The development and application of solid state nuclear track detectors for in-core γ-dosimetry

Intsrnlrtimal 0 Pergamon

Journal

oJ Applied

Radiarion and I.woroper

Vol.

0020-708X/80/0601-0363SO2.WO

31. pp. 363 lo 369

PressLtd 1980. Printedin GreatBritain

The Development and Application of Solid State Nuclear Track Detectors for In-core y-Dosimetry* K. NADEEM, R. A. AKBER, A. HUSSAIN and H. A. KHAN? Nuclear Engineering Division, Pakistan Institute of Nuclear Science & Technology (PINSTECH), Nilore, Rawalpindi, Pakistan (Receiwd

15 September

1979)

Attempts have been made to develop solid state nuclear track detectors (SSNTDs) for the measurement of y-doses in the megarad (Mrad) range, such as exists in and around a nuclear reactor core. The changes brought about in (a) track etching parameters and (b) U.V.and i.r. transmittance have been studied as y-dose measuring indices. Effects of various core conditions such as (a) thermal/fast neutrons (b) j-particles (c) water (d) temperature and (e) y-energy spectra etc., have been investigated for their influence on th above-mentioned two y-dose measuring indices. Finally, some results concerning in-core y-dose measurements of the Pakistan Research Reactor (PARR) have been given.

1. Introduction THE TECHNIQUE of solid state nuclear track detection has now become very popular due to the many intrinsic properties of SSNTDs. They are inexpensive, very versatile in shape and dimensions and easy to analyse. Advantages of their use in nuclear reactor cores are their compactness, and the fact that they introduce only minimum flux perturbations and reactivity changes.“) Earlier, KHAN et ~1.‘~*‘)reported the effect of high y-doses on the SSNTD. It was observed that y-doses of several megarads (Mrad) not only changed the general etching velocity V, of the plastic SSNTD-like cellulose nitrates and polycarbonates, but also that track dimensions such as average and maximum track length and width were considerably modified. Since these changes took place in a very systematic manner, their use as an index for high y-dose measurement was also stated. In the present paper, we describe some experiments which were carried out to explore the possibilities of extending the scope of these detectors for in-core y-dose measurements. In this connection, both the track etching parameters and U.V. and i.r. light transmittance through various solid state nuclear track detectors have been tried as y-dose measuring indices. Initially, the measuring index vs y-dose curves have been obtained by using a large “OCo y-source. Later, the effects of reactor core environments on these curves have been studied. We *Work sponsored by the IAEA under Research Contract No. 1662/RB. Financial support is gratefully acknowledged. t Chief Scientific Investigator of the contract, to whom all correspondence should be addressed. * Manufacturers: ’ General Electric Company, U.S.A., b Kodak Pathe, France, ’ Bayer Chemicals Ltd. Germany and d Chance Brothers, England.

have tried to observe the effects of (a) accompanying radiations such as neutrons and p-particles, (b) water, (c) temperature and (d) y-energy spectra such as exist around a reactor core.

2. Experimental Details A 15,OOflCi 6oCo y-source was used for the large y-exposures of SSNTDs. A Fricke dosimeter’4) was employed to find out the does rate at the geometry of exposure which came out to be about 1 Mrad h-‘. All the doses were measured in terms of absorbed dose in a unit density material (water). The detectors employed in the present studies were L.exan,*8 CA80-15,b Makrofol-NC and soda lime glass.d For track parameters, the modifications in the etched track width of 252Cf spontaneous fission fragments were studied as a function of the absorbed y-dose. The y-exposure was given after producing the latent damage trails. In one of the experiments the modification in the latent damage cores of “‘Pb ions of 7.1 MeV/nucleon incident normally upon the detectors were also observed and compared with those of fission fragments incident in 2x-geometry. The etchant used by us was 10.7 NaOH maintained at 50 + 1°C. The detectors which were to be analysed for their optical properties were not exposed to any charged particles prior to their y-exposure. In order to observe the effect of water upon the dose-measuring indices, the detectors were exposed in a glass tube containing distilled water. Spacers were used to prevent them from sticking to each other and blocking the water column between them. The results were then compared with those obtained by simultaneous exposure under dry conditions. The effect of elevated temperature was studied by annealing the detectors at various temperatures after exposure to high y-doses. Thermal neutron exposures

363

K. Nadeem et al.

364

Gamma

l2o8Pb

x252 Cf

dose-40Mrod

I 7. I MeV/amu) fission fragments

I..._ 10

20 Etching

30

40

time tmln)

FIG. 1. Enhancement in the average track width of ‘Wf spontaneous fission fragments falling in 2x-geometry and normally incident 7.1 MeV/nucleon *‘*Pb ions with y-dose. It is quite evident that the ratio between the track widths of y-exposed and unexposed detectors is more for ‘OsPb ions. The y-dose was 40 Mrad and the etching was carried out in 10.7 N NaOH at 50 f 1°C.

3. Results and Discussion 3.1 The dose measuring indices

were made in the thermal column of the Pakistan Research Reactor (PARR) which is a swimming-pool

type reactor. The cadmium ratio was about 2ooO at the positions where the detectors were exposed. The maximum fluence attained in the present studies was about lOi neutron cmm2. The fast neutron exposures were carried out up to the maximum neutron fluence of 10’ 5 n cm-‘. The

CA ---

fast neutrons were obtained from the reaction 9B(3He, n)“C. In order to study the effect of B-particles, we used accelerated electrons of 1.2MeV which were normally bombarded upon the detectors. To observe the changes in the response of the detectors, if any, with variation of y-energy in and around a nuclear reactor core, the following experiments were performed. The SSNTDs were exposed to a y-dose through a pneumatic rabbit system for various time intervals. These exposures were carried out several days after the reactor had shut down so that the changes in the dose rates during the exposure time were very small. Chemical dosimeters were exposed in the same position. The dose rate during this experiment was about 0.4 Mrad h- ‘. Later the readings of the two types of detectors were compared.

Within the experimental errors, the results of 252Cf spontaneous fission fragment track widths in plastic detectors (CA80-15 and Makrofol-N) confirmed the results of KHAN et al. reported earlier.(2*3) Modifications in track diameter due to 7.1 MeV/nucleon 208Pb ions showed greater sensitivity to y-dose. Figure 1 shows a comparison between the fractional change in

80-15

D l60e

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I___.__-----1 400

300

(Mrad)

600

500

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L

dose

lengtll

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l

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90-15

I

1

600

900

I

;

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m 0 1 11

22 21 32.84

Mrad Mrod

44.15

Mrod

zo300

1500

I200 WOVO length

I

I900

I 2100

(nm)

FIG. 2a. Modifications created in the relative transmittance at different wavelengths of a CA80-15 cellulose nitrate SSNTD exposed to various y-doses. The inset shows that the peak value at about 525 nm can be used for measurement of y-doses as low as 0.25 Mrad.

In-core y-dosimetry

with SSNTDs

365

the average track width of fission fragments and 2osPb ions in CA80-15 at a y-dose of 40 Mrad. Further experimental work concerning use of the track parameters of heavy ions for y-dose measurements is in progress. The changes introduced in the transmitting properties of light of various wavelengths with y-dose are plotted in Fig. 2 a-c. The results have been obtained for CASO-15, Lexan and soda lime glass. One aspect worth noting is the range of y-dose covered (0.25-650.0 Mrad) by employing various detectors for their light-transmitting properties. One may select a suitable set of two or three wavelengths which cover the dose range involved in a specific experiment. The average value of the dose predicated by these measurements at these wavelengths can then be taken as a figure close to the actual y-dose.

ent from that of a “Co source,!‘) any possible changes in the relative variations of the energy absorption mass attenuation coefficients of the SSNTD as compared to that of water can affect the results. Energy absorption mass attenuation coefficients of polycarbonates (Lexan, Makrofol-N, Makrofol-E) and cellulose nitrate (CA80-15) were obtained by using the data already available for various ele-. ments at different y-energies.‘@ The results are shown in Fig. 3a. The broken line shows the energy absorption mass attenuation coefficients for water and have been drawn for the sake of comparison. It is quite evident that the discrepancies in the mass attenuation coefficients are not very large (within 10%). Thus the calibration curves as obtained for ‘j°Co can be used for the measurement of y-dose in and around a nuclear reactor core within the above-mentioned errors. The results of an experiment for the measurement of the dose in the rabbit system of the nuclear reactor (PARR) are shown in Fig. 3b. The continuous line gives the dose as measured by a chemical (Fricke) dosimeter which was previously used for the calibration of the 6oCo source. The points are for the dose, measured by employing the optical properties of CA80-15 and Lexan. The results of these measurements by two different dosimeters are in fair agreement with each other and strongly prove the already drawn conclusion of y-energy independence of optical properties of SSNTD. Similar results (not shown here) were obtained for track parameters as well.

3.2 The effect of y-energy spectra

3.3 The e&cc of water and temperature

As stated earlier, the above-mentioned dose measuring indices have been obtained and calibrated against a 6oCo source for the absorbed y-dose in a unit density material (water). As the energy spectrum for the y-rays from a nuclear reactor is entirely differ-

For most of the research reactor cores, water is present as a coolant and moderator between the fuel element plates. The SSNTD used for incore measurements may therefore be subjected to y-dose under wet conditions. Moreover the reactor core temperature

Gannna dose (Wad) FIG. 2b. Relative transmittance vs y-dose curves plotted at various wavelengths for L.exan.

0.70

0.60

I t

300

400 Gamma

500 dose

600

700

600

900

Mmd)

FIG. 2c. Optical density at different wavelengths vs. y-dose absorbed in soda lime glass. An interesting feature is the linear behaviour of the detector.

366

K. Nadeem et al. .4’

x Polycorbonates 1.0-

0 Cellulose

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I I .6’

I I .2’

Polycarbonate

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(C12H601eN4)

CA 80-15

1

.4 I 0

I 4

2

-..

Gamma

energy

--_w

L,-__.__

(MeV)

FIG. 3a. Calculated value of the energy absorption mass attenuation coefficients of polycarbonate and cellulose nitrate as a function of y-energy. The broken line is for water and has been drawn for the sake of comparison. The inset shows the relative discrepancies in the mass attenuation coefficients of the above two materials as compared to that of water.

may also alter the readings. Our observations about the effect of water and temperature can be summarized as follows: (i) The measured y-dose (by using the average track width in CA80-15) is increased when the exposure is

Fricke

dosimeter

l

CAEO-I5

320nm

L EXAN IOOOnm

,LbL

0

I

2 Exposure

3 time (hl

4

i

5

FIG. 3b. y-Dose measurement in the rabbit system of a research reactor. The line is for the chemical (Fricke) dosimeter, while the data points are for the dose as measured by using calibration curves (Figs 2a and b) for the optical properties of y-exposed L.exan and CA80-15. The agreement between the two results shows the energy independence of the SSNTD y-dosimeters (see text).

made under water. Up to a y-dose of about 90 Mrad, the increase is averaged and lies close to about 14%. The y-dose readings as obtained for the exposures carried out under water are therefore to be corrected by a factor of l/1.14 for the true values. (ii) No significant enhancements have been observed in W,,, for Makrofol-N up to these doses. (iii) Even for the same y-exposure, the fractional variation in the track width starts falling down after a certain temperature limit. This temperature limit is at about 80°C for Makrofol-N (Fig. 4) and about 65°C for CA80-15. The use of SSNTD for y-dose measurements above these temperatures, therefore, requires the application of necessary corrections to the calibration curves. (iv) Both for Lexan and CA80-15, the optical properties are perturbed in an irregular manner due to temperature and wet exposures conditions (Fig. 5). Unfortunately, the smaller discrepancies take place at wavelengths which are not very sensitive to y-dose. Consequently either entirely new calibration curves are to be obtained by considering the specific core conditions, or otherwise the discrepancies (which may be up to about 20% or even more) are to be tolerated as such.

In-core y-dosimetrywithSSNTDs

> 2.0-

I :: $

come out of the fuel plates. The presence of neutrons becomes particularly important when the reactor is in operation. Due attention has therefore to be given to their effect on SSNTD dosimeters. The results of our observations are as given below:

MAKROFOL-N

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I .o -

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0.01 0

(i) The CA80-15 is to be discarded for the in-core measurements during reactor operation due to the neutron recoil tracks which may mix up with the original tracks of the fission fragments. (ii) Up to a fast neutron fluence of 10’ 5 n crne2 (the maximum in the present experiment), the track width in Makrofal-N is not affected. (iii) Neutron fluence up to the above-mentioned limit does not change the optical properties of L.exan to a significant degree. The effect on CA80-15 is, however, quite large and the relative transmittance vs wavelength curve has almost the same pattern as that for y-rays (Fig. 6).

min

50

loo

Anneding

150

temperature

(‘0

FIG. 4. The effect of annealing temperature on the dose

measuring index Wm,JW~Xofor Makrofol-N. 3.4 Chunges due to accompanying radiation Mixed radiation originates from a nuclear reactor core. Apart from y-rays, neutrons (fast and thermal) and (although only a small fraction) B-particles also I.

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CA.

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:

x x l,oo_,_~_____“o__~__~__~__‘b____~ Bb 68

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:

0

-----'y&Y

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IO min

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0.808 1.05I

0 : b ~~___~~~~~~~~~~

B

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367

0

0

0

0

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a --g____8__.

X

o.ss-

x=y+

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A= y + 125 “c 30 min -----'y&Y

I

I

700

1100

0.85 300

orwet

Wave

I

I

1500

1900

length

FIG. 5. The effect of annealing temperature and water on the behaviour of optical properties of Lexan

and CASO-15 as a y-dose measuring index. The curves have been drawn for a y-dose of 5.0 Mrad. I 25~10’~ / _I 00x IO’5

MN

^_ ._I

r?

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5%w

T::_

l-

2 5Mrad _._

.-.---._(

’ II00 Wave

,

1300 l~co length (ny)

I

1700

1900

2lO(

FIG. 6. Modifications caused in the optical properties of Lexin and CA80-15 exposed to fast neutrons. The broken lines are for y-rays and are drawn for the sake of comparison.

K. Nadeem et al.

368

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Fuel

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20

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\

0-0 40-

/’

\

-40 i

t 20-

/

/

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.-

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20

: 0

0

20

40

60

0

20

40

1

60

0

Distance from the top (cm)

FIG. 7. Results obtained from measuring y-dose inside the fuel plates of a swimming-pool reactor.

(iv) B-particles exposed CA80-15 also show similar effects. (v) The thermal neutron fluence up to about lOI n cmm2 in the thermal column (with cadmium ratio up to 2000) does not change the behaviour of polycarbonate SSNTD. 3.5 Application for y-dose measurement The above-mentioned SSNTD y-dosimeters were used for y-dose measurements in and around a nuclear reactor core both during operation and after shut-down. The same detectors were also used for the measurement of y-doses in the spent fuel elements. An interesting application was that of the dose measurement between the fuel plates of a swimmingpool type reactor. The spacing between the two plates is less than 3 mm. The detectors attached to the stringers were lowered between this spacing and the y-dose was measured later by analysis of the detectors after recovery. The track parameters were used as a y-dose measuring index. Figure 7 shows the results of some of these measurements carried out at different times in various fuel elements. The typical behaviour of the y-dose variation along the fuel plates can be regarded as a consequence of the non-uniform burn-up along their lengths.

type nuclear

4. Conclusions The following conclusions can be drawn from the above mentioned experimental work: (i) The changes produced in the optical properties and the modifications caused in the etched track parameters of SSNTD can be utilized for the measurement of high y-doses in and around a nuclear reactor core. (iir The above-mentioned dose measuring parameters do not show any significant energy dependence from a few KeV up to several MeV. (iii) The fractional change in the average track width of fission fragments in CA80-15 reads the y-dose to a higher value by 14% if the exposure is made under wet conditions. A correction factor of l/1.14 is therefore required for finding the true y-dose. No such changes have been observed in the behaviour of Makrofol-N up to a y-dose of 90 Mrad. (iv) For annealing times of 10-30 min, the fractional change in the maximum track width of Makrofol-N starts falling from about 75°C onwards. This sort of temperature limit occurs at about 65°C for CA80-15. (v) The presence of water at temperatures above 100°C for L.exan and 60°C for CA80-15 causes considerable changes (above 10% for some wavelengths)

369

In-core y-dosimetry with SSNTDs

in the light transmitting

properties. These changes

occur in an irregular manner. (vi) Accompanying radiation (such as fast and ther-

mal neutrons) does not change the behaviour of polycarbonate SSNTDs, while the use of CASO-15 becomes impossible due to the presence of recoil tracks. The optical properties of CA80-15 are also modified by the absorption of neutrons and fl-particles. Acknowledgements-The authors acknowledge with thanks the financial help of IAEA under their contract 1662/RI/RB to meet the expenses of the present research work. The help of various centres for neutron, electron and heavy ion irradiations is also gratefully acknowledged.

AA*.31/6-c

Thanks are also due to Mr Fida Hussain Qureshi for carefully typing the manyscript.

References 1 PRICE P. B., FLIESCHERR. L. and WALKW R. M. ’ Nuclear Tracks in Solids: Development and Applications. University of California Press, Calif. (1975). KHAN H. A. Nucl. Instrum. Meth. 127, 105 (1975). 3. KHAN H. A., AK~JZRR. A., HUSSAING., HUSSAINA. and JAMILM. Int. J. appl. Radiat. Isotopes 29,49 (1978). 4. Proc. Symp. Dosimetry in Agriculture, Industry, Biology and Medicine. IAEA, Vienna (1973). 5. BENEDICTM. and PIGFORDT. H. Nuclear Chemical Engineering. McGraw-Hill, New York (1957). 6. ETHERIGNTONH. (Editor) Nuclear Engineering Handbook, 1st edn. McGraw-Hill, New York (1958).

2.