o

AE-109

The Properties of CaSO4: Mn

Thermoluminescence

Dosimeters

Bengt Bjärngard

AKTIEBOLAGET ATOMENERGISTOCKHOLM, SWEDEN 1963

AE-109

THE PROPERTIES OF CaSO^Mn THERMOLUMINESCENCE DOSIMETERS

Bengt Bjärngard

Summary;

The properties of the radio the rmolumine s cence of CaSO,:Mn,

used for dosimetry of gamma and roentgen radiation, have been in-

vestigated. The light yield of the luminophor has been determined

to 1. 5 percent for 1 MeV gamma radiation. The dependance of the

the rmolumine s cence light sum on the exposure, the exposure rate,

and the exposure time can qualitatively be described by the first

order process model, modified by a broad energy distribution of

the electron trap depths. Some applications are discussed. The

cheapness of the dosimeters, the convenient read-out, and the

broad range of measurable exposures suggest that CaSO,:Mn can

be a valuable complement to other dosimetry systems.

Printed and distributed in June 1963.

LIST OF CONTENTS

Page

1. Introduction 3

1.1. Thermoluminescence 3

.1.2. The thermoluzainophor CaSO,:Mn 5

2. Experimental 5

2. 1. Preparation of the CaSO4;Mn 5

2.2. The luminophor samples and the read-out

apparatus 7

3. Results 9

3, 1. Emission spectrum 9

3.2. Glow curve 9

3, 3. Stability of the thermoluminescence signal 9

3. 4. Light yield 1 1

3,5. Electron shielding 14

3. 6. Variation of the sensitivity with the photon

energy 15

3, 7. Dependence of the sensitivity on exposure,

exposure rate,and exposure time 16

3. 8. Influence of large exposures on the sensitivity 19

3,9. Influence of the irradiation temperature 19

3. 10. Background phenomena 19

4. Discussion 20

References 22

Table I 24

Figures 25

- 3 -

1. Introduction

This report deals with the properties of the CaSCK:Mn thermo-

luminescence dosimeter. These have been studied in connection with

an effort to develop a system for measurements of small exposures

of gamma radiation. A preliminary notice on the latter subject has

been given [ i] o

1. 1. Thermoluminescence

Thermoluminescence is a special aspect of phosphorescence. A

phosphorescent material emits light after exposure to ultraviolet light,

ionizing radiation, or some other exciting agent. The properties of

this phosphorescence can often be described with a simple model, in

which the excited electrons are trapped in metastable states, from

which they can escape and return to ground state under emission of

luminescence photons. The probability that an electron, which is caught

in such a trap, leaves it in unit time is

p = v eXp (-E/kT) (1).

Here k is Bolzmann's constant, T is the absolute temperature, and

v and E are parameters of the trap. E is identified as the trap depth

and v is a frequency factor, which can be assumed to be independent

of the temperature T.

If n is the number of electrons in (monoenergetic) traps in a

phosphor sample, n will vary with the time t and the temperature T as

^ = - n . P(T) (2).

If the probability that a photon is emitted when an electron leaves

a trap is c, the number of photons emitted per unit time by the phos-

phor sample is

I=~ c | S = C e n . p ( T ) ( 3 )

- 4

If the temperature of the sample is raised, the value of p in-

creases. This results in a momentarily increased phosphorescence

emission, provided that c, the number of photons emitted per emptied

trap, is temperature independent. In this way the phosphorescence

can be thermally stimulated. A special case occurs when the tempe-

rature of the sample must be raised considerably above the tempera-

ture during the excitation for the phosphorescence light to be emitted.

This kind of thermally stimulated phosphorescence is generally called

the rmolumine s cence.

According to eq. (2) the number of trapped electrons at the time

t is, if the temperature T has been constant,

n(t) = nQ exp (-pt) = nQ exp (-vt e"E/kT) (4)

where n is the number of trapped electrons when the excitation was

finished at t = 0. If the temperature now is raised sufficiently, all

these electrons will leave their traps in a comparatively short time.

The associated number of photons or a fraction of them can be measu-

red as a "thermoluminescence signal". This signal is independent of

the temperature variation T(t) of the sample, if the number of emitted

photons per escaping electron is 'ndependent of the temperature. Of

course, the variation of the emission with time will be different for

different temperature cycles T(t).

A special way of heating the luminophor sample is when the

temperature increases linearily with time. The variation of the light

emission with temperature is then called a "glow curve". This curve

shows one or more "glow peaks", corresponding to the emptying of

electron traps of different depths. The interpretation of glow curves

is generally complicated by continuous and often broad distributions

of the trapping levels.

- 5 -

1. 2« The thermoluminophor CaSO :̂Mn

CaSCKsMn has a siraple glow curve. Above room temperature

there is only a single glow peak, which lies at about 100 C, which

implies that the decay of the excited thermoluminescence signal is

considerable already at room t-jmperature. This limits the number of

possible applications in dosimetry but is not solely a disadvantage

since the read-out temperature can be moderate, which simplifies

the read-out procedure as well as the dosimeter construction. Other

advantages of CaSO4:Mn are that it has a high sensitivity to ionizing

radiation and that it is simple to prepare, CaSO ,!Mn has a mean atomic

number that is high compared to that of air, and a rather strong depen-

dence of the signal per unit exposure on the photon energy can be ex-

pected,

CaSO.sMn was one of the first thermoluminophors used for

dosimetry of ionizing radiation. Its thermoluminescent properties

were known before 1900, and it had been used for measurements of

ultraviolet radiation, e.g. by Watanabe [2] , when Burger, Lehmann

and Mayer [ 3J in 1954 tried it for dose measurements in radiation

therapy,

Paterson and Friedman[ 4] found CaSO.sMn favourable for

milliroentgen dosimetry, and Nosenko, Revzin,and laskolko measured

5 mR of gamma radiation with CaSO4:Mn powder [ 5] » Archangelskaja,

Weinberg and Razumova devised a method to propare monocrystals

of CaSO ,;Mn [ 6j , Recently, Spurny used monocrystals of CaSO .:Mn

in an emergency dosimeter [ 7J .

2, Experimental

2, 1, Preparation of the CaSO,sMn

Several methods to prepare thermolumine scent CaSO.;Mn have

been described in the literature. The one that has been most commonly

used [ 2, 8] is to heat a mixture of CaSO,. 2HOQ and MnSO ,• H_O to

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about 1000 C, The activating Mn ions are captured in and possibly

also diffused into the anhydrite crystals as these grow at the elevated

temperature. In common with the precipitation technique of Medlin [ 9l,

this method gives a powder of small crystals. Monocrystals of CaSO,;Mn

can be grown in a melt of CaSO,, MnSO, and NaCl [6, 7J .

The CaSO^:Mn used in this investigation was prepared by heating

a mixture of the sulphatess This method was chosen at an early stage

because it was simple and gave the largest sensitivity in a series of

samples prepared with the three methods. This result is far from de-

finite, however, since the Influence of the preparation variables on the

sensitivity was not investigated at this time.

Such a study was made only for the glow method. As a whole the

results agree with those of other Investigators. As recommended by

Watanabe [ 2] it was found favourable to mix the sulphates with diluted

sulphuric acid. If the heat treatment is performed in a slightly reducing

atmosphere, as was used by Peter [ 8] , the sensitivity can be made

3 - 4 times higher than if the sample is heated in air. The optimum

Mn concentration was found to be about one mole MnSO * per hundred

moles of CaSO,. This figure applies to the original mixture of the sul-

phates. Only a fraction of the Mn ions present is incorporated in the

crystals formed L.9I » but no effort was made to determine this fraction»

The following preparation method has been used for the CaSO ,;Mn

with which this investigation has been performed. The ingredients have

been of pro analysi purity.

CaSO.»2H«O and one mole percent MnSO.»H?O are mixed with

dilute (25 %) H-SO, to a slurry. The mixture is left at room temperature

for 16 hours. The water is driven off at 80-90 C and the sulphuric acid

at 250 - 300 °C. The mixture.is then heated at 900 °C for 45 minutes in

a slightly reducing atmosphere (N~ and SO_). The crucible is rapidly

cooled and the powder ground by hand In a mortar.

— 7 -

A glow temperature between 900 and 1000 C gives the best re-

sult and in this temperature region the glow time is not critical within

1 5 to 135 minutes. At higher temperature or longer glow time the sen-

sitivity of the luminophor decreases. This is due to the formation of

manganese oxides, which give the powder a brownish tone. When these

oxides are washed away, the sensitivity of the powder increases some-

what, but the washing has no effect for powders prepared under opti-

mum conditions.

Most of the crystals prepared in this way have a largest size of

about 50 [i. No difference in sensitivity with the size could be detected

for crystals from the same preparation. Small spots of a sample of the

crystalline powder show a greater brightness than the rest of the sample

when heated after irradiation. No satisfactory explanation for this has

been found. Evidently the sensitivity varies considerably between cry-

stals of the same preparation, but this is not determined by the dif-

ferences in size. From this it can be suspected that the sensitivity of

the luminophor can be increased, presumably with a method that allows

more sharply defined values of the preparation variables, as the mo-

nocrystal method does.

The CaSO.:Mn powder was fastened to 0. 1 mm thick Kanthal '

strips, 53 mm by 20 mm, with silicon resin (Dow Corning MC 805).

The powder was mixed with the diluted re.sin, and the slurry was pain-

ted onto the Kanthal strip. The solvent was allowed to evaporate at

room temperature for at least an hour. The phosphor samples were

then baked at 200 C for some hours. This layer is not damaged by

the heating during read-out, provided that the temperature does not

exceed 250 C.

The luminophor has been heated during read-out by a current

Kanthal is an alloy of Fe, Cr, Al and Co. The quality used has aresistivity of 1.35 ohm. mm /m.

- 8 -

through, the Kanthal strip. The temperature increase of the sample

was not measured directly; it was found to be sufficient to maintain

constant the current in the strip by manual regulation. The tempera-

ture of the sample increases with time to a limiting value that nor-

mally has been about ZOO C. The temperatu

30 seconds after the current is switched on.

mally has been about ZOO C. The temperature reaches 100 C about

The light that is emitted by the heated sample has been detected

by an EMI type 6097 S photomultiplier, selected for its relatively

small dark current. Normally the anode current of the multiplier has

been integrated by a polystyrene condenser, the voltage of which has

been measured with a Keithley model 610 electrometer. In some spe-

cial cases (glow curves, large exposures) the anode current was

amplified by the Keithley instrument and recorded. The area under

the curve was then graphically evaluated. All measurements have

been intended to integrate the thermally stimulated emission of .light

from .all the traps associated with the 100 C glow peak, i.e. a repea-

ted read-out for the same dosimeter should give a signal that is

negligible compared to the first one.

By means of a slide the sample strip was introduced in a light-

proof box containing the photomultiplier. The strip was then clamped

to the two heating current electrodes in a well reproducible position

beneath the photocathode, - The reproducibility of the measurements

of the thermoluminescent signals was within;! 3 % with condenser

integration. For large exposures the sensitivity of the photocathode

may change because of the heavy illumination. This effect was studied

and corrected for, but some uncertainty remains and with graphical

integration, used for large exposures, the reproducibility was within

:t 8 %.

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3, Resu l t s • • • .

3, 1, Emission spectrum

The spectrum of the thermally stimulated phosphorescence of

CaSO ,:Mn in the region of visible light was studied with a spectrq-

photometer (Zeiss type PMQ II), As is' shown by Figure 1 the emission

spectrum extends from about 4500 A to about 5900 A with a maximum

at 4970 Å. The full width at half maximum, is about 500 Å. Archangelskaja

et alii [ 10] reported a very similar spectral distribution, only some-

what broader. This might be caused by different temperatures of the

samples during the measurements. In our case the temperature was

about 80 °C..

Our CaSO ŝMn shows however a small component of light with

maximum intensity at about 5400 A, which has a considerably higher

thermal stability than the main phosphorescence. It showed up in the

spectrum recordings only when the main phosphorescence had decayed.

3,2, Glow.curve

The glow curve of our CaSO. :Mn was registered with the EMI

6097 S photomultiplier. The temperature of the sample was measured

with a thermocouple and the heating rate was manually adjusted. The

results agree with those of other inve stigator ss e.g. Medlin[9l»A

single glow peak with maximum at about 100 C Was found. From

the results with the spectrophotometer the glow peak associated with

the 5400 A emission was expected to show up slightly under 400 C.

However, it could not be separated from the main peak and the back-

ground of red heat glow. Probably it is identical with the peaks re-

ported by Moore [ 1 11 at about 350 C.

3. 3. Stability of the thermoluminescence signal

As was mentioned above there is an appreciable decay of the

excited thermoluminescence signal of CaSO.tMn at room tempera-

ture. This decay manifests itself as a phosphorescence. If the num-

ber of photons emitted per released electron is independent of the

- 10 -

temperature, the phosphorescent light emission is exactly equal to

the rate of decrease of the thermoluminescence light sum. Experi-

mentally, this was found to be the case for the phosphorescence at

room temperature, - The phosphorescence of CaSO .;Mn has been

studied by Medlin [l2] ,

The most important parameter for the decay of the thermolu-

minescent signal, i.e. for the phosphorescent light emission, is

the temperature after the irradiation. Because of the broad energy

distribution of the relatively shallow traps, the irradiation tempera-

ture, time, and exposure influence the rate of decay. So does the

time that has elapsed since the irradiation. Figure 2 illustrates how

the signal decays at 25 and 37 C. The initial rates of decay are 6

and 15 % per hour respectively. At -25 C the decay is very smallj

less than 10 % are lost during the first 70 hours. These data were

obtained with short irradiation times (5 minutes), the irradiation

temperature 22 C, and an exposure of 300 mR, If 10 hours are

used to give the same exposure, the initial rate of signal decay is

less than 1 % per hour at room temperature. This shows the depen-

dence of the decay on the irradiation time, and it reflects how the

distribution of trapped electrons is shifted to deeper traps when the

irradiation time increases. - The influence of the dose with constant

irradiation times was not large enough to be measured and therefore

should be negligible for most purposes.

In the measurements described below, the normal procedure

has been to read out the dosimeters one hour after the end of the

irradiation. This time includes the transport from the irradiation

laboratory, which has taken about 10 minutes at an approximate tem-

perature of 22 C. During the rest of the time the dosimeters have

been stored at 25 C. If the dosimeters for some reason could not be

read immediately after this standard storing procedure, they were

deposited at -25 C until read-out, though never for more than 8 hours.

~ 11 -

3. 4. Light

The amount of light per unit exposure that is emitted by an irra-

diated dosimeter per unit coated area must increase with the thickness

of the layer up to a saturation value. To study this, and especially to

find out at which thickness saturation occurs, a set of dosimeters were2

prepared with varying amounts of CaSO ,:Mn covering an area of 1 cm .137These dosimeters were irradiated with gamma radiation from Cs .

After the normal storing at 25 C for 1 hour, the dosimeters were

heated and the light emitted in the forward direction was measured with

a photomultiplier. The amount of light emitted through the surface of

the layer per unit mass of the luminophor varies with the thickness

of the layer in the way shown by Figure 3. It is constant for thin layers,

increases to a maximum level, and finally decreases as the thick-

ness of the layer is increased. The increase must be the result of

reflection of light by underlying grains, and the final decrease is

caused by absorption of light within the grains and in the remains of

the silicone resin. This absorption is not large. Above 100 mg/cm ,

which was the thickest layer studied, the emission per unit area con-

tinues to increase when the layer is made thicker, which means that

light from the deepest portions of the layer still reaches the surface. -

The dosimeters were covered with 4 mm perspex. during the irradiation

(cf. below)o Qualitatively the same behaviour as in Figure 3 for Cs

gamma radiation was measured for roentgen radiation with the half

value layer thickness 3. 2 mm aluminium.

For very thin layers the reflections by neighbouring grains can

be neglected. The effective reflection factor for the Kanthal support,

which was measured separately, was also found to be negligible. This

makes it possible to determine with a simple method the number of

photons emitted by the luminophor per unit exposure,

A photomultiplier was calibrated against a 1 candle tungsten

wire lamp with the colour temperature 2700 K , The solid angle of

the photocathode, which has the effective area S, could be written

as S/b steradians for the distance b used between the cathode and the

i) The lamp was calibrated by Lumalampan AB, Stockholm

- 12 -

.wire of the lamp. A current I amperes was measured. It can be shown

that

g(X) dX(5).

N(X) V(X) dX

Here N(X) is the photon spectrum of the lamp* g(X) is the spectral

sensitivity of the photomultiplier in coulombs/photon, and V(X) is the

spectral sensitivity of the eye in lumen, sec/photon. L, finally," is the

source intensity of the lamp in candles.

A thin luminophor sample was then heated in front of the photo-

multiplier. The solid angle of the cathode was in this case S/a • Since

the reflection of the luminescence photons by support and grains can

be neglected, n. S/a photons reach the photocathode if n photons are

emitted per unit solid angle. The photomultiplier gives a charge Q

coulombs at the anode and

n o (X) g(X) d\ (6)

where n (X) is the photon spec t rum of the luminescence , normal i sed

to one photon totally.

From eqs. (5) and (6)

,CN{X) g(X)

i b

2 ,CN{X) g(X) dX L. J_; ^

J N(X) V(X) dX U (X) g(X) dX

In this way the light yield of the CaSO. ,'Mn was de termined. The

function V(X) was calculated from data in a r e p o r t by Condas [ i 3 j ,

- 13 -

the function g(^) was taken from data in EMI's manuals, and. the spectrum

of the lamp N(X) was calculated from data in de Vos's tables [ 14J . For

g(X.) and N(\) the absolute magnitude drops from the final results, which

simplifies the calculations. The luminescence spectrum was taken from

Figure 1 „

12The light yield was found to be 0. 31 x tO photons emitted per;60 •

gram and roentgen of Co gamma radiation. During the irradiation

the phosphor strip was covered with a 4 mm thick per spex sheet. The

normal storing at 25 C for t hour preceeded the measurement, and

6 % of the stored light has been emitted during this time (Fig. 2).

For Co gamma radiation most of the excitation is caused by

electrons, generated in the perspex but passing into the CaSO.:Mn.

This can be seen from Figure 4. Some of these electrons are back-

scattered by the Kanthal support. This effect was studied with beta

•radiation and the increase in excitation because of the presence of the

Kanthal support should be less than 10 %. The dose to the luminophor

layer can be estimated as

D. = D • -g . l u m . . . (8)lum perspex S

if the backscattering of electrons is. neglected. D is the dose per unit

exposure and S is the mass stopping power for electrons, Perspex

can be considered as air equivalent, and the dose per unit exposure

D is then 0, 88 rad/R. The ratio of stopping powers has beenperspex ' ^r & *

estimated to 0. 92. The energy absorbed by the CaSO,:Mn when a

dosimeter is irradiated with 1 R of Co gamma radiation is then

0. 81 rad, and the yield of the luminophor is

120.31 x 10 2_photons/g.R 1 , . / , ,

- _• £ /-&:.— = _ — photons/eV.0.81 rad/R f ö

- 14 -

Since the energy of a luminescence photon is about 2. 5 eV and. one

photon is emitted per 160 eV absorbed, the energy yield is about

1.5 %, i.e. 1.5% of the energy absorbed is re-emitted as visible

light.

For LiF, Morehead and Daniels [i5J determined that 0. 005 %

of the gamma radiation energy absorbed was stored in electron traps.

Our comparisons between CaSO ,:Mn and L.iFj like that of Morehead

and Daniels from Harshaw Chemical Inc. but of a later date (1962),

give the same figure. A natural fluorite, modified by MBLE in Brussels

and kindly put at our disposal, was found to give 1. 6 times as much

light (sum of three glow peaks) as our CaSO^Mn. This figure agrees

with the results of measurements at MBLE that 100 eV are absorbed

per emitted photon for the fluorite [ 16] .

3, 5, Electron shielding

For measurements of exposures it is important that the response

of the dosimeter is determined by the flux of photons through the boundary

surfaces of the dosimeter. The electrons that pass through these sur-

faces from outside must not be allowed to give any contribution to the

signal. If they do, the signal per unit exposure will be influenced by

the geometrical arrangements of the irradiation. This is a principle

that is sometimes overlooked. Figure 4 can be used as an illustration.

It shows the signal per unit exposure for a luminophor strip covered

by various thicknesses of perspex in a collimated and in an uncollimated

beam of gamma radiation. The high value without covering perspex for

the uncollimated beam is due to electrons, generated in the environment

by the gamma radiation. The curves show that it is necessary to use

some kind of covers, thick enough to stop the most energetic electrons

present in an irradiation experiment. We have used 4 mm thick perspex

covers in all irradiations described below.

- 15 -

3, 6. Variation. of_the_ sensitivity with the photon energy

The variation of the sensitivity, i. e. signal per unit exposure,

with the energy of the electromagnetic radiation for a CaSO.sMn dosi-

meter, consisting of a Kanthal strip coated with luminophor and co-

vered with perspex, is complicated by the discontinuities in medium.

Let us restrict the discussions to the case when the photons are incident

perpendicularly upon the phosphor coated side of the strip. At low photon

.energies the exciting electrons are generated within the luminophor, but

at high photon energies, say t MeV, most of the exciting electrons are

generated within the perspex (Fige 4). However, it can be shown that

the ratio between the sensitivities for 1,25 MeV radiation (Co ) and

0.027 MeV radiation (I )[i7] is approximately given by the same re-

lation as would have been valid if the sample had been irradiated under

electron equilibrium conditions in both cases?

' °27)airs(0.027)

Here (J. is the mass energy absorption coefficient, and it is

assumed that the light yield of the luminophor is energy independent

and that the nuclear electron backscattering by the Kanthal support

can be neglected. The measured ratio between the sensitivities is

about 10 % larger than the value calculated from_eqt, .{9), The diffe-

rence can partly be ascribed to the influence of electron backscat-

tering, and it can be concluded that the light yield of the CaSO.jMn

is practically energy independent between 25 keV and 1.25 MeV.

With the 4 mm thick perspex covers, the sensitivity has a

maximum at about 30 keV that is ten times that for 1 MeV photons.

This large sensitivity for low energy photons can in some appli-

cations be annoying. It can be depressed by surrounding the dosi-

meter in its perspex cover with a metal filter» In Table I are listed

the relative sensitivities for various primary radiations with 4 mm

perspex covers and with 4 mm perspex and a 1 mm tin filter.

- 16 -

3. 7, Dependence of the sensitivity on exposure, exposure rate, and

Let us assume that a phosphor sample contains N monoenergetic

electron traps of which n are filled. This sample is irradiated with

the exposure rate R roentgens/sec. The differential equation for the

increase in the number of trapped electrons can be supposed to be

| £ ~ = aR(N-n) - np (10).

Defined by eq. (t), p is the probability that a filled trap is

emptied per unit time. The constant a can be interpreted as the frac-

tion of empty traps filled per unit exposure.

Provided that n = 0 for t = 0 and that the irradiation temperature

and the exposure rate are constant, this equation has the solution

or, if the exposure D = Rt is introduced,

n = N , -i 1 - exp l-aD(i+p/aR) | }• (12)

1 + p/aR ' r \ \ if/ i i i \ i

For large doses (aD»l) and/or long irradiation times

(aDp/aR = p t» l ) n approaches a saturation value that depends on

the exposure rate as well as the irradiation temperature. This sa-

turation value is

l i m i t 1 + p/aR

On the other hand, for small exposures (aD«i) and short

- 17 -

/irradiation times (pt«i) eq.'(12) can be approximated with

NaD (1 + p/aR) = aDN (14).

1 + p/aR

The sensitivity of this ideal luminophor with monoenergetic

traps is independent of exposure rate as well as irradiation tempera-

ture if the dose is small and the irradiation time short.

However, the CaSO.iMn has traps of a broad energy distribution.

This would not have caused any deviation from the behaviour suggested

by the equations above, if the value p = v . exp(uE/kT) had been very

small for all traps at the irradiation temperature. But this condition is

not fulfilled when CaSO t̂Mn is irradiated at or above room temperature.

To study these conditions, the constant a in eq« (10) was first de-

termined. From eq. (12) it follows that at high exposure rates, the

response can be supposed to vary with the dose as

S = constant, i 1 - «xp(-aD) 1- (15)

Dosimeters were irradiated with an exposure rate of 134 R/sec

in a Co gamma irradiation cell. The thermoluminescence signal

after the standard storing was measured. It increases with the expo-

sure according to Figure 5. The experimental points suggest a function

S = constant . j l - exp (-3. 5 x 1 0~ D) >•

where D is the exposure in roentgens. Since p is of the order of magni-

tude 0, 06 hr (Fig» 2) and R = 134 R/sec» the requirements for the

use of the approximation eqe (1 5) are fulfilled and consequently

a = 3.5 x 10~5 R~1.

- 18 -

Figure 6 shows values of the response per unit exposure, S/D,

as a function of the irradiation time for high and low exposure rates.

The solid lines are calculated from eq. (T2), For the high exposure«g , ...

rate, the choice of p is not critical since aR = 3,5x 10 x 134>0

- 19 -

3. 8, Influence of large exposures_qnj;he_sens_itivity

Bräunlich and Scharman E 1 8] found that the sensitivity of ther-

moluminescent LiF increased considerably for a sample that had been

irradiated with a gamma exposure exceeding 500 R, When the LiF sample

was subjected to a heat treatment (700- C for 1 hour) the sensitivity re-

turned to the original value.

To discover whether a similar effect exists for CaSO.sMn a set

of dosimeters were calibrated with a small exposure, irradiated with

Co gamma radiation with exposures up to 70 MR, carefully zeroed,

and re-calibrated. The new values agreed with the originals within the

measuring error» which in this case was about-- 8 %. If present at all,

the effect is cured by the zeroing process, which consisted of heating

at 300 C for a few minutes and at 250 °C for 2 hours. Since no devia-

tion from linearity between signal and exposure other than that implied

by eq. (12) has been detected, it can be concluded that the effect does

not exist for CaSO4:Mn.

temperature

The' irradiation temperature can influence the sensitivity in two

ways. When the temperature changes, the probability that an electron

is released per unit time changes. There is also a possibility that the

probability that an excited electron is trapped changes with the tempe-

rature.

Dosimeters vere irradiated with a standard dose at temperatures

between -20 and +50 C. The signal was measured immediately after

the excitation. The exposure time was short (3 minutes). No change of

the signal with the temperature during the irradiation could be detected^

and the probability that an excited electron is trapped can be assumed

to be temperature independent in this interval.

3^10. Backgroundj>henomena

One of the possible applic?tions of CaSO,sMn in dosimetry is for

measurements of small exposures. It is possible to measure 50 fiR of

1 MeV gamma radiation [ 1] . The factors governing the minimum mea-

- 20 -

surable exposure are the dark current of the photomultiplier, the

emission of incandescent light from the dosimeter when heated for

read-out, and excitation by light and mechanical disturbancies (tri-

bothermoluminescence). The dark current can be reduced by various

means; the most efficient are cooling of the photomultiplier and mo-

dulation of the light signal. The influence of the incandescent light

emission of the dosimeter can be reduced with optical filters. Exci-

tation by light and by mechanical disturbancies seems to give signals

not exceeding the equivalent of 1 mR, but make certain precautions

necessary [ 11.

4. Discussion

The reproducibility of the exposure measurements with CaSO,:Mn

has during the course of this investigation been improved to about i 3 %

for exposures of the order of magnitude t R. The absolute accuracy of an

exposure measurement is limited by the energy and the angular dependanc«

of the sensitivity and is only exceptionally as good as the reproducibility.

Unless exposures less than 1 mR shall be measured, the CaSO.;Mn

dosimeters are very convenient to read out. This 'makes them useful in

many experiments where a wide range of measurable exposures is impor-

tant. Spurnf reported [7] that CaSO.tMn can be used for measurements

of gamma radiation in reactors, since the excitation because of neutron

irradiation is small. For such a use the Kanthal support is not well

suited, however.

The large sensitivity of CaSO,:Mn can be utilized in special appli-

cations. One of these is measurements of small exposuresC i] . Another

is to construct small dosimeters for measurements of exposures in the

usual radiation protection range. There is, for instance, a need for small

finger dosimeters that can be worn without inconvenience.

In such applications as those mentioned it is essential that the irra-

diation time is short or known. Otherwise the shallowness of the traps

complicates the evaluation of the exposure, as has been described above.

- 21 -

For personnel monitoring* the use of CaSO,;Mn is restricted to short

sampling periods of the order of magnitude of one day. On the other

hand the shallowness of the traps can be made use of in emergency do-

simeters* Irradiated at a constant and small exposure rate the thermo-

lumine scene e signal will reach a saturation level after some days. This

signal is about equal to the one that would have been excited if the ex-

posure during 100 hours had "been given momentaneously, This will

normally be an acceptable background for emergency purposes.

- 22 -

References

1. BJÄRNGARD BCaSO.(Mn) thermoluminescen.ee dosimeters for small dosesof gamma radiation.Rev. Sci0 Instr. 33 (1962) 1129

2. WATANABE KProperties of a CaSO^fMn phosphor under vacuum ultravioletexcitation.Phys. Rev0 83 (1951) 785-791

3. BURGER H, LEHMANN J and MAYER UDie Anwendung der Simultandosimetrie bei der intravaginalenRöntgenbestrahlung der Parametrien.Naturwiss. 4 (1954) 209-210

4. PATTERSON D A and FRIEDMAN HMilliroentgen dosimetry with thermoluminescence.J. Opt. Soc, Am. 47 (1957) 1136-1137

5. NOSENKO B M, REVZIN L. S and IASKOLKO VUse of CaSO,?Mn for dosiraetry,Soviet Phys.-Tech. Phys. 1 (1956) 1983-1985 (English transl.)

6. ARHANGEL^KAYA V A, VAYNBERG B I and RAZUMOVA T KThermoluminescent monocrystals of CaSO.-Mn.Opt. i Spektr. 4 (1958) 681-683 (In Russian)

7. SPURNY ZEin The rmolumine szenzdo sim eter.Kernenergie 5 (1962) 611-615

8. PETER HThe rmolumine szenz und Dosimetrie von samariumaktiviertemKalziumsulfat.Atomkernenergie 5 (i960) 453-455

9. MEDLIN W LThermoluminescence in anhydrite,J. Phys. Chem. Solids 18 (1961) 238-252

10. ARCHANGELSKAJA V A, VAJNBERG B I, KODJUKOV V Mand RAZUMOVA T KLiumineszenzdosimeter fiir yStrahlen, (3-Teilchen und Neutronenauf der Grundlage des Phosphors CaSO , . Mn.Kernenergie 4 (1961) 149-152 (German transl.)

1 1. MOORE L EThermolixminescence of sodium sulfate and lead sulfate andmiscellaneous sulfates, carbonates,and oxidesJ. Phys, Chem. 61 (1957) 636-639

- 23 -

12. MEDLIN W L,Decay of phosphorescence from a distribution of trappinglevels .Phys. Rev. 123 (1961) 502-509

13. CONDAS G ATechniques for evaluating the commercial photo cathode.UCRL-6740 (1962)

14. DE VOS J CThe émissivity of tungsten ribbon.Dissertation, Amsterdam 1953

15. MOREHEAD F f ana JJANIELS F °Storage of radiation energy in crystalline lithium fluorideand metamict materials.J. Phys. Chem. 56 (1952) 546-548

16. SCHAYES RPersonal communication.

17. BERONIUS P, FORBERG S, HENRIKSON C-O andSÖREMARK RThe use of iodine-125 as an X-ray source in roentgen diagnosis,Intern. J. Appl. Radiation Isotopes 13 (1962) 253-254

18. BRÄUNLICH P and S CHARM ANN ADosimetrie mit Hilfe der Thermolumineszenz von Lithium-fluorid.Nukleonik 4 (1962) 65-67

- 24 -

Table I

Relative sensitivity of a dosimeter for perpendicularly incident

radiation

radiation

I1 2 527keV1 %l

Cs 662 keV

Co60 1.25 MeV

Roentgen?

25 kV h. v. t.

50 kV h.v. t.

1 00 kV h. v. t.

145 kV h. v. t.

175 kV h. v. t.

190 kVh.v.t .

190 kV h.v.t .

0. 6 mm

T. 35 mm

3.2 mm

0.38 mm

1,04 mm

1, 8 mm

3,3 mm

Al

Al

Al

Cu

Cu

Cu

Cu

Cover

4 mm perspeX 41

9.6

1.00

1.00

5.06

5.85

6.05

4.78

3.06

1.98

1.07

mmmm

0.

0.

0,

0.

0.

0.

0.

0.

0.

perspextin

-

96

96

007

021

11

33

64

78

88

1) h. v. t. = half value thickness

B/E'L

4500 5000 5500Wavelength, A

6000

Fig. 1. Emission, spectrum of CaSO '̂.Mn at about 80 C

after excitation with 10 kR of gamma radiation.

j I T I T

0.3

0.2

0.1 -

0.0 - J I I I 1 I l _ _ J I 1 I I i500 1000 1500

Storing time, minutes

Fig. 2. The decay of the thermoluminescent signal.

Irradiation time 5 minutes, exposure 300mR.

10fo

u

teai?o1 05CL

"5

.2

QO

I | I | i i i i i i i I I i i

i i i i i i 1 i i i i i i t i

10Thickness of layer, mg/cm^

100

Fig. 3. The light emitted per unit area and per mg of

luminophor as a function of the thickness of

the luminophor layer.

0 100 200 300 400

Thickness of perspex, mg/cm2

Fig, 4. Relative signal per exposure unit of gamma radiation

from a Co source. A uncollimated, and B collimated

radiation.

102 103 K)« 10 5Exposure, roentgens

106

Fig. 5. The thermoluminescen.ee signal as a function of the

exposure at the exposure rate 134 R/sec. The circles

show experimental values, the solid line represents

S = constant. J 1 - exp(-aD) i- with a = 3. 5 x 10"5 R~ ,

1.0

10 102 103Irradiation time, minutes

Xfi

Fig, 6. The signal per unit exposure as a function of the expo-

sure time for two different exposure rates. The circles

are experimental values, the solid lines are calculated.

LIST OF PUBLISHED AE-REPORTS

1—29. (See Ihe back cover of earlier reports.)

30. Metallographic study of Ihe isothermal transformation of beta phase inzircaloy-2. By G. Östberg. 1960. 47 p. Sw. cr. 6:—.

31. Calculation of the reactivity equivalence of control rods in the secondcharge of HBWR. By P. Weissglas. 1961. 21 p. Sw. cr. 6:—.

32. Structure investigations of some beryllium materials. By I. Fäldt and G.Lagerberg. I960. 15 p. Sw. cr. 6:—.

33. An emergency dosimeter for neutrons. By J. Braun and R. Nilsson. 1960.32 p. Sw. cr. 6:—.

34. Theoretical calculation of the effect on lattice parameters of emptyingIhe coolant channels in a D2O-moderated and cooled natural uraniumreactor. By P. Weisglas. 1960. 20 p. Sw. cr. 6i—.

35. The multigroup neutron diffusion equations/1 space, dimension. By S.Linde. 1960. 41 p. Sw. cr. 6:—.

36. Geochemical prospecting of a uraniferous bog deposit at Masugnsbyn,Northern Sweden. By G. Armands. 1961. 48 p. Sw. cr. 6:—.

37. Spectrophotometric determination of thorium in low grade minerals andores. By A.-L. Arnfelt and I. Edmundsson. 1960. 14 p. Sw. cr. 6:—.

38. Kinetics of pressurized water reactors with hot or cold moderators. ByO. Norinder. 1960. 24 p. Sw. cr. 6:—.

39. The dependence of the resonance on the Doppler effect. By J. Rosén.1960. 19 p. Sw. cr. 6:—.

40. Measurements of the fast fission factor (j) in UO2-elements. By O. Ny-lund. 1961. Sw. cr. 6:—.

44. Hand monitor for simultaneous measurement of alpha and beta conta-mination. By I. U . Andersson, J. Braun and B. Söderlund. 2nd rev. ed.1961. Sw. cr. 6:—.

45. Measurement of radioactivity in the human body. By I. O . Anderssonand I. Nilsson. 1961. 16 p. Sw. cr. 6:—.

46. The magnetisation of MnB and its variation wiih temperature. By N.Lundquist and H. P. Myers. 1960. 19 p. Sw. cr. 6:—.

47. An experimental study of the scattering of slow neutrons from HjO andD2O. By K. E. Larsson, S. Holmryd and K. Otnes. 1960. 29 p. Sw. cr. 6:—.

48. The resonance integral of thorium metal rods. By E. Hellslrand and J.Weitman. 1961. 32 p. Sw. cr. 6:—.

49. Pressure tube and pressure vessels reactors; certain comparisons. By P.H. Margen, P. E. Ahlström and B. Pershagen. 1961. 42 p. Sw. cr. 6:—.

50. Phase transformations in a uranium-zirconium alloy containing 2 weightper cent zirconium. By G. Lagerberg. 1961. 39 p. Sw. cr. 6:—.

51. Activation analysis of aluminium. By D. Brune. 1961. 8 p. Sw. cr. 6:—.

52. Thermo-technical data for DjO. By E. Axblom. 1961. 14 p. Sw .cr. 6:—.

53. Neutron damage in steels containing small amounts of boron. By H. P.Myers. 1961. 23 p. Sw. cr. 6:—.

54. A chemical eight group separation method for routine use in gammaspectrometric analysis. I. Ion exchange experiments. By K. Samsahl.1961. 13 p. Sw. cr. 6:—.

55. The Swedish zero power reactor R0. By Olof Landergård, Kaj Cavallinand Georg Jonsson. 1961. 31 p. Sw. cr. 6:—.

56. A chemical eight group separation method for routine use in gammaspectrometric analysis. I I . Detailed analytical schema. By K. Samsahl.18 p. 1961. Sw. cr. 6:—.

57. Heterogeneous two-group diffusion theory for a finite cylindrical reactor.By Alf Jonsson and Göran Näslund. 1961. 20 p. Sw. cr. 6:—.

58. Q-values for (n, p) and (n, a) reactions. By J. Konijn. 1961. 29 p. Sw. cr.6:"—.

59. Studies of the effective total and resonance absorption cross section forzircaloy 2 and zirconium. By E. Hellstrand, G. Lindahl and G. Lundgren.1961.26 p. Sw. cr. 6:—.

60. Determination of elements in normal and leukemic human whole bloodby neutron activation analysis. By D. Brune, B. Frykberg, K. Samsahl andP. O. Wester. 1961. 16 p. Sw. cr. 6:—.

61. Comparative and absolute measurements of 11 inorganic constituents of38 human tooth samples with gamma-ray spectrometry. By K. Samsahland R. Söremark. 19 p. 1961. Sw. cr. 6:—.

62. A Monte Carlo sampling lechnique for multi-phonon processes. By ThureHögberg. 10 p. 1961. Sw. cr. 6:—.

63. Numerical integration of the transport equation for infinite homogeneousmedia. By Rune Håkansson. 1962. 15 p. Sw. cr. 6:—.

64. Modified Sucksmith balances for ferromagnetic and paramagnetic mea-surements. By N. Lundquist and H. P. Myers. 1962. 9 p. Sw. cr. 6:—.

65. Irradiation effects in strain aged pressure vessel steel. By M. Grounesand H. P. Myers. 1962. 8 p. Sw. cr. 6:—.

66. Critical and exponential experiments on 19-rod clusters (R3-fuel) in heavywater. By R. Persson, C-E. Wikdahl and Z. Zadworski. 1962. 34 p. Sw. cr.6:—.

67. On the calibration and accuracy of the Guinier camera for Ihe deter-mination of interplanar spacings. By M. Mölfer. 1962. 21 p. Sw. cr. 6:—.

68. Quantitative determination of pole figures with a texture goniometer bythe reflection method. By M. Möller. 1962. 16 p. Sw. cr. 6:—.

69. An experimental study of pressure gradients for flow of boiling water ina vertical round duct. Part I. By K. M. Becker, G. Hernborg and M. Bode.1962. 46 p. Sw. cr. 6:—.

70. An experimental study of pressure gradients for flow of boiling water ina vertical round duct, Part I I . By K.M. Becker, G. Hernborg and M. Bode.1962. 32 p. Sw. cr. 6:—.

71. The space-, time- and energy-distribulion of neutrons from a pulsedplane source. By A. Claesson. 1962. 16 p. Sw. cr. 6:—.

72. One-group perturbation theory applied to substitution measurements withvoid. By R. Persson. 1962. 21 p. Sw. cr. 6:—.

73. Conversion factors. By A. Amberntson and S-E. Larsson. 1962. 15 p. Sw.cr. 1 0 : - .

74. Burnout conditions for flow of boiling water in vertical rod clusters.By Kurt M. Becker. 1962. 44 p. Sw. cr. 6:—.

75. Two-group current-equivalent parameters for control rod cells. Autocodeprogramme CRCC. By O. Norinder and K. Nyman. 1962. 18 p. Sw. cr.

76. On the electronic structure of MnB. By N. Lundquist. 1962. 16 p. Sw. cr6

77.

78.

79.

80.

81.

82.

83.

84.

85.

86.

87.

88.

89.

90.

91.

92.

93.

94.

95.

96.

97.

98.

99.

100.

101

102

103

104.

105.

106.

107.

108.

109.

The resonance absorption of uranium metal and oxide. By E. Hellstrandand G. Lundgren. 1962. 17 p. Sw. cr. 6:—.

Half-life measurements of »He, « N , " O , mf, »Al, "Sem and '"Ag. By J.Konijn and S. Malmskog. 1962. 34 p. Sw. cr. 6:—.

Progress report for period ending December 1961. Department for ReactorPhysics. 1962. 53 p. Sw. cr. 6:—.

Investigation of the 800 fceV peak in the gamma spectrum of SwedishLaplanders. By I. ö . Andersson, I. Nilsson and K. Eckerstig. 1962. 8 p.Sw. cr. 6:—.

The resonance integral of niobium. By E. Hellstrand and G. Lundgren.1962. 14 p. Sw. cr. 6 : - .

Some chemical group separations of radioactive trace elements. By K.Samsahl. 1962. 18 p. Sw. cr. 6:—.

Void measurement by the (y, n) reactions. By S. Z. Rouhani. 1962. 17 P.Sw. cr. 6:—.

Investigation of the pulse height distribution of boron trifluoride pro-portional counters. By I. O. Andersson and S. Malmskog. 1962. 16 p.Sw. cr. 6:—.

An experimental study of pressure gradients for flow of boiling waterin vertical round ducts. (Part 3). By K. M. Becker, G. Hernborg and M.Bode. 1962. 29 p. Sw. cr. 6:—.

An experimental study of pressure gradients for flow of boiling waterin vertical round ducts. (Part 4). By K. M. Becker, G. Hernborg and M.Bode. 1962. 19 p. Sw. cr. 6:—.

Measurements of burnout conditions for flow of boiling water in verticalround ducts. By K. M. Becker. 1962. 38 p. Sw. cr. 6:—.

Cross sections for neutron inelastic scattering and (n, 2n) processes. ByM. Leimdörfer, E. Bock and L. Arkeryd. 1962. 225 p. Sw. cr. 10:—.

On the solution of the neutron transport equation. By S. Depken. 1962.43 p. Sw. cr. 6:—.

Swedish studies on irradiation effects in structural materials. By M.Grounes and H. P. Myers. 1962. 11 p. Sw. cr. 6:—.

The energy variation of the sensitivity of a polyethylene moderated BF)proportional counter. By R. Fräki, M. Leimdörfer and S. Malmskog. 1962.12 p. Sw. cr. 6:—.

The backscattering of gamma radiation from plane concrete walls. ByM. Leimdörfer. 1962. 20 p. Sw. cr. 6:—.

The backscattering of gamma radiation from spherical concrete walls. ByM. Leimdörfer. 1962. 16 p. Sw. cr. 6:—.

Multiple scattering of gamma radiation in a spherical concrete wallroom. By M. Leimdörfer. 1962. 18 p. Sw. cr. 6:—.

The paramagnetism of Mn dissolved in a ond ft brasses. By H. P. Myersand R. Westin. 1962. 13 p. Sw. cr. 6:—. H

Isomorphic substitutions of calcium by strontium in calcium hydroxy-apatite. By H. Christensen. 1962. 9 p. Sw. cr. 6:—.

A fast time-to-pulse height converter. By O. Aspelund. 1962. 21 p. Sw. cr.6:—.

Neutron streaming in D2O pipes. By J. Braun and K. Randen. 1962.41 p. Sw. cr. 6:—.

The effective resonance integral of thorium oxide rods. By J. Weitman.1962. 41 p. Sw. cr. 6:—.

. Measurements of burnout conditions for flow of boiling water in verticalannuli. By K. M. Becker and G. Hernborg. 1962. 41 p. Sw. cr. 6:—.

. Solid angle computations for a circular radiator and a circular detector.By J. Konijn and B. Tollander. 1963. 6 p. Sw. cr. 8:—.

. A selective neutron detector in the keV region utilizing the "F(n, y)MFreaction. By J. Konijn. 1963. 21 p. Sw. cr. 8:—.

Anion-exchanqe studies of radioactive trace elements in sulphuric acidsolutions. By K. Samsahl. 1963. 12 p. Sw. cr. 8:—.

Problems in pressure vessel design and manufacture. By O. Hellströmand R. Nilson. 1963. 44 p. Sw. cr. 8:—.

Flame photometric determination of lithium contents down to 10-3 ppmin wafer samples. By G. Jönsson. 1963. 9 p. Sw. cr. 8:—.

Measurements of void fractions for flow of boiling heavy water in avertical round duct. By S. Z. Rouhani and K. M. Becker. 1963 32 p.Sw. cr. 8:—.

Measurements of convective heat transfer from a horizontal cylinderrotating in a pool of water. K. M. Becker. 1963. Sw. cr. 8:—.

Two-group analysis of xenon stability in slab geometry by modal expan-sion. O. Norinder. 1963. Sw. cr. 8:—.

The properties of CaSOirMn thermoluminescence dosimeters. B. Bjärn-gard. 1963. Sw. cr. 8:—.

Förteckning över publicerade AES-rapporter

1. Analys medelst gamma-spektrometri. Av Dag Brune. 1961. 10 s. Kr 6:—.

2. Bestrålningsförändringar och neutronatmosfär i reaktortrycktankar —några synpunkter. Av M. Grounes. 1962. 33 s. Kr 6;—.

Additional copies available at the library of AB Atomenergi, Sludsvik, Nykö-ping, Sweden. Transport microcards of the reports are obtainable throughthe International Documentation Center, Tumba, Sweden.

EOS-tryckerierna, Stockholm 1963