HS-RP/008/PP CERN HS DIVISION REPORT

PERSONNEL NEUTRON BY MEANS OF CELLULOSE NITRATE FILM COMBINED WITH LiF AS BOTH RADIATOR AND TLD

B.J. Tymons *) and J.W.N. Tuyn

Geneva - 23 August 1976

(Submitted to "")

*) Now at Chalk River Nuclear Laboratories, Canada. PERSONNEL NEUTRON DOSIMETRY BY MEANS OF CELLULOSE NITRATE FILM COMBINED WITH LiF AS BOTH RADIATOR AND TLD

B.J. Tymons *) and J.W.N. Tuyn Health and Safety Division, CERN, 1211 Geneva 23, Switzerland

(. 1. INTRODUCTION

The systems currently in use for personnel neutron dosimetry in general

do not perform satisfactorily, owing to fading or an incorrect response

as a function of neutron energy. The aim of the present study was to develop

a system which would perform correctly in the strongly varying neutron spec-

tra found behind the shielding of high-energy proton accelerators, covering

a wide neutron-energy range. This requires at a minimum a double element

device detecting low-energy neutrons and fast neutrons separately.

A system developed at Rutherford High Energy Laboratory (Ha71) makes

use of the high sensitivity of 6LiF thermoluminescence detectors (TLD)

to low-energy neutrons and the NTA emulsion response to fast neutrons.

• The system described here similarly uses 6 LiF, but as a radiator and TLD, combined with cellulose nitrate foil for fast neutron detection. The main

advantage of this combination is that the 6 LiF slow-neutron and fast-neutron

responses are both permanently recorded on one element, the cellulose-

nitrate foil, since the 6 LiF in contact with the foil acts as a radiator

producing holes in the foil from the 6 Li(n,a)T reaction. The foil has a

good sensitivity to fast neutrons and has the advantage, for the purposes

*) Now at Chalk River Nuclear Laboratories, Canada. - 2 -

of neutron dosimetry, of being completely insensitive to gamma-rays. A

7 LiF TLD was also included in the for estimation of gamma-ray

dose. It was found that the dosimeter could provide information on:

a) fast-neutron dose equivalent from the holes created by interaction of

neutrons in the foil;

b) slow-neutron dose equivalent from the holes created in the foil by

the (n,a) reaction in the 6 LiF radiator;

c) slow-neutron dose equivalent, after correction for gamma-rays, from

the 6 LiF TLD reading with gamma-ray dose estimation from the 7 LiF TLD

reading.

An initial investigation concerning the response and sensitivity of

the system with calibration sources has been previously described (Ty73).

2. PRACTICAL DETAILS

The system, consisting of LiF material, foil and holder, is shown in

Fig. 1. The holders were constructed of polyethylene. The TLD materials

used were 6 LiF and 7 LiF Teledyne Teflon discs and Harshaw chips. However

only the Teflon discs would be used in the final system, because the

chips -- although producing a higher hole density from alpha-particles in

the foil than the discs -- do not cover a sufficient area of the cellulose

nitrate foil to achieve reasonable statistics for hole counting. It was

observed that the ratio between the hole density in the foil under the

chip and disc was not always related to their relative 6 LiF content. It

was also influenced by the difference in the self-shielding effect which

exists for the chip and disc at low neutron energies. The cellulose nitrate

foil, Kodak LR115, consisted of an 8 µm thick layer of cellulose nitrate,

incorporating a red dye, which was attached to a transparent polyester

backing. - 3 -

The polyethylene , as shown in Fig. 1, were exposed on

a water-filled elliptical cylinder 35.5 x 21.6 x 60 cm used as a phantom.

The discs and chips were read on a Teledyne Isotope 7300B, with a

modified heating plate for the chips. After 90 min etching in a 10% NaOH

solution at 60°C the holes in the cellulose nitrate were counted under a

microscope. A green filter was used to make the holes clearly visible as

light green spots on a dark background.

3. RESULTS AND DISCUSSION

( Fifteen sites were chosen for the exposures. Most of these were

areas of well-known radiation composition at the CERN 28 GeV Proton

Synchrotron. The neutron spectrum at these sites varied from spectra

' containing a large fraction of low-energy neutrons, such as those found

opposite access labyrinths, to hard spectra, such as those found behind

the shielding of target areas. The dose equivalent was estimated for

each of these sites with the CERN radiation survey instrumentation (Ho72).

It was found that, in general, the neutron incidence was not isotropic,

so that for a better estimate of dosimeter response to neutrons unmoderated

by the phantom it should have been rotated through 180°. Some indication

of the 1sotropy of the radiation field could be obtained by attaching

dosimeters to the front and the back of the phantom.

The hole density per rem found in the foil after exposure to a PuBe

source was a factor of 3 lower than that found for a typical area behind

the shielding of the accelerators. However, 14 MeV neutrons produced a

hole density per rem close to that of the typical radiation field. From

the results, it was possible to establish for the cellulose nitrate foil

a semi-empirical formula giving an approximation to neutron dose equivalent: - 4 -

Neutron dose (mrem) = 5.2[:o~ + N0 ]

N = holes/cm2 under 6 LiF disc u N holes/cm2 in outside area. 0

The factor, 200, by which N is divided, was de~ived from the relative u response of a simple albedo dosimeter containing 6LiF at different neutron

energies (Al74).

The above equation gave an average neutron dose equivalent which was

within ±25% of the survey measurement. This can be seen in Fig. 2, where

the ratio of neutron dose equivalent measured with the foil(~), and

also nuclear emulsion to neutron dose equivalent measured with the survey

instrumentation (H ), is plotted against spectrum index (hardness). s

The spectrum index was obtained from the ratio N /N • The values 0 u

of ~/Hs for the LR115 foil are grouped around the value unity across

the spectrum. This shows rather conveniently that an acceptable neutron

dose equivalent estimate can be determined independently of the neutron

spectrum, while the NTA emulsion may show considerable deviations, owing

to its poor neutron energy response (not included in its error bars).

With an accepted minimum count level of 10 holes/cm2 and with a

background level of approximately 15 holes/cm2 (the foil was not annealed

before exposure), it can be seen that a minimum of approximately 50 mrem neutron dose equivalent can be detected. This value can be measured with an accuracy within the limits given by the ICRP (ICRP 68).

When used for personnel dosimetry different processing routines could be followed depending on radiation environment. One suggested routine is as follows: - 5 -

a) Each month read out the TLD discs. From these results obtain an indica­

tion of neutron dose {from a suitable calibration) and gamma-ray dose.

b) A decision is then made whether to develop the cellulose nitrate foil

or, because there is a low neutron dose indication, leave it in the

dosimeter to integrate for a further period.

c) After six months, or even one year, the cellulose nitrate foil would

be developed and neutron dose estimated from the equation given pre­

viously.

It should be emphasized that the equation for determination of the

neutron dose equivalent is only valid when the device is used behind the

shielding of high-energy proton accelerators. The use of the dosimeter

in different radiation environments, for example nuclear reactors, would

require simple recalibration.

4. CONCLUSION

From the results it can be seen that the system operates satisfactorily

as a personnel dosimeter for neutrons with improved accuracy for neutron

dose estimation compared with the NTA film now used at CERN.

The problem of adapting the proposed system to routine evaluation

would require further study. With present technology it would appear

possible to adapt some method to assess the hole density automatically.

The Quantimet image analyser seems most promising {Po75), or alternatively,

assuming that the cellulose nitrate layer can easily be removed from the

polycarbonate backing, a spark counting technique could be employed {Be74).

Acknowledgements

The authors wish to thank Dr. J. Baarli for his interest and guidance

throughout the project. The cooperation of Mr. J. Dutrannois in the film­

badge evaluation and Dr. M. Hoefert in the survey dose measurements is grate­

fully acknowledged. - 6 -

REFERENCES

Al74 Alsmiller R.G. Jr. and Barish J., 1974, The Calculated Response of 6LiF Albedo Dosimeters to Neutrons with Energies $ 400 MeV, Health Phys. ~' 13.

Be74 Becker K. and Abd-el-Razek M., 1975, Automatic Spark Counting of Fast Neutron Induced Recoil-particle Tracks in Polymer Foils, Nucl. Instrum. Meth. 124, 557.

Ha71 Hack R.C., 1971, Personal Fast Neutron Dosimetry around Nimrod -­ a Third Note, Rutherford High Energy Laboratory Report, RHEL/M-R8.

Ho72 Hoefert M., 1972, Dose Equivalent and Quality Factor of Radiation from High Energy Accelerators, Proc. First Symp. on Neutron Dosimetry in Biology and Medicine, Munich, 1972; p. 873. (Luxembourg: Connnission of the European Connnunities).

ICRP 68 International Commission on Radiological Protection (ICRP), 1968, Publication 12, General Principles of Monitoring for the Protection of Radiation Workers (Oxford: Pergamon Press).

Po75 Polig E., 1975, Automatic Determination of High a-Track Densities in Polymer Detectors, Int. J. Appl. . Isotopes 26, 519.

Ty73 Tymons B.J., Tuyn J.W.N. and Baarli J., 1973, System for Personnel Dosimetry in Mixed Radiation Fields, Proc. Symp. Neutron Monitoring for Purposes, 1972, Vol. II; p. 63 (Vienna: IAEA). holder BASE PLATE 5 FOIL

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