha

electron vacancies. If an electron filled one ovacancies, it would have an energy in the BIntroduction

Conduction 

Band

Valence 

Band

Band Gap

e

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e

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e

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+

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When radiation energy is absorbed by a silv

bromide crystal, some of the electrons are pfrom the Valence Band to the Conduction BOnce in the Conduction Band, these electromobile Some will move to the sensitivity spIntroductionmobile. Some will move to the sensitivity sp

fill the electron vacancies.
Introduction

Conduction 

Band

As a result of an exposure to radiation, an ein the Valence Band is promoted to the ConBand.

Valence 

Band

Band Gap

e

-

e

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e

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e 

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e

- e

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e

- e

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e

Se

S

(A

+

e

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If an electron reaches a “sensitivity speck” t

is converted into a “latent image” that consisneutral silver atoms, i.e., silver metal.Introduction

Conduction 

Band

Valence 

Band

Band Gap

e

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e

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+ e

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e

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e

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LIm

(Ag 

e

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This shows two electrons moving to “sensiti

specks.” The greater the radiation energy din the crystal, the greater the number of elecmoving to the sensitivity specks.Introduction

SILVER BROMIDE (AgBr) CRYSTAL

e

-

e

-
Two sensitivity specks have now been conv

“latent images.”Introduction

SILVER BROMIDE (AgBr) CRYSTAL

Latent Image
The silver ions at the latent image have bee

chemically reduced, i.e., converted to metalIntroduction

SILVER BROMIDE (AgBr) CRYSTAL
Film Processing
After it has been exposed to radiation, the 

photographic film is processed.

The processing consists of three or four ste1 Submersion of the film in a developing sFilm Processing

. Submersion of the film in

a developing s

2. Submersion of the film in a stop bath 3. Submersion of the film in a fixative solut4. Rinsing the film in water.
The developer solution contains chemicals t

speed up the reduction of the silver bromideIf the radiation produced 10 or so atoms of msilver (reduced Ag+ ions) in a crystal, the decan catalyze the complete conversion (reduDevelopercan catalyze the complete conversion (redu

the AgBr crystal into a grain of metallic silveEach interaction of a 10 keV photon might pone “developable crystal. The interaction of photon might produce 5-10 “developable” cr
The stop bath, usually a solution of dilute ac

accomplishes two purposes:

– It stops the action of the developer

Stop Bath (optional)

– It minimizes the contamination of the f

by the developer
The purpose of the fixative (containing sodiu

thiosulfate) is to dissolve any remaining unresilver bromide crystals. Unless this is done, would darken following a subsequent exposlight. As the name implies, the fixative stabil“image.

”

Fixative

Once the film is in the fixative, it can be expambient light. As a rule, used fixative is collected so that thcan be recycled.
The purpose of the rinse (e.g., 30-40 minute

running water) is to remove the fixative soluAfter rinsing, the film should be dried in such as to avoid drying spots.Rinse
To ensure a consistent relationship between 

darkening of the film and the absorbed dose• The times that the film is in the developer 

min) and fixer (e.g., 10-15 min) must not c

Quality Control• The temperature of the developer (e.g., 2

must be consistent.

• The developer and fixer solutions should • Agitation of the film during developing (e.

seconds every minute) should be consiste
Assessing the Dose
The greater the radiation dose, the darker th

will be after processing.Until the 1950s, the darkening of the film waassessed by eye. One procedure was to try a newspaper through the processed film If tAssessing the Dosea newspaper through the processed film. If t

newspaper could be read, the dose was conacceptable.In the mid 1940s, electronic instrumentation (densitometers) began to be used to quantifdarkening of the film. 
A densitometer is a device that is used to m

the darkening of the film. The following diagillustrates a typical densitometer.Densitometer

1

2

Photomultiplier

Optica

0

Filter/Lens

Film

Light Source

Photomultiplier

Tube (PMT)
A photomultiplier tube is used to measure th

amount of light that gets through the film.The process involves comparing the intensitlight that gets through the exposed film with intensity of light that gets through unexposeDensitometerintensity of light that gets through unexpose

The densitometer reads out in units of “opticdensity” which ranges from 0 to 4 or so.
The “optical density” is calculated as follows

Optical Density

O.D. is the “optical density”I

0 is intensity of light that gets through unexpose

I

x is intensity of light that gets through the expos
If ten times more light gets through the unex

film than gets through the exposed film, the density is 1.If one hundred times as much light gets throunexposed film as gets through the exposed Optical Densityunexposed film as gets through the exposed 

the optical density is 2.If one thousands times as much light gets ththe unexposed film as gets through the expofilm, the optical density is 3.
The optical density of film exposed to differe

doses is measured. Optical density is the pla function of absorbed dose. Calibration Curve

Useful Range

OD

Absorbed dose (rads/Gy)
The optical density of an emulsion for a give

exposure (the sensitivity) depends on severthings: 

– Thickness of emulsion

Calibration Curve

– Density of AgBr crystals– Density of AgI crystals– Size and shape of crystals (grains)– Nature of defects/impurities in crystals
Calibration Curve

A high sensitivity emulsion, known as a “fasemulsion,” is better able to assess low doseA low sensitivity emulsion, known as a “slow emulsion,” is better able to assess high dosOD

Absorbed dose (rads/Gy)

Fast Emulsion

Slow Emul
Kodak Film Types for Therapeutic Ra

Film Type Range Satura

PPL 0.25 – 5 rads (0.0025-0.05 Gy) 10 rads (

1 15 d

XTL

– 15 ra

d

s

(0.01-0.15 Gy) 30 rads (

XV-2 5 – 100 rads (0.05-1 Gy) 200 rads 

EDR2 25 – 400 rads (0.25-4 Gy) 700 rads 
Kodak Film for Personnel Monitorin

Kodak Type 2 Film

Approximate useful Range 30 mrems - 2500 

Bt RB

e

ta 

Response 

(maximum) 1.71 – 5 MeV

Photon Response 5 keV – 3 Me
Kodak Type 2 Film for Personal Mon

Support

Fast EmulsionSlow Emulsion

Light tight 

packet

So that the two sides can be easily distinguthe fast emulsion has a matte surface while slow emulsion has a glossy surface.
Although no longer used, this film was once 

commonly employed to estimate neutron doThermal neutrons were absorbed by nitrogefilm. The resulting n, p reaction produced ptracks in the developed emulsion

Kodak Type A NTA Film for Neutrons

tracks in the developed emulsion.

Fast neutrons with energies above 500 keV produced tracks due to proton recoil, i.e., escattering collisions with hydrogen nuclei in The film was insensitive to neutrons with epenergies up to 500 keV.
Fast 

Neutron

Slow 

Neutron

Fast 

Neutron

Slow 

Neutro

Cadmium

Kodak Type A NTA Film for Neutrons

Support

Proton

Track

Proton

Track

Proton

Track Emulsion
The tracks were counted under a microscop

the track density was related to the absorbeA cadmium filter was used to distinguish the number of proton tracks produced by fast nfrom the number of tracks produced by slow

Kodak Type A NTA Film for Neutrons

from the number of tracks produced by slow

neutrons.
Film Dosimeter Badges
The film holder (aka badge) usually ha

open “window.” The film in the window area responds tBeta Doseparticles as well as x-rays and/or gammrays.The film covered by the plastic film holonly responds to x-rays and gamma ra
Energy Dependence of Film Respon

The sensitive component of the film dosimesilver bromide crystals, contains a high Z msilver (Ag).For a given exposure (e.g., 1 rad), the opticis greater for low energy photons (e g 100 is greater for low energy photons (e.g., 100 

for higher energy photons (e.g., 662 keV).
Energy Dependence of Film Respon

OD

Optical density as a function of photon energiven exposure (e.g., 1 rad)

Photon Energy (keV)

10 100 1000
Energy Dependence of Film Respon

This means that the calibration factor, which the optical density to the absorbed dose, vathe photon energy. As such, film dosimeteruse some method to estimate the energy of photons to which they are exposed.They do so by analyzing the ratios of the opdensity of the film underneath metal filters wdifferent atomic numbers (e.g., Al, Cu, Pb).
Window

Typical Film Badge Design

Front View Side View 

Cu FilterAl Filter

Pb/Sn Filter
In some cases, the filters on the front of the 

and those on the back have different shapes square vs round). In this case, the image of filter on the developed film can identify whetexposure was from the front or back.Exposure Direction

Exposure from front, 

square image

Exposure from bac

round image

Film Film
If the exposure occurred for a very short tim

image of the filters on the developed film shhave sharp well defined edges. If the exposwas prolonged, the images should be less d

Acute vs Prolonged Exposure

Acute Exposure Prolonged Exposur
An irregular unusual image might indicate th

badge (or the film) had become contaminateContaminated Badge

Contaminated 

Badge/Film
Advantages and Disadvanta

of Film Dosimetry
Advantages of Film Dosimetry

• The dosimeter itself serves as a perman

record

• The film can be reread• Film is available in a wide ran

ge of sizes

• The photographic image can indicate the 

presence of contamination on the badge

• The photographic image can distinguish 

short term exposures from chronic expos
Disadvantages of Film Dosimetry

• The response of film is very dependent o

energy of the photons.

• Accurate estimations of the dose require 

and consistent processing of the film.