gate frame question - femaunit four external dosimetry . 4- 9 answers question circle the correct...

4-1

UNIT FOUR

EXTERNAL DOSIMETRY

GATE FRAMEQUESTION

Unit Three reviewed the biological effects andinternal hazards of radiation exposure. You learnedabout the early and late effects of radiationexposure, the factors affecting radiation damage, theconcept of radio-sensitivity, and the pathways ofdeposition and distribution of radioactive material inthe body. Now that you have reviewed themechanisms of radiation damage and the biologicaleffects of that damage, it is an appropriate time tocover the selection of external dosimetry methodsfor detecting radiation in emergency responsesituations.

The radiological emergency responder must monitorhis/her total exposure to radiation during theoperation. This unit will review the fundamentalconstruction and operation of three commonly usedmethods of external dosimetry: pocket ionizationchambers, film dosimeters, and thermoluminescentdosimeters. The advantages and disadvantages ofeach dosimeter type will also be covered briefly.

You have responded to an accident involving a truckcontaining radiopharmaceuticals packages. TheIncident Commander tells you that a package foundon the ground indicates that it contains 0.2 Ci ofcesium-137 (Cs-137). Cs-137 is a beta and gammaemmitter, with a radioactive half-life of 30 years.

What type(s) of radiation dosimetry device shouldyou be using to monitor your radiation exposure atthe site? __________________________________________________

__________________________________________________

Unit Four External Dosimetry .

4-2

ANSWER

Your answer should includethe adjacent information.

The pocket ionization chamber, film dosimeter (orfilm badge) and thermoluminescent dosimeter (TLD)would all be appropriate in this situation. Cs-137 isa beta and gamma emitter, and all three of thosemethods are capable of measuring gamma radiationexposure. The film dosimeter and TLD could alsodetect beta radiation.

To determine (i.e. read) the total gamma exposurewhile at the site, the direct reading pocket ionizationshould be used. None of these dosimeters willeffectively detect exposure from inhaled radioactivematerial.

If your answer included all or most of the abovepoints, you should be ready for the SummaryQuestions at the end of this unit. Turn to page4-18.

If your answer did not include these points, it wouldbe advisable for you to complete the instruction forthis unit. Turn to page 4-3.


4-3

RADIATION EXPOSURE RECORD

NAME________________________________________________ADDRESS_____________________________________________CITY/STATE/ZIP_______________________________________

Social Security Number__________Dosimeter Serial No.________

Date Initial Reading Final Reading Dose______ ____________ ___________ _______________ ____________ ___________ _______________ ____________ ___________ _______________ ____________ ___________ _______________ ____________ ___________ _______________ ____________ ___________ _______________ ____________ ___________ _______________ ____________ ___________ _______________ ____________ ___________ _______________ ____________ ___________ _______________ ____________ ___________ _______________ ____________ ___________ _______________ ____________ ___________ _______________ ____________ ___________ _________Date________________ Total Exposure_____________________Signature ______________________________________________

INTRODUCTION TO DOSIMETRY

Dosimetry is the monitoring of individuals toaccurately determine their radiation dose equivalent.When radiation interacts with the human body, thereare no perceptible sensations and usually noimmediate effects. We could therefore receive anamount of radiation that could injure our tissuesseverely without realizing it at the time. To protectourselves and others, we must use and rely uponinstruments to quantify and qualify radiationmeasurements.

Radiological emergency responders must be able tokeep track of their total exposure to radiation duringa radiological emergency operation. To do this, aninstrument called a dosimeter is used. Thedosimeter keeps track of the total charges createddue to radiation interactions in it. The very sameproperty of radiation that damages human tissuesmay be used to detect it - ionization. The concept ofionization was explained in Unit Two.

In order to properly use radiation detectioninstruments, it is essential to fully understand theirmethod of operation. Only a radiological emergencyresponder who understands the instrumentation anduses it correctly will attain maximum effectivenessfrom it. The major dosimetry systems used by thoseinvolved in radiation work are pocket ionizationchambers, thermoluminescent dosimeters (TLDs),film dosimeters, and combinations of systems (TLDplus pocket ionization chambers.)

To test your understanding of these concepts,answer the question on the following page


4-4

QUESTION

Circle the correct answer

_______ is the monitoring of individuals toaccurately determine their radiation dose equivalent.

a. radiotherapy

b. dosimetry

Turn the page to check your answer.


4-5

ANSWERS

QUESTION


a. No, radiotherapy is the use of radioisotopesfor medical purposes. Dosimetry involvesspecial radiation detection instruments thatmeasure radiation exposure of individualsusing the device.

Try another question.

b. Exactly. You understand the meaning andpurpose of dosimetry. During a radiologicalemergency, responders keep track of theirtotal exposure to with radiation dosimeters.

Move on to the next section.

What property of radiation is utilized by manydosimeters to detect radiation?

a. ionization

b. radioactive half-life



4-6

ANSWERS a. Correct. You understand that certaindosimeters keep track of the total chargecreated due to the ionization created byradiation interactions with matter. Ionizationis what also damages human tissue after ithas been exposed to radioactive material.

Proceed to the next section.

b. Wrong answer. Recall from Unit Two thatradioactive half-life is the time it takes for aradionuclide to decay to one-half of theradioactive atoms that were present at thebeginning of the time period. This propertyis not measured by radiation detectioninstruments. It is an inherent nuclearproperty of nuclides. Ionization is theprocess of removing an electron from anatom, leaving two charged particles. Theseelectrical charges may be detected andmeasured by the dosimetry system.

Reread page 4-3.


4-7

0 20 40 60 80 100 120 140 160 180 200MILLIROENTGENS

POCKET IONIZATION CHAMBERS

Pocket ionization chambers rely on ionization todetect radiation. A pocket ionization chamberconsists of a small, air-filled chamber in which aquartz fiber is suspended. A small microscope and agraduated scale enables one to read the shadow ofthe quartz fiber. The quartz fiber is displaced bycharging it with about 200 volts; at this point, thedosimeter scale reads 0. Exposure to radiationdischarges the fiber by creating ions; the dosimeterscale then reads that amount of ionization.

There are various types of pocket ionizationchambers. Some are direct, or self-reading, whileothers are indirect, or nonself-reading. There is alsoa variety of pocket ionization chambers that read atdifferent rates (0.01-200 mR and 1-500 R). Pocketionization chambers, primarily measure whole bodygamma exposure (with some x-radiation).

There are several advantages to using pocketionization chambers.

• A cumulative exposure can be read at anytime or location without ancillary equipment.

• The chambers can be used repeatedly bysimply recharging or rezeroing.

• They have a long shelf-life with little to nomaintenance requirements.

• The individual can directly read his/herexposure.

• Pocket ionization chambers measure gammaexposure accurately.

• They are sealed at the time of manufactureand are relatively insensitive toenvironmental conditions.


4-8

QUESTION


There are also possible disadvantages to the use ofpocket ionization chambers.

• The exposure readings on the devices may besensitive to a significant mechanical shock(for example, if dropped more than a fewfeet to a concrete surface).

• The initial cost of a pocket dosimeter is high. Let’s pause now and test your comprehension ofhow pocket dosimeters operate.

Pocket ionization chambers measure ______radiation exposure.

a. gamma

b. alpha



4-9

ANSWERS

QUESTION


a. Yes, pocket ionization chambers primarilymeasure whole body gamma exposure.


b. No, pocket ionization chambers primarilymeasure whole body gamma exposure.

Try the next question.

It is important to closely monitor and documentyour radiation exposure from responding to aradiological emergency. If you are using a pocketionization chamber, how would you document yourexposures?

a. Manually record the exposure readings.

b. Photocopy and store the computer printouts.



4-10

ANSWERS a. Very good. You remembered that pocketionization chambers are generally read onsite. These exposure readings can andshould be recorded on a log. Since there isno printout or film required to get themeasurement result, the exposure should bemanually recorded at the time of the reading.

You are ready to proceed to the next section.

b. Incorrect answer. There are no computerprintouts coming from pocket ionizationchambers! Users must manually record theexposure readings.

Review page 4-6.


4-11

Badge front with window Filters

Films

Badge coverand clip

Disassembled Film Badge

FILM DOSIMETERS

Film dosimeters, or film badges, consist of layeredcomponents. Imagine a sandwich with the followinglayers starting from the top: the badge front, with awindow for exposure; filters that selectively filterout certain types of radiation; films to detect theradiation; more filters; then the badge cover and clipto attach the dosimeter to the individual’s clothing.

After a designated period of exposure, the film istaken out of the “sandwich” badge, developed, andread on a densitometer, which reads the amount ofdarkening on the film. The amount of darkening isproportional to the radiation exposure.

There are many advantages to using a film dosimeterfor personnel monitoring.

• The dose measurements for various filmbadges range between 10 mrem to 1500mrem for gamma and x-radiation, and 50mrem to 1000 rem for beta radiation.

• Film badges can distinguish betweenpenetrating radiation (high, medium, and lowphoton energies) and non-penetratingradiation (beta and x-ray radiation less than20 keV).

• Film dosimeters are practical because theyare small, lightweight, and relativelyinexpensive.


4-12

QUESTION


There are also possible disadvantages associatedwith the use of film badges to monitor individualradiation exposure.

• The response of the film to radiation isenergy dependent; at energies less than 300keV, the response tends to increase.

• The films cannot be read immediately.

• Environmental conditions such as heat andhumidity will affect the film’s response toradiation.

• Film badges may be left or lost at the site ofthe radiation accident

• They may be contaminated with radioactivematerials, which will lead to a false higherresult.

Exposure to radiation of the film in the dosimetrybadge causes the film to darken. The darkening isproportional to the amount of

a. radiation released at the site.

b. radiation exposure of the individual wearingthe badge.



4-13

ANSWERS

QUESTION


a. No, the film dosimeter, or badge, cannotmeasure all the radiation released at the siteof the radiological response operation. Thebadge is only useful for the person wearingit.


b. Correct answer! A densitometer measuresthe amount of darkening of the film. Thisamount of darkening is proportional to theamount of radiation exposure of the film.


Film dosimeters are capable of detecting beta,gamma, and x-radiation.

a. True

b. False



4-14

ANSWERS a. Correct answer. The dose measurements forvarious film dosimeters range between 10mrem and 1500 mrem for gamma and x-radiation, and 50 mrem to 1000 rem for betaradiation. Furthermore, film badges candistinguish between penetrating radiation(high, medium, and low photon energies) andnon-penetrating radiation (beta and x-rayradiation less than 20 keV).

Proceed to the next section.

b. Wrong answer. The statement is true.Various film dosimeters can measureradiation doses ranging between 10 mremand 1500 mrem for gamma and x-radiation,and 50 mrem to 1000 rem for beta radiation.

Reread page 4-10.


4-15

CrystallineMaterials Inside

Clip

THERMOLUMINESCENTDOSIMETERS

Thermoluminescence is a property, possessed bycertain crystals, of emitting light upon heating afterhaving been exposed to ionizing radiation. Theemitted light can be measured. The amount of lightis directly proportional to the radiation absorbeddose. Thermoluminescent dosimeter (TLD) badgesare issued to radiation workers and emergencyresponders who have the potential to be exposed toionizing radiation. The badges are usually issued foruse on a monthly or quarterly basis to record thedose received by these personnel from beta, gamma,x-ray , and neutron radiation. Their range ofmeasurement spans anywhere from less than 1 mremup to as much as 105 rem.

There are several advantages to using TLDs forpersonnel monitoring.

• TLDs can be used with radiation fields ofwidely varying energy and intensity.

• They can store information for long periodsof time without fading.

• TLDs are reusable, and can be used for manyapplications because of their small size.

• TLDs are less energy dependent than filmdosimeters or pocket ion chambers.

There are also some possible disadvantages andproblems with TLDs.

• They cannot be analyzed immediately.

• Environmental factors such as humidity andheat may affect the results.


4-16

QUESTION


• They may be left or lost at the site of theradiation incident.

• They may be lost, and they may becontaminated with radioactive materials.

To check your understanding of these concepts,answer the following question.

Thermoluminescent dosimeters contain crystals thatrespond to ionizing radiation and emit ------.

a. radioactive particles.

b. light.



4-17

ANSWERS

QUESTION


a. Wrong answer. The crystals do not emitionizing radiation. They measure radiationby emitting light in response to having beenexposed to ionizing radiation.


b. Correct. You understand thatthermoluminescence is a property possessedby certain crystals. It is the process ofemitting light upon heating after exposure toionizing radiation. The amount of emittedlight is directly proportional to the absorbedradiation dose.

Move on to the Summary Question.

One must be cautious with interpreting readingsfrom thermoluminescent dosimeters used during atypical summer day in Florida because

a. of the daily lightning and thunderstorms.

b. the humidity and heat affect the results.



4-18

ANSWERS a. No, unless the thermoluminescent dosimeteris hit directly by lightning, use of thisdosimeter should yield relatively reliablereadings.

Reread page 4-14.

b. Correct answer. You clearly understand thatenvironmental factors such as humidity andheat may affect the results ofthermoluminescent dosimeters.

You may now proceed to the Summary Question.


4-19

SUMMARY QUESTION

Complete the following table:

METHOD

TYPE OFRADIATIONMEASURED

ONEADVANTAGE

ONEDISADVANTAGE

Pocket IonizationChamber

Film Dosimeter

ThermoluminescentDosimeter

Turn the page to check your answers.


4-20

ANSWER

METHOD

TYPE OFRADIATIONMEASURED

ONEADVANTAGE

ONEDISADVANTAGE

Pocket IonizationChamber

gammax-ray see page 4-7 see page 4-8

Film Dosimetergamma, x-rayand beta see page 4-11 see page 4-12

ThermoluminescentDosimeter

gamma, x-ray,beta, and neutron see page 4-15

see pages 4-15and 4-16

Once you are able to complete the chart correctly, move on to Unit 5.

gate frame question - femaunit four external dosimetry . 4- 9 answers question circle the correct...

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