On the determination of alpha activity in airborne particulate from zircon sand using cellulose nitrate track detectors

Nuclear Tracks, Vol. 12, Nos I-6, pp. 833-836, Int. J. Radiat. Appl. Instrum., Part D Printed in Great Britain. ON

THE

DETERMINATION

FROM

ZIRCON

A.

SAND

OF

ALPHA

USING

CECCHI

a)

ACTIVITY

CELLULOSE

, C.

0191-278X/86 $3.00+.00 Pergamon J o u r n a l s Ltd.

1986.

GORI

a,b)

IN

AIRBORNE

NITRATE

and

G.

TRACK

PARTICULATE DETECTORS

ZATELLI

b)

a)Istituto Nazionale Fisica Nucleare, Largo E. Fermi 2, Firenze b)Dipartimento Fisica Sanitaria, U.S.L. IO/D, Firenze

(Italy)

KEYWORDS Nuclear

Track

Detectors

- Cellulose

Nitrate

- Alpha

activity

- Zircon

Sand

ABSTRACT An application of cellulose nitrate a~3 ~ nuclear 2 3 2 track detector for determinig the alpha activity from natural radionuclides U, Th and their daughters in zircon sand was studied. The air suspended particulate having been deposited onto Milllpore filters was examined. Using a new experimental procedure we were able to obtain accurate track detection 6 Depending on the working site, measured alpha activity ranged from 0.4 to 4.5 . i 0 0 Bq . m

INTRODUCTION Radiological hazard of alpha emitters, particularly in insoluble chemical forms, is associated mainly with the fact that radioactive particulate can be inhaled and subsequently deposited in the lungs. Therefore it is extremely important to monitor the working sites where alpha radioactive particulate may be dispersed in air. As a consequence of the radiotoxicity of alpha emitters, monitoring systems require high sensitivity to detect low concentration of alpha emitters. In addition very simple detection systems are needed since the monitoring has £o be carried out in working areas where no specialized personnel and equipment may be locally available. The choice of plastic track detectors et al. 1981; HASHEMI - NEZHAD et al.

(PARETZKE 1977; SOMOGYI 1980; 1982), in particular cellulose

KHAN 1980; HENSHAW nitrate as used in

our experiment, satisfies the two above-mentioned requirements of sensitivity and semplicity. The object of our work was to investigate if cellulose nitrate detectors could be used to determine the air concentration of alpha activity from zircon sands, commonly used by the ceramic industry. Other methods of detection of alpha activity in zircon sand have been studied (BOOTHE et al. 1980; BERGAMINI et al. 1985). The method we have developed is based on a previous study carried out by our group (CECCHI et al. 1985). In that paper the reproducibility of the tracks produced by alpha particles impinging onto the cellulose nitrate detectors with angles of incidence ~ 4 5 ° and energy values > 1 MeV was established. We gives the most reliable results.

MATERIALS

AND

also

proved

that

an

etching

time

of

about

i00

minutes

METHODS

A factory, which produces materials used in the ceramic industry, in the suburbs of Florence was selected for our measurements because of the large amount of zircon sand employed.

833

834

A. C E C C H I et a~.

Three

work

different sand a

sites

c~loring

7

cm

two

within

manipulation

the

factory

procedures

components).

area

of

months.

At

Millipore

The

layer

were

(i.e. each

filters,

collected

identified.

sand site

the

0.45

was

storage,

airborne

~ m

< 5 mg

Each

them

milling

was and

particulate

pore~size,_ . cm

of

sand

characterized mixing

was

of

collected

for 4 - 12 hours_l every

. A i0 - 15

1

. min

by

diverse onto

day

pumping

for

system

was u s e d for the c o l l e c t i o n of the samples. The air volume was m e a s u r e d with a gas flowm e t e r c o n n e c t e d to the pump; v a l u e s r a n g e d from 5 to 15 m 3 (MARSHALL et al. 1980). The conce n t r a t i o n of a l p h a a c t i v i t y in 46 samples o f a i r b o r n e p a r t i c u l a t e was determined. ly

available

200

detectors. To o b t a i n from

45 °

angle and

reproducible

to

90°(normal

selection the

~ m thick

was

the

of

its

the

filters

tracks

only

obtained

detectors.

The

low

particulate

of c e l l u l o s e

incidence) by

nitrate, particles

allowed a

to

of

a

with

reach

collimator

consists

CA 8 0 - 1 5

at

angles

the the

stainless

in

plastic a

6N

radioactivity,

exposed

at

a

60

°C,

for of

detectors

NaOH

/

onto to

constant in

105

a

the

steel

/

were

(blanks). approx.

The

also

air

ranging

between

1 mm

the

thick

Limit filters

plate

with

collimator

of bath

each

run

airborne particulate filter

irradiated processed

background

was

5 tracks per mm .

The the was

In non

incidence detectors.

cellulose nitrate detector

etched

temperature

some

as track

maintained

thermostatic

minutes.

measurement

detectors

were

solution,

of

contact

Commercial-

were used

plastic

d e t e c t o r s for a b o u t one week. The

type,

(Fig. i).

deposited

was

alpha

were

placing

collimator

n i n e h o l e s 1 mm in d i a m e t e r Because

sheets

Fig. 1

concentration

alpha activity (Bq . m determined from the total

c e l l u l o s e n i t r a t e detectors,

E x p l o d e d view of the e x p e r i m e n t a l setup.

_~f ) number

of

tracks

counted

u s i n g an o p t i c a l m i c r o s c o p e

in

the

irradiated

area

of

the

(Fig. 2).

50~m

1

Fig.

2.

Optical

t h r o u g h the was a m i x e d the

zircon

showed

(B).

can be seen.

microscopic

p h o t o g r a p h as e x a m p l e of c e l l u l o s e n i t r a t e d e t e c t o r

c o l l i m a t o r 239 described241in the nuclides ( Pu Am sand The

samples

and

variation

which

~er

produces

of tracks

forms

.... !

irradiated

(A) . The source u s e d in this e x p o s u r e Cm) source w h i c h has a h i g h e r a c t i v i t y than more

numerous

in f u n c t i o n

tracks.

A

of the d i v e r s e

In spite of the d i f f e r e n t forms all tracks are detectable.

particular angles

is

also

of i n c i d e n c e

=-ACTIVITY IN AIRBORNE PARTICULATE

835

The alpha particles with angles of incidence within the stated range were all detectable in a single field of view. In order to increase the counting statistics, nine irradiated fields of view, corresponding to the number of holes, were examined. The air concentration of alpha activity was calculated using the following formula: S 4~ 1 A = C .... . (1) s [~" TV where: C S s V T ~=

= total number of tracks in the irradiated area = total area of the deposited particulate layer = detector area irradiated through the collimator sampled air volume (m ) = irradiation time (seconds) average solid angle subtending the source through the collimator onto plastic detector. Details concerning t h e ~ calculation are reported in our previously mentioned paper.

RESULTS

sampling site

AND

DISCUSSION

n. of samples

Table 1 air volu~e rsr~e particulate concentraticn (m-)

i n a i r (rag . m"-3)

aactivity o~centrati~n i n a i r (10

Bq . m - )

Min.

Max.

Mean

Storage

15

6 - 15

0.41

0.4

2.3

1.0

Milling

19

5 - 12

0.49

0.8

3.6

1.9

Mixing

12

5 - 13

2.30

1.2

4.5

2.1

The minimum, maximun and mean concentrations of alpha emitting activity air samples, at the different working sites, are reported in Table i. The spread of concentration values found in the airborne particulate of plastic detectors, exposed to the the same i n e a c h of the two detectors

in the

collected

was due to the diverse daily content of zircon sand samples. In fact comparisons between several couples same samples, showed that the number of tracks was about within the limits of statistical errors.

The a ~ r a g e concentration of the alpha activity in the collected particulate was 21 Bq g The analysis of a sample of zircon sand, using Ge(Li) gamma-spectroscopy routinely employed in our laboratory, gave results in agreement with those reported by Boothe et ~ J ~ and Bergamini et al. These data permitted the calculation (according to the decay -i of U and 23~2Th) of the total alpha activity as 35 Bq . g . The difference in values found using plastic detectors and gamma-spectroscopy was probabily due to the fact that t h e zircon sand airborne particulate contained inactive material, thus explaining the lower values of the cellulose nitrate detectors. This method could therefore permit the determination of alpha activity of zircon sand in spite of the presence of inactive components. However our data are based on the fact that the layer of deposit was always < 5 mg.cm -2 so that the energy from the alpha particles never decreases below 1 MeV. On the other hand the validity of our method was proved for energies up to 5.5 MeV; therefore the calculation of the alpha activity may be slightly under estimated because of the presence of alpha particles of higher energy. The

sensitivity

of

our

method

was

defined

as

the

airborne

alpha

activity

capable

of leaving a track density equal to two standard deviation of the background. The limit of sensitivity for an airborne particulate sample, with a volume of i0 m 3 and a3one wee~ irradiating time, was calculated according to equation (i), and was equal to 2 . i0 Bq . m . The angle of incidence range (45° - 90°), which establishes the 10% efficiency for detection of all alpha particles emitted in a 2 ~ solid angle, must be said to constitute the principal limitation of sensitivity of our method.

A. CECCHI et al.

836

CONCLUSION By means of CA 80-15 cellulose nitrate detectors the effectiveness and simplicity of a collimator as used in our experimental procedure for the determination of alpha activity in a thin layer of alpha emitting materials, were proved in relation to zircon sand. Based on the choice of optimal etching conditions in relation to energy and angle of incidence of alpha particles on the CA 80-15 cellulose nitrate detector, the method will be further studied utilizing diff~erent types of cellulose nitrate detectors. We are planning to evaluate their practical application in those fields where high sensitivity is required,

such as radiation protection.

REFERENCES Bergamini

M., Borio R., Campos-Venuti

G. et al., in press, Radiation protection

aspects o f t he

use of zircon sand, Sci. Total Environ. Boothe G.F., Stewart-Smith

D., Wagstaff D. and Dibblee M., 1980, The radiological

aspects

of

zircon sand use, Health Phys. 38, 393-398. Cecchi A., Gori C. and Zatelli G., Quantitative determination of superficial alpha a c t i v i t y by means of plastic track detectors, submitted to Nucl. Instr. and Meth., 1985. Hashemi-Nezhad S.R., Green P.F., Durrani S.A. and Bull R.K., 1982, Effect of etching conditions on the bulk-etch rate and track-etching response of CA 80-15 cellulose nitrate, Nucl. Instr.

and Meth. 200, 525-531.

Henshaw D.L., Griffith N. and Landen O.A.L., response of CR 39 plastic

track detector,

Khan H.A., 1980, Track registration detectors,

Nucl.

Instr. and Meth.

Marshall M. and Stevens D.C., rne particulate

radioactive

1981, Effect of various etching solutions

Nucl.

Instr. and Meth.

and development

efficiencies

on the

190, 415-421. of solid state nuclear track

173, 43-54.

1980, The purposes,

materials,

methods and accuracy of sampling for airbo-

Health Phys.

39, 409-423.

Paretzke H.G., 1977, Results of an international alpha particle registration intercomparison with solid state nuclear track detectors, Nucl. Instr. and Meth. 147, 133-137. Somogyi G., 1980, Development

of etched nuclear tracks,

Nucl.

Instr. and Meth.

173, 21-42.