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The energy response of LiF, film and a chemical dosemeter to high energy photons and electrons

This content has been downloaded from IOPscience. Please scroll down to see the full text. 1976 Phys. Med. Biol. 21 414 (http://iopscience.iop.org/0031-9155/21/3/007) View the table of contents for this issue, or go to the journal homepage for more

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PHYS. MED. BIOL.,

1976, VOL. 21, NO. 3, 414-421.

@ 1976

The Energy Response of LiF, Film and a Chemical Dosemeter to High Energy Photons and Electrons M. BISTROVIC, MSC., and 2. MARIGIC, PH.D. Central Institute for Tumours and Allied Diseases, Ilica 197, 41000 Zagreb, Yugoslavia

M. A. GREENFIELD,

PH.D.

Department of Radiology, University of California, Los Angeles, U.S.A.

B. BREYER, PH.D., I. DVORNIK, PH.D., I. SLAUS, PH.D. and P. TOMAS,PH.D. Institute “Ruder Boslovi6”, Zagreb, Yugoslavia

Received 4 June 1975, infinal form 26 January 1976 ABSTRACT.The physical responses of LiF, film and a chemical dosemeterinhigh energy photon and electron beams has been studied. It is shown that the response for film and the chemical dosemeter does not change significantly up to 42 MeV, while for LiFit decreases byabout 10%. Analysis of calculations and measurements showed that this decrease could not be explained by existing theories.

1. Introduction

In order to determine absorbed dose in a medium (water, lucite or tissue) we can use cavities with known radiation response properties as dosemeter. The absorbeddose in the cavityfor a given irradiation is relatedto theabsorbed dose in the medium by D,(cavity) = DE(medium)SgzJfTm(E)

(1)

where S ( E )is the ratio of collision stopping powers for the cavity andmedium under consideration at anaverage energy E throughout the cavityindependent of the origin of the electron, whether in the medium or cavity. This expression is valid when the cavity is small compared with the average range of electrons (or with the average range of secondary electrons in thecase of photon beams) : thus it is also valid for 60Co secondary electrons: Dco(cavity)= Dc,(medium) SgzJt:,

(CO)

(2)

where SgzJizrn (CO)is the electron stopping power ratio for the 6oCoradiation. Since we use a 6OCo source for calibration we divide these two equations: D,(cavity) = F D,(medium) EDc,(medium) * Dc,(cavity)

Energy Response of Lip, Film and a Chemical Dosemeter to Photons

415

(Johns and Cunningham 1969) takes into account the difference between the types of irradiation, normalized to unity at6oCoelectron energies. The FE factor is the rate of absorbed dose in the cavity per rad in the medium at a specific energy. FE depends on the average energy of electrons inside the cavity and on the materials of the medium and the cavity respectively. Measurements indicate (e.g. Paliwal andAlmond 1975) that there is a decrease of the thermoluminescent response of LiF for high energy electrons and photons relative to the response at 6oCoenergy. Almond and McCray (1970) try to explain these results using D,(cavity) = fEDE(medium) where in the case of high energy electrons

while in the case of photons, following Burlin (1970),

Here, E is the average energy of incident electrons, d is the weighting factor for the contribution of primary radiation and (1 - d ) for the contribution of secondary electrons generated inside the cavity. These contributions depend on the mean path length of electrons crossing the dosemeter and therefore the size of the cavity should be taken into account. Now FEis given by E’

=f E / f C o *

This FE was calculated by Almond and McCray (1970) for an LiF cavity. Burlin (1970) argues that Almond’s formula for electrons is incorrect and he suggested that fE for electrons should be given by f E

avity - d s cmedium’

The actual physical response of cavities will be determined by the following comparative method. We compare the physical response of cavities to radiation of energy E with the response to 6oCoradiation at the same ionization chamber reading, I ,

pE ---

rE Tco

where rE is the response for radiation of energy E and rco is the response for 6OCo radiation. Since the absorbed dose in the medium is related to the

M . Bistrovic‘ et al.

416

units by the C, or C, factors, the absorbed

ionization chamber reading in ‘R’ dose in the medium is

D,(medium) = C,(E)I(E) D,(medium) = CE(E)I ( E ) .

l-

(7)

If the physical response of the cavity, r , is related to the absorbed dose in the medium by a factor +E a t energy E , then r,(cavity)

=

+,DE(medium)

r,,(cavity) = +CoDc,(medium) and r,(cavity) = Q D,(medium) r,,(cavity) EDc,(medium) where Q E = Factor @ E is the physical response (thermoluminescence, density, extinction) of the cavity per rad in medium a t energy E , normalized to unity against 6oCoradiation. Using formulae (6), (7),(8) and (9) one obtains

Knowing the factors C, and C, and measuring PE,Q E can be determined. If the physical response of a cavity is proportional to the absorbed energy in it, is different , from FE then the then QE should be nearly equal to FE. If @ physical response is not proportional to theabsorbed energy in the cavity. 2. Measurementsandresults

We used threetypes

of material for the cavities:LiF,X-ray

film with

80% AgBr, and a chemical dosemeter (Dvornik, Zec, Bari6 and Razem 1969).

The composition of the chemical was: Contentbyvolume ; 10% ethanol 10% chlorbenzene 80% izooctane

Elementalcontentbyweight 79.62% C 13.89% H 1 * 6 0 ~00 4.87% cl.

:

Extinction was measured a t a wavelength of 0.552 pm. The LiF was used in the form of chips (0.32 x 0.32 x 0-09 cm). The beam directionwas perpendicular to the plane in which the chips lay. The chemical dosemeter was a liquid in a container of dimensions 1 x 1 x 5 cm, and again the beam was perpendicular to the largest dimension. The film samples were 3 x 3 cm cut from one film sheet andtheir thickness was disregardedincalculations. For simplicity, the measurements were carried out in a lucite phantom (25 x 25 x 25 cm) and the

Energy Response of LiF, Film and a Chemical Dosemeter to Photons

417

experimentalarrangementis shown in fig. 1. All cavitymaterials and the ionization chamberwere embedded in smaller lucite blocks so that theirpositions were interchangeable and air spaces were avoided. Electron (photon 1 beam

m

All shaded ore lucite

l Ionization chamber

1

\

/

.

1

'

1

Cavity dosemeter

I

,

lb)

1 cm

Ionization

chamber

Cavity dosemeter here, 10 LiF chips)

Fig. 1. The experimental arrangment shown (a) in plan and (b) elevation. The cavity may be LiF chips (shown), film or chemical dosemeter. The cavity and ionization chamber can be interchanged. The overall dimensions of the lucite phantom are 25 x 25 x 25 cm.

The FE factors for LiF, chemical dosemeter and film emulsion have been calculated, firstly (table l a ) using electron stopping power data (eqn 3(a)) and data forpercentagereduction of collision energy loss due to density effect (Berger and Seltzer 1964), and secondly, (table lb), from eqns (4) and ( 5 ) calculating d from

d = 1 -=P ( -BP") BP"

where B = 14.0/E&Txfor LiF (Paliwal and Almond 1975), E,,, is the maximum electron energy in MeV and x is the mean path length. For our chemical dosemeter jl was calculated from the relation (Burlin and Chan 1969) exp ( -PR)= 0.01

LW.BistroviC et a,l.

418 Table la. The

Energy ~~

LiF/ lucite

film/ lucite

chemical/ lucite

LiF/ Ha0

film/ H20

chemical/ Ha0

1.000 1.007 1.008 1.009 1.009 1.009 1.009 1.010

1.000 1.096 1.116 1.133 1.141 1.144 1.146 1.125

1.000 0.998 0.999 0.999 0,997 0.996 0.994 0.999

1.000 1.001 1.001 1.009 1.010 1.011 1.011 1.005

1.000 1.090 1.114 1.134 1,143 1.146 1.148 1.120

1.000 0.996 0.999 1,003 1.001 1.000 0.999 1.002

~

soco

Electrons Electrons Electrons Electrons Electrons Electrons X-rays

FE factors calculated from eqn (3a)

5 MeV 10 MeV 20 MeV 30 MeV 35 MeV 40 MeV 42 MeV

Table lb. The

3 ' ' factors calculated

Energy

LiF/ lucite

Electrons 5 MeV Electrons 10 MeV Electrons 20 MeV Electrons 30 MeV Electrons 35 MeV Electrons 40 MeV X-rays 42 MeV

1.000 0.981 0.978 0.976 0.977 0.977 0.977 0.979

coco

from eqns (4) and ( 5 ) film/ lucite

chemical/ lucite

1.000

1.000 0.982 0.993 1.001 1.003 1.003 1,003 1.005

1.096 1.116 1.133 1.141 1.144 1.146 1.125

Table IC. The F ' factors calculated from eqns (4a) and (5)

Energy + W O

Electrons Electrons Electrons Electrons Electrons Electrons X-rays I

5 MeV 10 MeV 20 MeV 30 MeV 35 MeV 40 MeV 42 MeV

LiF/ lucite 1.000 0.973 0.974 0.974 0.976 0.976 0,976 0.979

I I

lucite film/ 1.000 1.096 1,116 1.133 1.141 1.144 1.146 1.126

1

lucite chemical/ 1.000 1.011 1.013 1.013 1.010 1.009 1.008 1.005

I

where R (g cm-2) is the range of the electrons. For the film we have taken d = 1 for all energies. Again, data for percentage reduction of collision energy loss due to density effect have been taken into account. Each cavity wasexposed to radiation simultaneouslywith an ionization chamber (diameter 0.63 cm), and the cavity and the ionization chamber were symmetrically placed in the beam in the region of its nominal homogeneity. Possible inhomogeneity and asymmetryof the beam were to some extent taken

Energy Response of L i F , F i l m and a Chemical Dosemeter to Photons 419 into account by interchanging the position of the ionization chamber and the cavity. The beam was found to be homogeneous in this region within 0.1%. Table l ( a ) shows that no significant differences should be expected when one measures in lucite instead of water. C,(Co) and C,(42 MeV) for lucite were taken from SCRAD (1971). The C, factors for lucitehave been calculated by the formula

where the average electron energyisestimated by taking into account an average electron energy loss of 2.5 MeV cm-l for lucite. 5 MeV electrons and 60Co radiation have been measured at a depth of 0.5 cm in lucite, and 10 MeV electrons a t 2 cm depth. 42 MeV X-rays were measured at 6 cm depth. We exposed 10 LiF chips (see fig. l ) , and have taken film and chemical dosemeter measurements at every chosen energy. Air spaces were avoided in all cases as previously described. values is within 2%. The results are shown in table 2. Theerror for

3. Discussion Measurement of absorbed dose in water or in a water-like medium using the physical response of cavities in the medium raises the question of the energy dependence of the physical response of the cavity per rad absorbed in the medium. It was shown (e.g. Paliwal and Almond 1975) that LiF thermoluminescent response per rad in waterdecreases 7-10% for high energy X-rays 2) and electronsrelative to the response a t 6oCoenergy.Ourresults(table confirm these data. Using eqns (4) and ( 5 ) from Almond and McCray (1970) we have obtained (table l b ) only a 2% decrease in the LiFabsorbed dose. The use of formula (4a) with d = 1 gives essentially the same results (table IC). It must be pointed out that ina medium with density near to that of a cavity, d must be taken as unityfor two reasons: firstly, because now only electrons play a role in the energy absorption, and secondly, the same reasoning about the weighting factorsshould be applied toboththe medium andthecavity. Holt, Edelstein and Clark (1975) have shown that a significant decrease of energy absorptioncould be expected onlyat relatively low electron energies, less than 10 MeV, in the case of LiF samples of thickness 2 1 mm. Since FE for LiF is equal to unity for all energies considered (see tables l a and lb) it seems that the decrease of nearly 10% in the thermoluminescent response for LiF could notbe explained by the decrease of the absorbed energy in it. This would mean that the assumption about the proportionality of the physical response of the cavity with the absorbed dose in it is probably not correct. The physical responses of our film and chemical dosemeter (i.e. the density and the extinction) have notundergone a significant decrease per rad in lucite.

420

M . Bistrovic' et al.

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Energy Response of LiF, Film and a Chemical Dosemeter to Photons

421

In the case of a film dosemeter a high absorbed energy in the cavity (see FE values in table la, lb or IC) is compensated by the decrease of its physical response. In thecase of our chemical dosemeter the absorbed energy in it follows very closely the absorbed dose in lucite (see e.g. table l a , l b or IC). The physical response seems to be (within 2%) energy independent and proportional to the absorbed dose in it.

RASUM~ La rBponse BnergBtique de LiF, du film et d’un dosimbtre chimique aux photons et Blectrons haute Bnergie La reponse physique de LiF, dufilm et d’un dosirnetre chimique dans des faisceaux de photons e t d’blectrons a haute Bnergie a BtB BtudiBe. On montre que la rBponse pour le doaimetre chimique et pour le film ne change pas sensiblement jusqu’a 42 MeV, alors que pour LiF elle diminue d’environ 10%. Une analyse des calculs et relevBs a montrB que cette diminution ne peut pas &re expliquBe par les theories existantes.

ZUSAMMENFASSUNG Die Energiereaktion von LiF, Film und einem chemischen Dosimeter auf Hochenergie-Photonen und -Elektronen Untersucht werden die physikalischen Reaktionen von LiF, Film und einem chemischen Dosimeterin Hochenergie-Photonen- und -Elektronenstrahlen. Es wird aufgezeigt, dass das Ver. halten von Film und dem chemischen Dosimeter sich his auf 42 MeV nicht wesentlich verandert, wahrend die Reaktion von LiF um rund 10% abnimmt. Eine Analyse der Berechnungen und der Messungen ergibt, dass sich diese Abnahme mit bestehenden Theorien nicht erkliiren lasst.

REFERENCES ALMOND, P. R.,and MCCRAY,K., 1970, Phys. Med. Biol., 5 , 335. BERGER, M. J., a n d SELTZER,S. M., 1964, Tables of Energy Losses and Ranges of Electrons and Positrons, NASA SP-3012. BURLIN,T. E., 1970, Phys. Med. Biol., 15, 558. BURLIN, T. E., a n d CHAN,F. K., 1967, Br. J . Radiol., 40, 556. DVORKIK, I., ZEC,U,, B A R I M., ~ , and RAZEM,D., 1969, in Handling of Radiation Accidents, STl/PUB/229 (Vienna: IAEA) p. 225. HOLT,J. G., EDELSTEIN G. R . , a n d CLARK,T. E., 1975, Phys. Med. Biol., 20, 559. JOHNS, H. E., a n d CUNNINGHAM, J. R.,1969, ThePhysics of Radiology (Springfield: Charles C. Thomas) p. 291. PALIWAL,B. R., and ALMOND,P. R.,1975, Phys. Med. Biol., 20, 547. SCRAD, 1971, Phys. Med. Biol., 16, 379.