Influence of the decay products of 222Rn on the background counting rate of a sensitive whole-body radioactivity monitor

NUCLEAR

INSTRUMENTS

AND

METHODS

i36 (I976) 585-597;

©

NORTH-HOLLAND

PUBLISHING

CO.

I N F L U E N C E OF T H E DECAY P R O D U C T S OF 222Rn O N T H E B A C K G R O U N D C O U N T I N G RATE OF A S E N S I T I V E W H O L E - B O D Y R A D I O A C T I V I T Y M O N I T O R * K.S. PARTHASARATHY

Division of Radiological Protection, Bhabha Atomic Research Centre, Trombay, Bombay-400 085, India Received 28 N o v e m b e r 1975 and in revised form 16 March 1976 The background counting rates of a sensitive whole-body radioactivity monitor and the decay products of 22ZRn have been measured simultaneously. The background counting rate and the concentrations do not show a simple relationship because of the deposition of decay products on the surfaces in the shielded space of the monitor. "Plate o u t " of decay products at concentrations of the order of I pCi/l has been clearly demonstrated. Contributions from airborne radioactivity and deposited radioactivity to the background counting rate are shown to be of the same order. Deposition of radioactivity due to the presence of electrostatic charges on the surface of polythene is s h o w n to be reduced by covering it with a conducting foil. The increase in background counting rate when uncovered polythene p h a n t o m s are used in calibration work is demonstrated. It is recommended that the use o f bare polythene p h a n t o m s must be discontinued in the light of this study. The advantage of high turnover rates o f air into the shielded space to prevent accumulation of decay products of 222 Rn is likely to be offset by the deposition of radioactivity on surfaces of synthetic materials used in the monitor. The small but variable contribution to the background counting rate from decay products of 222Rn is undesirable when scrupulous precautions are taken to reduce the traces of radioactive impurities in the materials used in fabricating the monitor.

1. Introduction

Whole-body radioactivity monitors are most frequently used in the measurement of internal contamination of occupationally exposed groups. Studies of 4°K and diagnostic investigations with the help of radionuclides administered to patients and animals have increased over a hundred percent in the past few years as shown by the I A E A (1970) directory'). If this trend continues, these instruments will be used very often to evaluate very low activity in a reasonable period of time. The lower limit of activity which can be determined by whole-body radioactivity monitors is in the range where the counting rate from the subject is barely distinguishable from the background counting rate. For isotopes used in clinical research, the activity to be assessed is in the nanocurie range/). For highly sensitive counters it is more important to keep the back ground counting rate and counting efficiency stable than to keep the background counting rate low and the counting efficiency high3). The background counting rate of a sensitive wholebody radioactivity monitor arises from the following sources: 1) radioactivity (man-made or natural) in the crystal units, in the electronic components and in the shielding materials, * This work was done in the Department of Medical Physics, The University of Leeds, England, when the author was in receipt of a Colombo Plan Study fellowship.

2) cosmic rays, 3) airborne radioactivity. Trace amounts of many radionuclides are present in any monitor. *°K and members of 238U and 232Th series constitute the important natural radioactivities. Steel is a popular shielding material and modern steel is reported to contain 6°C0. Cosmic ray produced isotopes such as Z2Na and 7Be may be ignored because of their low rate of production. Radioactive impurities in the materials used in fabricating the monitor contribute a fairly constant component to the background counting rate. The contributions from cosmic rays is a slightly varying one. G a m m a rays emitted by the decay products of 222Rn in the air surrounding the detectors have been mentioned as important contributors to the variable part of the background counting rate of different gamma ray detectors4-7). The background counting rate of a large NaI(T1) detector in the energy range 25 keV to 1.575 MeV increased by 100 c.p.m, for an increase in concentration of every pCi per liter of 222Rn in a heavily shielded steel room of 8 ' x 7 ' x 6 ' volumeS). The concentrations of Z22Rn in this laboratory varied from 0.01 pCi/1 to 2.5 pCi/1 s). In one of the investigations, the background counting rates of a gamma ray spectrometer [10.2 crux x 10.2cmNaI(T1) crystal] and the concentrations of 2zz Rn in the counting space were measured simultaneauslyg). ~t was found that the normal concentrations of a2aRn ranging from 0.1 pCi/1 to 0.4pCi/1

586

K. S. PARTHASARATHY

would affect the background counting rate only by a few percent. Equilibrium between various decay products was assumed and the filtration technique was used to obtain the concentration of 2Z2Rn in air. In another study a more accurate method of measuring 222Rn in air in the counting room was employed~°). Increase in background counting rate with increase in the concentration of 222Rn was demonstrated. A review of this topic showed that there is scope for a modified approach to the subject. 222Rn is not a gamma emitter. 214pb and 214Bi, the decay products of 222Rn are the principal gamma emitters. Measurement of the concentration of 222Rn alone without any idea of the concentrations of 2~4Bi and 2t4pb is probably of limited value. The concentrations of 2~4Bi and 2~4pb in the laboratory vary very widely. This paper deals with the influence of the decay products of 222Rn on the background counting rate of a sensitive whole-body radioactivity monitor.

3.00

~

<

,<

Fig. 1 is a plan of the laboratory. The laboratory contains two steel rooms each enclosing a whole-body radioactivity monitor, a steel room for a sensitive gamma spectrometer, a store room, instrument room and five other rooms used as offices. The laboratory has a concrete floor covered with vinyl tiles, plastered concrete walls and the partitions between the rooms are made of wood and glass. A fan (F) outside the building draws air at a high flow rate through coarse filters and a thermostaticaly controlled heater before it is distributed inside the laboratory. Each monitoring room is equipped with a separate fan (f) which draws air from the laboratory through another coarse filter. This fan maintains a flow rate of 14 000 l/min of recirculating laboratory air. There is an electrostatic precipitator fitted to the cubicle11).

3. Experiments 3.1. CONCENTRATIONSOF 214Bi, 2~4pb AND BACKGROUND

COUNTING

RATE

The experiment consisted of measuring the background counting rate of a multicrystal whole-body radioactivity monitor and the concentrations of 214Bi and 214Pb simultaneously. The monitor has been described previously 1. ~2). The concentrations of 2[4Bi and 214pb have been measured accurately by the method of least-squares analysis t 3). The concentrations of the decay products of 222Rn varied from day to day, however, if background counting rates were to be observed for a very wide range of

_

i

R

SC~,LE $1

$2

0 I

Shielded rooms for wholebody

radioactivity

2. Plan of the laboratory

-,, 2-25

IZ..O0

S 3

Stee| room meter

F

External

f

Fan f i t t e d

L

Lift

monitors for

a gamma

ventilation on the

1 2 3 I I METRES I

" P

spectro-

fan roof

of S t

o

$2

well

R

Rooms

C

Corridor

from

which air is drawn spQce

into the shietded

Fig. 1. Plan of the laboratory. concentrations, measurements had to be done for several days. This was a tedious procedure. Instead of this, the concentrations of the decay products of 222 Rn were increased by switching off the ventilation unit providing air for the laboratory, permitting 222Rn diffusing from the walls and the floor of the laboratory and that which was in air to build up significantly. The concentrations of 2~4pb and 2~ 4Bi may be increased nearly ten times by this method. It was assumed that the concentrations of the decay products do not change during the sampling period or that they can be satisfactorily described as averages ~s). Nonsystematic changes are difficult to be dealt with analytically. During the first experiment the concentrations of the decay products of 222Rn were increased by leaving the ventilator fan of the basement laboratory switched off overnight. The decay products of 222Rn built up in air. The fan fitted to the shielded rooms circulated the contaminated air around the detectors. A water-filled polythene phantom was positioned over the couch which was covered with a sheet of polythene. The background spectra of the monitor and the concen-

INFLUENCE

OF D E C A Y

PRODUCTS

1400

d

587

OF 2 2 2 R n

oj

jJ

1289

LLI

J

[]

[]

¢r 1.9 "7

B

1160

A

~

j

j

J

[]

-

[]

•

•

O O Z O 13: O C) I]3

1040

~

~

~

C

•

920

I 20

I

I 60

I

I

I

100

CONCENTRATION

I 140

OF

2148i

I~__1

I

183 ( pCi

t

220 per cubfc

I 260

t

) 300

metre)

Fig. 2. Background counting rate o f the m o n i t o r in the energy band from 530 keV to 2560 keV. Lines through triangular symbols are obtained by least-squares fit.

trations of 2148i and Z14pb were determined simultaneously for several intervals of 30 rain. Fig. 2 relates the background counting rates of tbe monitor in the energy band 0.53-2.56 MeV with the concentrations of 2148i in the shielded space. All the points in graph ' A A ' were obtained after allowing 22ZRn and decay products to build up overnight by switching off the fan. The graph 'BB' gives the result of measurements done during the period of build-up and later. The following conclusions are drawn frem this experiment.

part of the bockground counting rate. The mean barometric pressure during the measurement in which the graph ' A A ' was obtained was 75.9 cm of mercury; the graph 'BB' was obtained when the atmospheric pressure was 74.6 cm of mercury. As the zero intercept of ' A A ' is larger than that of 'BB', the difference in the value of the intercept was not due to variations in atmospheric pressure. The difference was neither due to changes in electronic equipments or in the cMibration of detectors.

1) The background counting rate increased with the concentration of 214Bi" 2) The points obtained during the build-up lie on a straight line. After the period of build-up, the background counting rate obtained did not show any simple relationship with concentration. 3) The zero intercepts which represent the background counting rate in the absence of the decay products in air are different for the two sets of results. The zero intercept consists of the background counting rate due to cosmic rays and radioactive impurities contained in the crystal units, electronic components and in the shielding materials and was expected to be a constant. The variations in cosmic ray intensity due to changes in barometric pressure will account for a small

The difference in intercepts may be due to the deposition of the decay products of 222Rn on the couch, phantom and walls of the cubicle. The part contributed by radon decay produc;s arose from that remaining in air and that deposited on various surfaces in the cubicle. When the experiment was started after leaving the ventilator unit switched off for several hours, a significant part of decay products might have deposited on the surface. The contributions from deposited activity probably lead to the large value of the zero intercept of graph 'AA'. The presence of decay products on large surfaces of the polythene phantom and polythene sheet close to the detectors lead to significant contributions to the background counting rate. The experiment was repeated after removing the polythene phantom and the couch. Graph ' C C '

588

K. S. P A R T H A S A R A T H Y

represents the relationship between the background counting rate in the energy band from 0.53 MeV to 2 . 5 6 M e V (Zl+Bi) and concentration of Z~+Bi. In this case also points obtained during the period of build-up lie close to a straight line. The zero intercept which represented the backgrotmd counting rate of the monitor with the concentrations of 2~4Bi in air reduced to zero was lower than that of the graphs 'BB' and 'AA'. This was presumably due to the absence of surfaces close to the detectors. The variation in the background counting rate of the monitor during the build-up of the decay products of 2 2 2 R n and also during their removal from the shielded space was observed. The third interesting observation was taken after the laboratory was isolated by switching off the ventilation fan providing air for the basement laboratory. Figs. 3, 4 and 5 represent the variation in different energy bands for the three experimental conditions. The variations of the concentrations of 2t+Bi with time are also indicated. The variations in background-counting rate closely

followed those of the concentrations of the two r a d i o n u c l i d e s , 214Bi and 214pb. Large fluctuations shown in fig. 5 occurred in spite of the fact that the laboratory was isolated by switching off the ['an and closing all the doors and windows of the basement laboratory. Leakage of a large mass of air bearing the decay products may account for the large variations in background counting rate. But we are aware of no mechanism which would produce such air movements. 3.2. DEPOSITION OF DECAY PRODUCTS

Experiments to study the relationship between the concentrations of decay products of Z22Rn and the background counting rate of the who!e-body monitor, revealed the importance of the deposition of the decay products of 222 R n inside the monitor. Before fabricat-

210

170 0 385

88 {-

•

O-o-OO

"O

.L:- L

110

o • - °,e , ° ' ° ' ° -

I

I

1620 kcV

TO 1980 kcV

I

I

I

I

L

~ <

0 L) c~ Z D 0 cr (D

~5

#X+X X

; o o o°

I

530

I

I

keV

I

I

.

x

"x"

I

0

I

I

I

I

t

I

I

I

I

x •x ' - ' x - x " x

1

I

I

~(_x_..x.X -x"

1

I

t

I

I

I

I

ZC~

•,9 ,,a

I

I

I

I

560

t

key

]

t

~r~

600

TO

I

I

I

kcV

~" ~

12

14

TIME

I

I

I

16

TO

I

i

I

I

i__

809 keY 1620 key

x ~ x.x+x.X,x.~x.×. il

I

J

I

x-x-x,

~

~ I

I

1620 kcV

I

TO

1980 key

"X

7 5

"X-x- -X,x.X_x,X-x-x-x

I I 0 ! CL ×.x /

I

I

I

I

[_-L_

x' i

I~00

TO

i

~1 x ~

530

k~V

2560

kcV

TO

1050 ,

x,

1000 0

I

I

I

I

~

I

x, ,x. ,x x x I

I

I

300 x

re

x LLI L) ,~ Z U 0 r.,

p,-

I

keV

200

J

10

I 18

[

I 20

I

22

( hr )

Fig. 3. Variation of background counting rate and z~,*Biin air with time, during build-up,

of

u.'~ o E

I

.o.-

./

~. ,k-&''l

,I

"k

,o-

100

keY

600

X.Xx/

.X_x.-X,x.X- - -x"

-

re

,_1

"5 0

I

I

P

560

TO

'X'x ~x.~ x.X~xX~x~x ~" I

SO

Z 0 o

t

I

1

x

~

0

Z ~-

<

_

~

490

ke:V

1

i

re

X'-x- .x.×X o -x-x

345

6

.x-.x..x-x- - -x :,c" 8 0 0 keV TO 1 6 2 0

385 .x

530

0

I

.,x_ x. -x. x '

500

E

<~

key

x- "+x-x

53O

7

2560

x- -x'x'X- *X-x-x- + ~-x 1 0 4 0 keV TO 1330 kcV

I

I

I

I

TO

x x- "X'x" ,, "'×+x' .x"

I

keY k(zV

x'X'x~,

/~'x x - -xl

-x-x"

x ~

c'l

200

0 U

",x I

x /

I

1040 1330

x,X-xx

I., ~

x"

.xx

1050

0

Z

0

x- x .

0 .< m

I

I

x

3z,5

]

A

190 175

z

I

-

E

& d

o'~"

.o" +',," "

79

~x~x

concentration

~©0

b. 1

0 10

J

12

i

i

J~°~l

I

14 16 18 TIME (hr)

20

Fig. 4. Variation of background counting rate and concentration of 21+Bi when decay products were displaced from air.

INFLUENCE

46O

I"'

?j

- .,_x. x _ -x x-X,x.X

550 keY TO

400

E C~ u

_

i

i

,

I

I

I

x.

.x" Xl-X I

x-X~ I

x ~.

J~'

xx->.

t' x

/

I{eV

}x '

TO

I

[

"'-xI

I

1

l

I~

I "'1( L

I

. . . . .J_.

10LO keV TO I ~ 2 0 k~V .x_x_ - -Xx.x _ _x.X-Xx.X

1 Z-x x'

X'x-,

2O5 0

,

1

I

1

~

i

f

x x"

1620 keY

550

235

Z

i

600

re"

Z

x

~x~X.. x/' X~x~X, BOO

IM F-<

x.X -x x-x-x,x

600 keV

0

OF D E C A Y P R O D U C T S

l

x "x

~ x'X x~x

~:-x=

I -- L _ ~ ]

_'

x. 0 (..1

X'x'x-- x x"x, x x" ,

1300 (23 Z 0

1200

530

kcV

2560

keV

x

x '"

TO

x -x x'X'x

rn

0

I

I

[

.J

I

1620 k e y x. / "

115

%X~ X%X ~

I

I

I

I ~

30 1980 x - x . x' x-x

3.2.1.

I

I

I

I

I

t

l ~ - ~ - -

k~V

"x.

100 I

"X-x .x.X-Xx i

I

I

x"

I

x-x~X'x x-x~

I

I

I

I

I

I

200 /

(2_ 100 {3...

Z

v

I..lJ

0

L)

u ~

b"

E

U

1

0 9

I

I

12

I

I

I

15

1) Deposition of radioactivity on the phantom and couch. 2) The contribution of air-borne and deposited decay products to the background counting rate. 3) Decay of the activity deposited inside the shielded space. 4) The influence of electrostatic charges on the surface of synthetic membranes on the deposition of decay products. 5) Deposition of natural radioactivity on different materials. 6) Methods to reduce the deposition of decay products on the surfaces.

x "x"

L~ U ,<

The whole-body radioactivity monitor was used to demonstrate the following:

x ×, x X.x/

re"

589

OF 2 2 2 R n

I

I

18

t

I

21

24

TIME (hr)

Fig. 5. Background counting rate and concentration of ZJ4B measured after allowing build-up of decay products.

ing highly sensitive counting equipments, different materials used in construction are tested for radioactive contamination. The contribution from such contamination may be reduced by proper choice of materials. But if the materials used in the vicinity of the detectors have a tendency to collect natural radioactive aerosols the contribution arising from this will be a variable one and it depends on the concentration of these aerosols in air and on the nature of the material. Of allied interest is the fact that water-filled polythene phantoms are used in the calibration of whole-body radioactivity monitors. Also to avoid radioactive contamination of the floor and couch impervious sheets of polythene are popularly used14). More than 28% of the monitors whose details have been published in the Directory of whole-body radioactivity monitors 1) (1970 edition) have plastic sheets as inside finish to prevent corrosion and for ease of decontamination.

Deposition of radioactiHty on phantom and couch

The whole-body radioactivity monitor was calibrated to 10 keV/channel. The external fan supplying air to the laboratory was switched off and decay products built up to a value of more than 1 pCi/1. The fan which recirculates air through the shielded space functioned normally. Two different sets of experiments were done to confirm the deposition of natural radioactivity in the monitor. In the first set, a water-filled polythene phantom normally used in calibration work was positioned on the couch which was covered with a polythene sheet. In the second set the phantom and the couch were removed. In both sets, the background counting rates were observed in consecutive 30 rain intervals after switching off the external fan. Fig. 6 shows the background counting rates in the

1220

v y v V V V

V i i

B

•

1120 •

•

o

g

0

1020

D

0 0

Dn

O

Do

V V V V v v

D ALL SHADED SYMBOLS REPRESENT COUNTING RATES WITH POLYTHENE PHANTOM IN POSITION

o n

60

O

[]

• • 0 [] ~7 n • O O'P n

°

o

I 2':0

] ,'-80

I ?20

I 960

TIME ( minute )

Fig. 6. Background counting rate of whole-body radioactivity monitor with and without polythene phantom.

590

K. S. P A R T H A S A R A T H Y

energy band from 0.53 MeV to 2.56 MeV, observed on six different days, plotted against time elapsed after the fan was switched off. Similar symbols represent measurements done on the same day. Three of the measurements were done with a water-filled phantom in position and for the remaining three, the phantom and couch were removed. The graphs indicate the following: 1) All the graphs have two distinct regions. At the beginning the counting rates increase rapidly for the first few hours. In the second region the counting rates remain reasonably constant or fluctuate slightly. 2) The maximum background counting rates recorded were higher when the phantom and the couch were in position. 3) The rate of increase in counting rate was estimated for the build-up region for the two sets. For most of the measurements this rate was larger when the phantom was positioned on the couch. 4) The background counting rate of the monitor at the beginning of each experiment was estimated from the observations during the initial period. The values obtained were different on different days. This may be partly due to the different initial concentrations of decay products in air on different days and different amount of activity deposited on the phantom and the surfaces in the counter. Table 1 gives the results of ten measurements, five with the phantom on the couch and the rest with the phantom and couch removed. The maximum back-

ground counting rates, obtained after saturation, the rate of increase in counting rate during the period of build-up, and the estimated initial background count rates are shown. The conclusions mentioned above are valid in all but three measurements. In measurements 1 and 3 with the phantom and couch removed, the rate of increase in background counting rate was appreciably higher and in measurement 3 with water-filled phantom in position the rate of increase in count rate was lower. These might be due to the differences in the amount of electrostatic charges on the phantom and the free atem fraction of radionuclides on the three occasions. Deposition is more when the atoms of the decay products remain free. Atoms attached to aerosols will not deposit readily. The free atom fraction of 218po in normal air varies considerably from day to day15). 3.2.2. Contribution of air borne and deposited activity to the background counting rate The availability of a separate ventilation unit which supplied air to the whole-body radioactivity monitor helped in estimating the contributions of airborne and deposited radioactivity to the background counting rate. The fan (F) at the external ventilation unit which supplied air to the basement laboratory was switched off tsee fig. 1). The concentration of the decay products of 122Rn bnilt up to about 1 pCi/1 in about 12 h. During the period of build-up, the fan (f) on the cubicle operated continuously to ensure that the concentrations of the decay products inside the cubicle were the same as those outside. The electrostatic precipitator was switched off during the build-up.

TABLE 1 B a c k g r o u n d c o u n t i n g rate of the whole-body r a d i o a c t i v i t y m o n i t o r in the energy ba nd 530 keV to 2560 keV duri ng build-up.

No.

E x perimental conditions

Estimated m i n i m u m b a c k g r o u n d counti ng rate (c.p.m?)

Maximum background c o u n t i n g rate (c.p.m.")

Rate o f increase of c o u n t i n g rate (c.p.m./h")

1 2 3 4 5

W i t h o u t p h a n t o m and couch

908 4-2.7 916.24.3.7 954.55:6.8 921.9 4. 6.4 943.2 4. 4.9

1136.9±6.2 1114.94.6.1 1070.44- 5.9 1017.54.5.8 1023 4-5.8

26.74-0.85 11.84-1.2 24.54- 2.2 14.9+2 10.74.1.6

1 2 3 4 5

With p h a n t o m and couch

964.5-4-6.9 908.1 + 5.6 988.04-6.3 928.1 4. 7.8 988.14- 8.9

1228.5 4. 6.4 1176.44-6.3 1150.44.6.2 1203.5 4. 6.3 1303.1 4-6.6

35.34- 2.2 37.1 4. 1.8 13.24.2 38.3 + 2.5 33.44-2.8

TI.e errors skc,,~n axe tl.e estinwAed s t a n d a r d deviations.

INFLUENCE

OF D E C A Y P R O D U C T S

The background counting rate of the monitor was computed by recording counts in successive 30 rain intervals during build-up. The background counting rate remained reasonably constant after about 12 h. The energy calibration of the monitor was 10 keV per channel. After the saturation values of the concentrations of the decay products of ZZZRn were reached, the air in the cubicle housing the monitor was isolated by switching off the fan (f) (see fig. 1) at the cubicle. The background counting rate of the monitor was recorded over successive intervals of 10rain. Measurements were continued for the next 2.5 h after the cubicle fan was switched off. A slow build-up in the background counting rate was observed during some of the experiments. After the elapse of nearly 2.5 h the external fan (F) was switched on. The fan (f) at the cubicle was left switched off. The external fan displaced the contaminated air from the corridor. After 1.5 h, the concentrations of the decay products in the corridor from where the cubicle draws air have been reduced by a factor of ten. At this moment the concentrations are the same as those in air outside the external fan. The background counting rate of the monitor remained constant during this interval indicating that there was no leakage from the cubicle to the corridor. When the experiment was repeated, a small reduction in background counting rate presumably due to leakage of air from the cubicle was observed during some of the experiments. Special precautions to prevent any leakage were not taken in any experiment. The door of the cubicle was opened wide, the fan at the cubicle and the electrostatic precipitator were

switched on. The contaminated air in the shielded space was thus replaced by relatively clean air. The precipitator had a collection efficiency of 82.5% for natural radioactive aerosol11). After leaving the door of the cubicle open for 5 rain, it was closed and the background counting rate was measured over successive intervals of 10 rain. Fig. 7 indicates the variations in background counting rate in the energy band from 0.53 MeV to 2.56 MeV (2X4Bi bands) during the three stages of this experiment. The reduction in background counting rate due to the removal of airborne radioactivity was seen. After the removal of the contaminated air with cleaner air the background counting rate was found to decrease with time. This was interpreted as due to the decay of the deposited activity. ]-he backgreund counting rate (B) of the monitor is made up of three components. B

=

Bo-FBair-FBde

B ~--- B O-t-Bale p.

1180

1300 • •

•

it

•

(1)

p .

Bo represents the contribution from cosmic rays and from long-lived radioactive impurities in the equipment itself. Bai r corresponds to the contribution from airborne radioactivity and Bdc;, is the contribution from deposited activity. When the contaminated air is replaced by relatively clean air, the contribution B,~r becomes negligible and we have

1400 00

591

OF 2 2 2 R n

/

(2)

T i

•

EXPER;,IENTALLY

0

POIHTS

OBTAINED

OBSERVED

BY LEAST

POINTS SCUARE'S

FIT

•

•

0

1120

uJ

1200

q

Q 0 0 1100

o~ lOGO[

L

1000

T

0L 16

,

I 17

i

I 18

t

I 19

i

I 20

i

I 21

TIME ( hr )

aT

I

I 20

I

I

TIME

Fig. 7. Effect of radon daughters on the background counting rate.

I

20.30 ( hr )

Fig. 8. Decay o f deposited activity.

I 21

[

592

K. S. PARTHASARATHY

The last group of measurements in fig. 7 represents the decay of the deposited activity. The background counting rate at any instant during decay may be expressed by

(3)

B = B o + B a o e -~t,

where Boo and 2 are the initial value of the contribution due to deposited activity before decay and decay constant of the deposited activity respectively. The total number of counts recorded by the monitor in an interval starting at time a and ending at time b are given by Total counts = B o ( b - a ) + TBdO (e- aa_ e-;.b) .

(4)

Eq. (4) was fitted to the total counts observed over several intervals by the method of weighted leastsquares to give estimates of B0 and Boo. Fig. 8 shows the decay of the deposited activity. Using the values of Bo and /?do, the contribution from airborne radioactivity B,~r was also computed. Table 2 represents the contributions to the background counting rate from airborne and deposited radioactivity B~ir and Bdo and the background counting rate of the monitor due to cosmic rays and radioactive impurities in the components of the monitor 80. Estimates of the decay constant of the deposited activity are also tabulated. The first two measurements were done with the water-filled polythene phantom on the couch and the last two with the couch and phantom removed. The results !ead to the following conclusions. 1) Contributions to the background counting rate from airborne and deposited radioactivity were clearly demonstrated. They were of comparable magnitude. Since the activities involved were very small the two contributions could not be measured mere accurately. 2) The value of the background counting rate of the monitor in the absence of airborne radioactivity and deposited activity was found to be different on different

days. This was possibly because contaminated air inside the shielded space was not removed satisfactorily. In these experiments, the contaminated air was replaced by cleaner air drawn from outside. Though this air was further cleaned by the electrostatic precipitator at the inlet of the cubicle, it is likely to c~)ntain different amounts of decay products on different days. This is because the collection efficiency of the precipitator is only 82.5%. A small variation in the background counting rate is likely to be due to changes in cosmic ray intensity with barometric pressure. 3) The decay constant estimated from different experiments was also found to be different. The deposited activity was expected to have a decay constant of 0.02 min -1 which is the value for mixed 222Rn daughters. In the experimental set-up this is obtainable only if the airborne component is absent. A small amount of decay products was always present in air. The airborne component might contribute in two different ~ays. Firstly, the decay products might build up in air and contribute positively to the lackground counting rates. Secondly the decay products from the apparently clean air would be deposited on surfaces near the crystal and this contribution may be significant. It is difficult to determine the decay constant more accurately because of the slnall amount of activity always present in air. For obtaining more accurate values, higher conce;ltrations of the decay products might be obtained by controlled release of activity from radon seeds. Also, the contaminated air must be replaced by activity free air, which may be obtained by storing air in cylinders for few months. 3.2.3. Deposition of radioacticity on d(fferent materials" Deposition of radioactivity was observed on sheets of nylon, polythene and PVC. A decorative plastic known as 'Fablon' is used to cover the walls of the shielded space in Leeds. Decay products were deposited appreciably when the concentrations were high. Fresh

TABLE 2 C o n t r i b u t i o n o f airb orn e and deposited radioactivity to the b a c k g r o u n d of the whole-body counter in the energy range 0.53 2.56 MeV. Date

C o u n t s per min."

Bo + Ba~r+ Bdo 5. 9.71 12. 9.71 26. 9.71 17.10.71

1355.5 4- 11.6 1089.24-10.4 1074.74-10.3 1213.55:11

Bo 937.6± 974.94966. 4886.3±

Bdo 4.6 6.5 3.8 13.5

" The errors shown are the estimated s t a n d a r d deviations.

258.74-40 51.1J=14.5 52.24-15 245.64- 11

B.~r 159.24-41,9 63.2+19 56.54-18,6 81.6±20,6

Decay constant" (min 1) 0.011964-0.004 0.029 4-0.018 0.04 4-0.024 0.0097 4-0.004

INFLUENCE OF DECAY PRODUCTS OF 222Rn sheets collected activity in all experiments. The collection efficiency was reduced as a result of prolonged exposure to air. Another synthetic membrane nown as 'Scotchtint' was tested in the same way. This is a polyster film of 0.001" thickness and it has a coating of a uniform |ayer of alumininm. Serial background spectra collected by keeping a sheet of Scotchtint on a detector revealed no deposition at normal concentrations of decay products. The vinyl tile used over the floors of the laboratory also did not collect any radioactivity under normal concentrations.

•

,"" •

LU

e.

"'"e /

z

OT 13

•

CLEAN

O

POLYTHENE ( antistatic

0""

1301

O{ 30

I

I

1

J

I I 180

10

--. O.O" l

I

I

.....

I

I 360

TIME

O ......

O .....

O .....

O ..... O--

I

I

[

I

I

( minutes

I

I 540

I

I 17

I

[ 19

I

[ 21

Part of this loss of charge may be due to neutralisation of the charge from other causes. After several days of exposure, the efficiency dinqinishes considerably. But if the concentrations of the decay products of Z2ZRn increase, the unneutralized charges help to collect radioactivity. Spraying the surfaces with an antistatic fluid reduces collection of radioactive aerosols. Fig. 10 shows the background counting rate of two detectors of the whole-body monitor. Two fresh polythene bags were placed on the two detectors. The two detectors had very similar characteristics. An antistatic aerosol spray was applied evenly on one of the bags. The background counting rate of the detector with the treated polythene bag was much less than that of the

o c.)

O..

I 15

Fig. 9. Reduction of background counting rate.

•

O"

t

T[ME [ hr )

...-4"

~p

13-9-'/1

9e0'

//'" O Z

•

~=~o60 =~

e.

•

8- 9-'71 9-9-7t

11~0,

O

150

•

• • •

3.2.4. Methods to reduce deposition on sur/aces Some atoms of the decay products of 222Rn carry charge. A large fraction of the decay products are attached to charged submicroscopic aerosols. A synthetic membrane such as polythene retains charge. The deposition on surfaces is aided by these electrostatic charges. The electrostatic charges are neutralized as a result of continuous exposure of polythene sheets to the atmosphere. Fig. 9 shows the variation of background counting rates of the whole body radioactivity monitor when a sheet of polythene was placed on the couch. The same sheet was positioned on the couch for several days. The observations taken on the same day are represented by the same symbol. The counting rate decreased after successive experiments. The efficiency of the sheet to collect radioactive aerosols was diminished by repeated exposures. This effect is likely to be the result of neutralization of charge on tbe surface of the sheet by the accumulation of charged aerosols.

(D

593

I

I

I

)

Fig. 10. Effect of antistatic spray on polythene on background counting rate.

POLYTH E N E )

594

~.. s. PARTHASARATHY

one with the untreated bag.The antistatic aerosol spray used in this experiment has an unpleasant smell. Also to give an effective coating on the surface of the polythene phantom large amounts of spray have to be used. The unpleasant smell inside the cubicle was considered a disadvantage. An alternative method is to give a conducting coating to the surfaces. Two identical sections of polythene phantoms normally used in calibration work, were left covered with a thin layer of aluminium foil for about 6 h. Decay products of 222Rn already present on the surface would have decayed by this procedure. These sections of the phantoms were positioned over two detectors. At the beginning of the measurement, the aluminium foil from one of the sections was removed. Fig. 11 gives the variation of the background counting rates of the two detectors at normal concentrations of Z22Rn taken over successive intervals of 30 min each. The detector on which the bare phantom was kept, indicated higher background counting rates. The background counting rates of the detectors were identical when they were operated without any phantom. Fig. 12 shows the background counting rates of two detectors when the concentrations of the decay products of 222Rn were increased to about 1 pCi/1 by switching off the ventilation unit of the basement laboratory. The detector carrying the bare polythene phantom recorded a higher background counting rate. Part of the gradual increase in background count-

I

'~

~

,'

210

~

/

/ ~

t /

i

•

O O

t..,,

i" ',,

,/'

2oo

=

BARE PHANTOM ALUMJNIUM COVERED

PHANTOM

i

i

i

•

,,4

"i,,,,

"~

,:

"I..1.

U-o°

190

I /

"(>---O

~'

o

: ',

",

,." ',

/ '; "/

o" ',, '~"

'b . . . .

~

" ""6'

'''~ " ',

<1> 0T 1

I 5

I 10 TIME [ hr ]

jig. 11. Effect of a cover of aluminmm on background counting ate at normal concentration of decay products.

2,sI o:

250

¢.)

M <{

225

~

z_ o°

[] []

/,

O

ALUMINIUM

COVERED

PHANTOM

200

•v

30

I 330

I 630 TIME

I 930

( rain )

Fig. 12. Effects of a cover of aluminium on the background counting rate during build-up of decay products. ing rate of both detectors initially is believed to be due to the activity build-up in air. 3.2.5. Background counting rate of the monitor To establish the background counting rate of the whole-body radioactivity monitor, the activity deposited on the phantom and the couch was reduced to a minimum by leaving the phantom and the polythene sheet on the couch covered with aluminium foil. This is expected to reduce significantly the deposited activity. The background spectrum was established by operating the monitor for 60 rain. For the next experiment the aluminium foils were removed, and the background spectrum was established for 60 rain. During both experiments, the ventilation units functioned normally. Table 3 gives the background counting rate of each of the eight detectors in the energy bands shown. The first row of numbers for each energy band represents the background counting rate of the detectors obtained with practically no deposited activity. The second set of numbers is the background counting rates when the bare polythene phantom and sheet were positioned on the couch. Table 4 contains the results of five experiments, three of them with bare polythene phantom and polythene sheet positioned on the couch and the rest with the phantom and couch covered with aluminium foil. All the five experiments were done sequentially. The first experiment was done with the covered polythene phantom and sheet, the second with the phantom and sheet bare and so on. Before each measurement with the covered phantom and sheet, an interval of four hours elapsed after covering the phantom to ensure decay of the deposited activity.

INFLUENCE

OF D E C A Y P R O D U C T S

595

OF 222Rrl

TABLE 3 Background counting rate of each o f the eight detectors (D1, D2, ..., D8) of the whole-body radioactivity monitor.

Energy bands (keV)

100-2000 530-2560 100-2560 100- 520

D1

281.74-2.2 295.1±2.3 117.24-1.4 123.54-1.4 289.74-2.2 303.74-2.3 172.54-1.7 180.24-1.7

Background counting rate o f the eight detectors (c.p.m.) a D3 D4 D5 D6

D2

314.14-2.3 327.34-2.3 136.34-1.5 137.84-1.5 323.14-2.3 336.14-2.4 186.84-1.8 198.34-1.9

314.44-2.3 318.04-2.3 139.54-1.5 139,94-1.5 322.64-2.3 327,3+2.3 183.14-1.8 187.44-1.8

266.74-2.1 272.24-2.1 108.94-1.4 112.84-1.4 274.44-2.2 279.44-2.2 165.54-1.7 186.64-1.8

283.14-2.2 295.94-2.2 116.34-1.4 120.24-1.4 291.44-2.2 304.44-2.3 175.14-1.7 184.2-/-1.8

283.54-2.2 291.44-2.2 119.54-1.4 121.34-1.4 292.34-2.2 298.94-2.2 174.84-1.7 177.64-1.7

D7

D8

296.74-2.2 307.74-2.3 122.84-1.4 129.64-1.5 304.44-2.3 316.24-2.3 181.64-1.7 186.64-1.8

299.34-2.2 303.14-2.3 123.64-1.4 124.74-1.5 307.24-2.3 311. 4-2.3 183.64-1.7 186.34-1.8

Errors shown are standard deviations. All numbers are rounded off to the first decimal place. The first row of n u m b e r s in each energy band are the background counting rates of the detectors with aluminium-covered polythene p h a n t o m in position. The second row represents counting rate with bare polythene p h a n t o m .

TABLE 4 Background counting rate of whole-body radioactivity monitor.

Energy bands (keV)

100-2000 100- 530 530-2560 100-2560

Background counting rate with bare polythene p h a n t o m s and sheet (c.p.m.) a

2406 • 1.5 1460.6 4- 1.2 1014.1 4- 1.1 2474.7 4- 1.6

2383.44-6.3 1446.04-4.9 1001.9 4-4.1 2447.1 4- 6.8

2411 4- 6.4 1467.44-4.9 1009.7 4-4.1 2477.1 4- 6.8

Background counting rate with p h a n t o m s and sheet covered (c.p.m.) a

2336.94- 2.8 N.C. 989.4 4- 1.2 N.C.

2339.6 ~: 6.2 1421 ~4.8 983.9±4.1 2404.9 • 6.3

a The errors shown are the estimated standard deviations. N.C. denotes that the values were not computed.

TABLE 5 Background counts of the whole-body monitor<

Energy bands (keV)

Mean counts with bare polythene phantom

Standard error

Standard deviation

530-2560 800-1620 560- 800 530- 730

29430 14210 9631 9316

±29.1 • 14.8 • 9.8 4- 16.56

4- 332.1 4- 168.9 4- 191 4- 188

Mean counts with aluminum-covered polythene

a The values shown are estimated from 130 spectra (30 min) of each category.

29040 14030 9384 9104

Standard error

Standard deviation

4- 19.96 4- 12.07 • 9.68 4- 10.15

4- 227.5 4- 137.6 4- 109.5 4- 115.7

596

K.S. PARTHASARATHY

those normally obtained. Covering the phantom and couch with a conducting layer reduces only the electrostatically deposited decay products of 2ZZRn. Diffusional deposition of decay products may occur under any condition. This component is larger when the amount of decay products remaining unattached is higher. The only report of the free atom ratio of 218po in the normal environment gave values varying from 7 to 40% 1s). Also, a variable contribution, comparable to that from deposited activity, arises from airborne radioactivity. Thus activity deposited due to diffusion of decay products and airborne radioactivity may account for the increased background counting rates even after reducing electrostatically deposited radioactivity.

These measurements are limited in number. Extreme variations in the concentrations of the decay products o f 2 2 2 R n may or may not have occurred during these measurements. Several hundred background spectra collected on different days showed variations in background counting rate which are not explainable in terms of counting statistics alone. To illustrate further, more than 130 background spectra were (of 30 rain duration) collected under the two conditions. Table 5 gives the mean values of background counts in the different energy bands. Statistical tests (t-test and Ftest) showed that the mean background counts and the standard deviation were low when the phantom was covered with aluminium foil. Fig. ! 3 shows histograms of the background counts in 30 rain in different energy bands when the phantom was used bare and when it was covered with aluminium foil. On some occasions the background counting rates with the covered phantom and sheet were higher than

4. Discussion

Diurnal variation in the concentrations of the decay products of 222Rn influenced the results of the

m

30 U3 Z 0

60

5 3 0 keY TO 2.56 NI~V

B

45

20

>

r'r" LU U') m 0

10

15

45 20

R

10 Z

~

z O

30

>

15

tlJ if) en 0

30

0

15

II tl.I m :~

:2

15

30

LL 0 n" LU 133

1040 k e y TO 1 3 3 0 keY

30

I.--

<

BOO kcV TO 1.62 McV

0

~

0

~

0

~

0

-/L

~

0

~

0

15

15

LO

0

LO

COUNTS

r

5 3 0 keY

0

I~

IN

0

0

30

0

0

0

0

E:)

0

0

0

0

0

560 keY TO

MIN.

TO 15

o

o

o o

o

o

o

~o o o

~o

oO

o ~ ~0 3 N g N ~ N N 0= o° o3 o

z COUNTS

IN

30

MIN.

F i g . 13. H i s t o g r a m s t o i n d i c a t e t h e effect o f a c o v e r o f a l u m i n i u m o n a p o l y t h e n e p h a n t o m

COUNTS

IN

30

MIN.

on background counting rate.

0

INFLUENCE

OF D E C A Y P R O D U C T S

experiments. Though the phenomenon of deposition of the decay products on various surfaces is well known, most of the earlier investigators did not demonstrate it, probably because they have attempted to correlate the increase in background counting rate with the concentrations of 222Rn in air. The use of polythene phantoms (employed in calibration work) on the couch must have helped deposition. But it is a normal practice in all laboratories. The importance of the contribution to the background counting rate from airborne and deposited radioactivity has been clearly demonstrated. For the same concentrations of 222Rn ' 214Pb and 214Bi may be present in different concentrations. Also, for the same concentration of 222Rn, the activity deposited depends on the type of surface left exposed in the cubicle and also on the nature of the decay product. Unattached decay products will deposit readily on any surface. Accurate quantification of the deposited activity was difficult because of the low activities and complicated geometries involved. It is customary to use high turnover rates of air into the shielded space to prevent accumulation of decay products. If surfaces which may collect natural radioactivity are left exposed, the resultant deposition of activity may off-set the advantage of high turnover rates. It is proposed that the use of bare polythene phantoms must be reconsidered in the light of this study. The small but variable contribution of deposited activity may be reduced by covering the phantom in an aluminium foil. After fabricating the shielded space, the inside finish including decorative plastics must be tested beforehand to ensure that they do not collect natural radioactivity. During some of the measurements a small increase in background counting rate was observed during the first few hours, after closing the shielded door of the cubicle. This increase is believed to be the result of build-up of decay products of 2 2 2 R n in stagnant pockets of air inside the shielded space. In the case of the monitor under study air entered a duct on the roof of the cubicle and was released at a single point. Releasing air uniformly at several points on the roof of the cubicle may prevent accumulation of decay products in the cubicle. It is known that the decrease in atmospheric pressure leads to increase in concentration of 222Rn ~6). This will be followed by an increase in the concentration of the decay products of 222Rn. As far as the background counting rate of a sensitive counter is concerned, the barometric pressure influences it in two ways. Firstly, background counting rate is increased because of the decrease in atmospheric pressure.

OF 2Z2Rn

597

Secondly by increasing the contribution from the decay products of ZZ2Rn. Since the contributions from these are of comparable magnitude, more detailed studies only can establish their relative significance. If the subjects whose body burdens have to be measured have been in rooms with high concentrations of radon, it is desirable to get them changed to new clothes after a shower. Alternatively they may be asked to remain in a room with good ventilation for an hour before measurement in order to let the radon daughter products decay. When the monitor is used in its most sensitive region, the stability of the background counting rate must be ensured by eliminating completely the decay products of 222Rn from air supply. The author acknowledges his gratitude to Prof. F. W. Spiers for providing all facilities during this work. He is grateful to Dr L. Burkinshaw and Dr D.H. Marshall for placing their knowledge of computer programming generously at his disposal. He appreciates the able assistance from his colleagues Mr R. Sadagopan and Mr R.N. Kulkarni in data processing. He is indebted to Dr U. Madhvanath for his suggestions at the time of preparing the manuscript. He is thankful to Messrs M.S. Kadam, V.P. Vartak and R.R. Lanjekar for their help in preparing the figures and to Miss M . L . Patankar for secretarial assistance. References i) Directory o[" whole-body radioactivity monitors (International Atomic Energy Agency, Vienna, 1970). 2) A. C. Morris, J. Nucl. Med. 6 (1965) 481. 3) B. Lindell, Proc. Symp. on Whole-body counting (I.A.E.A., Vienna, 1962) p. 235. 4) G. Joyet and A. Hauptman, J. Appk Math. and Phys. 16 (1965) 547. s) A. Stenberg and 1. U. Olsson, Nucl. Instr. and Meth. 61 (1968) 125. 6) S. R. Lewis and N . H . Shafrir, Nucl. Instr. and Moth. 93 (1971) 317. 7) K. V. H. Liden, Proc. Symp. on Whole-body counting (I.A.E.A., Vienna, 1962) p. 145. 8) C. E. Miller, Proc. Symp. on Whole-body counting (I.A.E.A., Vienna, 1962) p. 39. o) F. C. Kloke, E. T. Smith and B. Kahn, Nucl. Instr. and Meth. 34 (1965) 61. 1o) j. B. Corcoran and F. Markun, Argonne Natl. Lab. Report A N L 7060 (1965). ~J) K. S. Parthasarathy, Nucl. Instr. and Moth. 128 (1975) 569. ~2) A. R. Wilson, Thesis (University o f Leeds, U.K., 1964). 13) O . G . Raabe and M. E. Wrenn, Health Phys. 17 (1969) 593. i~) N. G. Trott, C. J. Parnell, H . J . Hodt and R. F. Entwistle, Brit. J. Radiol. 36 (1963) 592. ~5) M . J . Duggan and D. M. Howell, Health Phys. 17 (1969) 423. 16) N. Jonassen, Health Phys. 29 (1975) 216.