A@. Radm ht. Vol. 42, No 12, pp 121s-1221, 1991 Int J. Radial Appl Instrum Part A Prmted m Great Britain All rights reserved

0883-2889/91 $3 00 + 0.00 Copynght 0 1991 Pergamon Press plc

Studies on the Radiation-induced

Coloration Mechanism of the Cellulose

Triacetate Film Dosimeter

KOJI MATSUDA* and SIR0 NAGAI*

Osaka Laboratory for Radiatton Chemistry, Japan Atomic Energy Research Institute, 25-1 Mir-minamimachi, Neyagawa, Osaka 572, Japan

(Received 29 October 1990; in rewed form 5 April 1991)

The spectes responsible for the coloration of the cellulose triacetate (CTA) film dosimeter have been studied using ultraviolet, electron spm resonance, infrared and gas chromatographic techmques. The post-irradiation change in the optical density at 280nm indicates that the coloration occurs not only during Irradiation (in suu coloration) but also after irradiation (post-irradiation coloration) and that m suu coloranon is due to unstable and stable components. The species responsible for the unstable component of in .rrtu coloration are ascribed to the radicals produced from CTA molecules and those for the stable component to the radiolysis products from CTA and trrphenyl phosphate contained in the dosimeter. On the other hand, post-irradiation coloration is attributed to the formation ofcarbonyl groups m CTA molecules, which is induced by reaction with NO, produced by irradiation of air.

Introduction

Cellulose triacetate (CTA) film dosimeter is one of the widely used plastic dosimeters available for the “megarad” (10 kGy) region (McLaughlin et al., 1989). The CTA film dosimeter that is now commer- cially available and generally used for the absorbed dose range of 104-106Gy, is the one that has been developed in our institute in collaboration with CEA, France and manufactured by Fuji Photofilm Co.

The CTA film dosimeter contains 15% wt triphenyl phosphate (TPP) with no other additives. The ab- sorbed dose is determined from the optical density change at 280 nm before and after irradiation, with the help of the calibration curve associated with the

dosimeter. It is of importance to know the mechanism for the

increase in the optical density by irradiation not only for handling but also improvement of the dosimeter and further development of a new dosimeter as well. Although several research groups have reported the dosimetric properties of the CTA film dosimeter such as effects of dose, dose rate, temperature and humid- ity (Puig et al., 1974; McLaughlin et al., 1979; Tanaka et al., 1984). There are only a few reports that are concerned with the mechanism of the radiation- induced coloration. In our previous report (Tamura

*Deceased 23 June 199 1. *Present address: Takasaki Radiation Chemistry Research

Establishment, JAERI, Watanukimachi 1233, Takasaki, 370-12 Gunma, Japan.

et al., 1981). we described briefly the species that could be responsible for the coloration of the Fuji CTA film dosimeter. Gehringer et al. (1985) reported that oxygen plays an important role in the radiation- induced coloration from the results obtained for the effects of dose rate, moisture and presence or absence of oxygen during irradiation on the response of the CTA film dosimeter produced by Societe Numelec, France. This report described our detailed study on the coloration mechanism for the Fuji CTA film dosimeter, concentrating on the identification of the species responsible for the coloration.

Materials and Experimental

The CTA film dosimeter, FTR-125, manufactured by Fuji Photofilm Co. was used in the present study. Two other films were also used: additive-free CTA film of 60-100 pm in thickness (prepared by evapo- rating methylene chloride-methanol solution of cellu- lose tnacetate supplied by Nakarai Chemicals Co.) and cellulose film of 3638 pm in thickness (as sup- plied by Tokyo Cellophane Co.). Triphenyl phos- phate (Nakarai Chemicals Co.) was used after purification by the zone melting method. The ir- radiation experiments were carried out with electron beams of 1.5 MeV energy and 50 PA current from a Van de Graaff accelerator unless otherwise stated. The mean dose rate to the film was 7.5 x lo2 Gy/s. Irradiations at lower average dose rates were

1215

121h KOJI MATSLDA and SIKO NAGAI

performed either with the Van de Graaff accelerator at lower beam currents or with a 2000 Ci ‘“Co source.

Measurements of ultraviolet (u.v ) absorption spectra, Including optical densities were made with a Shnnadzu UV-2lOA double beam spectrophotometer The change m optical density at 280nm before and after Irradiation, AO.D.(280). was measured as a func- tlon of time after irradiation from 3 mm to 24 h. All the values of A0 D.(280) for samples of different thickness were normalized to those for I25 /1 m bhlch 1s the thickness of the commercial CTA film dosimeter.

Electron spin resonance (ESR) spectra were recorded on a JEOL ME-X spectrometer Product analysis for lrradlated TPP powder was carried out with a gas chromatograph. Yanaglmoto G-80. eqmpped with an FID detector and Tenax CC columns. Reactions Induced by NO? m the CTA tilm doslmeter. addltlve-free CTA film and cellulose film wet-c studied using the U.V. and ESR spectrometers and a Hltachr 215 grating 1.r spectrometer

Results and Discussion

(I) Post -rr.r.a&ition changr in optical dmtrtj~

As reported previously (Tamura et d.. I9 8 I ). when the CTA dosimeter IS n-radiated with electron beams at mean dose rate of 7 5 ‘* IO' Gyis m air at room temperature, AO.D.(280) decreases gradualI> with time up to about 20 mm after lrradlation and therc- after mcreascs with the storage time A typlcal curve IS shown in Fig. I for the CTA film doslmeter Irradiated to a dose of 100 kGy. These spontaneous changes m AO.D.(280) with time mdlcate that the coloration of the CTA film doslmeter consists of two different phenomena. one occurrmg during and im- mediately after lrradlatlon and the other proceedmg more slowly after Irradlatlon. These are referred to respectively as in sttu coloration and post-lrradlatlon coloration.

The decrease m A0 D.(280) with time up to 20 mm together with the fact that the extrapolation of the curve due to the post-Irradiation coloration to zero dose gives a sigmticant A0 D.(280) as shown m

07

0.4

In-situ coloration: Unstable component

.

\ln-situ coloration: Stable component

J I I I I I I 3 10 30 100 300 1000 3000

Time after irradiation (min)

Fig i Optlcal density change at 280nm before and after electron-beam lrradlation of the CTA film doslmeter m an at room temperature to 100 kGy as a function of stor- age time after lrradlatlon The different components of

radlatlon-Induced coloration are mdlcated

.”

a, I I I I 1 1

u -15 1 ’ 3 10 30 100 300 1000 3000

Time after irradlatlon (min)

Fig I. suggest that 111 vtu Laolor‘l!lon ,\ Lwllpo\cd ,rf‘

;I combination t)f unstahlc .lnd ,tabls componcnth. That the coloration of the C‘TA film dl>\lmetc‘r ion- slsts of these three component\. ( I I un\tahlc and (2) stable components of 111 i/m coloration and (3) un- stable post-lrradiatlon coloration. ~a5 Illustrated more distinctly by continuous recording of the optIcal density at 280 nm [O.D.(280)] after Irradtatmn The contmuous recordmg, m addltlon. shoned that the half-hfe of the unstable component IS ieverai mlnutcb Therefore, it can be expected that A0 D.(?XO) would not show apprcclahle lmtlal decrease due to thr <!cca> of the unstable c~omponcnt when 11 radlatlan tmie exceed:, (‘t,. 20 min. ‘IS confirmed h> the e\perrments described below,

Flgurc 2 shows the cttcct ~~I’ ,tbsorbcd don on the post-lrradiatlon change in A.0 D.(280) relatlbe to the nutlal value. AO.D.(?W), which IS dctined here as AO.D.(280) determlncd 3 mm after lr- radiation. As can be seen from the figure. the increase of absorbed dose reduces the amount of Initial decrease m AO.D.(380) and tmio reqmred for AO.D.(280) to reach the mlmmum value When Ir- radlatron was carried out at a relatively Ion dose rate, for Instance I Gy:s. to 40 kGy ( I I I h Irradiatlvn time). AO.D.(280) Increases monotonously with time without showmg any imtral decrease These results are consistent with the half-life of the unstable component cstlmated abow

Figure 3 shoas the post-lrradlntlun change In A0.D (280) relatlvc to the respective A0 D.(280), for the CTA film doslmeter m different atmospheres during and after Irradlatlon The samples In 0: and N2 atmospheres were prepared b! mtroduclng 150 torr 0,. and 760 torr N,. rcspectl\elq. to the quartz cells contatning the CT.4 film dosimeter whvzh

had been evacuated to IO ’ torr. .4s can be seen in Fig 3. the AO.D.(280) in I r1cuo shghtly decrcascs with time wlthout any increase at longer storage time. m contrast to the A.(> D (280) In air The A0 D.(280) In oxygen decreases with tlmc up to about 30 mm bl a greater extent than m air and doe> not Increase at longer tmles The A0.D (280) In mtrogen changes only slightly with tnne after Irradlatmn These find- lngs indicate that the unstable component of 111 vtu

Coloration of the CTA doslmeter 1217

u;

-20 1 ’ I I I I I 3 10 30 100 300 1000

Time after irradiation (min)

Fig. 3. Optxal density change at 280 nm for the CTA film dosimeter, relative to the value 3 mm after Irradiation, when the films are stored during and after lrradlation in different atmospheres: 0, in air; 0, in wcuo; A, m 150 torr 0,; 0,

in 760 torr N,.

coloration is related to the reaction between oxygen and color centers produced in the dosimeter. The fact that post-irradiation coloration proceeds in air and is negligible either in t:ucuo, nitrogen or oxygen suggests that some product other than ozone produced from air, most likely nitrogen oxides, may play a role in the coloration. In addition, since irradiated additive-free CT.4 film also exhibits post-irradiation coloration when irradiated in air as described below, the post- irradiation coloration may result from reaction of nitrogen oxides with CTA and/or TPP.

In order to investigate whether the coloration of the CTA film dosimeter arises from radiation effects of CTA or TPP, or both, changes in AO.D.(ZSO) with time after irradiation were studied for additive-free CTA film in vacua and in air. The AO.D.(280) in vucuo decreases slightly with time. The AO.D.(280) in air, on the other hand, decreases rapidly until cu 30 mm and then increases with time. These results strongly indicate that CTA itself plays an impor- tant role not only in m sztu coloration but also in the post-irradiation coloration of the CTA film dosimeter.

Figure 4 shows the post-irradiation change in AO.D.(280) agamst absorbed dose for the CTA film dosimeter and additive-free CTA film; the AO.D.(280)) values (at 3 min after irradiation) and the minimum values of AO.D.(280) at a later time after irradiation are plotted. The AO.D.(280) values for additive-free CTA film have been normalized to those for 106 pm in thickness by taking into account the fact that the CTA film dosimeter is 125pm in thickness and contains 85% wt CTA. As may be seen from the figure, in situ coloration of the CTA dosime- ter arises not only from TPP but also CTA. It is evident that the contribution of the unstable com- ponent, which corresponds to the difference between AO.D.(280), and the minimum value of AO.D.(280), is much greater in additive-free CTA film than in the CTA dosimeter. This fact indicates that color centers that are produced not in TPP but in CTA are

s 2 - l-

s I

0 100 200 300 400

Absorbed dose (kGy)

Fig. 4. Optical density change at 280 nm for the CTA film doslmeter (with TPP) and additive-free CTA film Irradiated in air as a function of absorbed dose. Solid lines, AO.D.(280) 3 min after irradiation; broken lines: mimmum AO.D.(280). 0 and 0, CTA film dosimeter; A and 0, additive-free

CTA film.

responsible for the unstable component of in situ

coloration.

(2) Species responsible for coloration

(a) In situ coloration: unstable component. Figure 5 shows the ESR spectra of the CTA film dosimeter and of additive-free CTA film after irradiation in

vacua at room temperature. Since both spectra are quite similar to each other, the radicals giving rise to the spectra must have been produced from CTA molecules, not from TPP. In order to obtain infor- mation about the radicals produced from TPP, an ESR spectrum of TPP powder irradiated in air at room temperature was recorded. The observed spectrum consists of two different components which are ascribed to phenoxyl radicals produced by the scission of a P-O bond and cyclohexadienyl-type radicals produced by H atom addition to a benzene ring, respectively, in TPP. These radicals were found to be relatively stable in air, which excludes the possibility that they would be the species responsible for the unstable component of in situ coloration.

a

Fig. 5. ESR spectra of the CTA film dosimeter (with TPP) (a) and additive-free CTA film (b) after irradiation in U~CUO

at room temperature to 200 kGy.

121X KOJI MATSUDA and SIRO NAGAI

1 .o

n

h

T----

Q

7 a

-I !L

.E 05 z I

1

a, I a ’ \.

\ \ i f I I I I I

0 30 60 90 120 150

Time after irradiation (min)

Fig 6 Decay curves of tadxals produced m the CTA film doslmcter and addltlve-free CTA film by lrrachatlon m r~uo and m ax at room temperature to 200 kGy. Closed symbols, m air, open symbols. 111 IYKUO l and 0. CTA film

doslmeter. A and &,. addltlve-free CTA film

When Irradlatlon of the CTA film doslmeter and

addltlve-free CTA film was carried out m air. their ESR spectra were analogous to those m Fig. 5 but decayed rapidly with time The decay curves of the radicals vz IYKUO and m air are shown m Fig. 6. It may be seen that the radicals produced in the CTA film doslmeter are somewhat less stable than those m additive-free CTA film both m IYKUO and m air. The decay behavior for the radicals in the CTA dosimeter ~1 IYKUO and m air agrees well with that found for the unstable component of VI SIZU coloration. In addition, the half-life of the radicals m air may be estimated to bc (‘(1 IOmm. in good agreement with the value of the unstable component obtained by the O.D.(280) measurement. From these results. the radicals pro- duced from CTA can be ascribed to the species for the unstable component of in situ coloration.

It was further found that the decay of the radicals from CTA m au proceeds through the reaction with oxygen to produce the peroxy radicals which are not htable at room temperature. Figure 7 shows the spectral change induced by exposure of the Irradiated CTA film doslmeter m air at - 30 C. The asymmetrlc spectrum (c) m Fig. 7 is characteristic of peroxy radicals (Rabek and Rinby. 1973) with the axially symmetric g factors, R, = 2 032 and <qL = 3.007.

Although extensive studies have been made on the

radicals produced by lrradiatlon of CTA (Florin and Wall. 1963). no defimte ldentlfication of the radicals stablhzed at room temperature has been successful m VIW of the complexity of the observed spectrum In an attempt to assign the radicals in question. ESR studies were extended to the radicals produced by lrradlatlon of cellulose film

Figure 8 compares the spectrum of additive-free CTA film Irradiated in L’UCUO (a) with the spectra of cellulose film Irradiated in cucuo (b) and m air (c). It IS noted that spectrum (a) IS almost identical to the central part of spectrum (b). Spectrum (b) changes Irreversibly to spectrum (c) on exposure of the sample

FIN. 7. ESR spectra of the C’TA iilm doslmetel recoldcd a~ low temperatures. the doslmeler being lrradlated 111 air at room temperature The madlated sample waj m~meraed Into hqmd mtrogen Immedlatrly after Irrndlatlon and \ub- qequently warmed to 30 C Spcctr,l a and h \$\erc recorded al - 30 C. spectrum c \~a5 at - 30 C ,tfter stor‘ige of the sample for 90 mln at this temperature and spectrum d was obtained on recoohng the sample to IO0 C The power shown on the right hand side of each spectrum Indxates the

mxrowave power employed in recordmg the spectrum

to moisture. Spectrum (b) ma! he intcrprcted by superposltlon of spectrum (c) and a doublet of (1” = I .6 mT as marked in spectrum (b). It wah found that spectrum (c) varies with the angle between the dIrectIon of the external magnetic field and the plane of the film. whereas the doublet m spectrum (b) dots not, showrng that the doublet splittmg may originate from the hyperfine interaction with one P-proton On this basis, the radicals giving rl\c to the doublet ma! be ascribed to the radical produced by H dtom abstraction from C, m a cellulose molecule. The same assignment has already been made hy Guthrlc t’f U/ (1972)

From these results. It wms reahonablc to assign the radicals dommantly produced from CTA

Fig. 8 ESR spectra of addltlve-free CTA tilm u-radiated ~1 t’acuo (a), and cellulose film lrradlated VI W~UO (b) and m

air (c)

Coloration of the CTA dosimeter 1219

CH, OAc I

i /r-O\. --c'\p"" HJ-"-

(1)

c3-c2

II H OAc

Scheme I

to radical I. The attribution of radical I to the species responsible for the unstable component of in situ coloration may be supported by the fact that ‘CHOH type radicals produced by H atom abstraction from aliphatic alcohols give rise to a U.V. absorption spectrum m 220-330 nm region (Simic et al., 1969).

(6) In situ coloration: stable component. As men- tioned already, the stable component of in situ color- ation may arise from the radiolysis products of both CTA and TPP.

Figure 9 shows the AO.D.(280) values for additive- free CTA film and cellulose film irradiated in air against absorbed dose. Since both curves are nearly the same up to a dose of 200 kGy, it may be reasonably assumed that the radiolysis products giv- ing rise to the AO.D.(280) would be about the same for CTA and cellulose. According to the extensive studies of radiation effects on cellulose (Blouin and Arthur, 1958) irradiation of cellulose induces the degradation of the polymer and the production of carbonyl and carboxyl groups. It seems highly prob- able that these productions give rise to significant optical density measured at about 280 nm. In fact, the U.V. absorption maxtma around 270 nm observed for irradiated carbohydrates have been ascribed to the products containing carbonyl groups such as enediols (Phillips and Criddle, 1960). In the present study,

2.5

r

I 0 100 200 300 400 500 600 700

Absorbed dose (kGy)

Fig. 9. OptIcal density change at 280nm for additive-free CTA film (A) and cellulose film (0) irradiated in air as a

function of adsorbed dose.

no further efforts were made on the identification of

the radiolysis products from CTA. There seems to be no reports concerning the radio-

lysis of TPP. By analogy with the results of radiolysis study on trialkyl phosphate (Wilkinson and Williams, 1961), which indicate that alkane and dialkyl phos- phate are the principal products, it may be expected that benzene and diphenyl phosphate would be pro- duced by irradiation of TPP. It is not likely, however, that conversion of TPP to benzene and diphenyl phosphate would result in any increase in O.D.(280), because sum of the molar extinction coefficients (E) of these products is lower than t of TPP at 280 nm as confirmed separately in our experiment.

In an attempt to evaluate the amounts of radiolysis products from TPP, GC analysis was made for TPP powder irradiated in air to IO3 kGy. Phenol and biphenyl were found to be among the products from TPP with the approximate G values of 0.12 and 0.046. respectively. Since phenol and biphenyl give U.V. spectra peaking at about 280nm (Jaffe and Orchin, 1962), a model experiment was carried out to assess the contribution of these compounds to the value of O.D.(280). In this experiment, the O.D.(ZSO) values were determined for methylene chloride sol- utions of the CTA film dosimeter and additive-free CTA film irradiated to 100 Mrad and methylene chloride solution containing phenol and biphenyl in known amounts, which were estimated to be pro- duced from TPP m the CTA dosimeter by lo3 kGy irradiation on the basis of the above GC result. The difference between values of O.D.(280) for the irradiated CTA dosimeter and additive-free CTA film was 1.34, whereas O.D.(280) for the solution containing the phenol and biphenyl was only 0.17. The result indicates that phenol and biphenyl pro- duced by radiolysis of TPP do contribute to the increase in O.D.(280) by irradiation of the CTA film dosimeter, though quantitative agreement is not satisfactory.

(r) Post-rrradiation coloration. As suggested above, the post-irradiation coloration may be caused by reaction between CTA and/or TPP with some product by irradiation of air. It is well known that irradiation of air produces ozone and nitrogen oxides, NO, (Lind, 1961). Among these, ozone may be excluded as possible species to react with CTA or TPP to induce the coloration, since the post- irradiation coloration was negligible in the CTA film dosimeter stored in oxygen atmosphere. On the other hand, NO, which is produced predominantly by irradiation of air, 1s known among NO, to be most reactive to various molecules. In order to know if NOz would participate in post-irradiation coloration of the CTA film dosimeter, changes in the U.V. and infrared (1.r.) absorption spectra on the exposure to NO2 were studied in some detail.

When the CTA film dosimeter was exposed to 19 torr NO,, the U.V. spectrum showed increase in O.D.(280), together with appearance of a new spec-

AR, 4;‘s ,2--G

trum composed of several bands around 350 nm. which IS designated spectrum X hereafter.

Figure 10 shows the U.V. spectrum observed lm- mediately after exposure of the CTA film dosimeter to NO, at 19 torr. The fine structure above 400 nm is due to NO, in gas phase (Hall and Blaset, 1952). Both 0.D (280) and O.D.(354), where 354 nm corresponds to the wavelength of the absorption maximum in spectrum X, increase gradually with time of exposure to NO,, as shown in Fig. 11. The Increment in O.D.(280) is about twice that in O.D.(354). Similar changes in O.D.(280) and O.D.(354) were also ob- served for additive-free CTA film as well as cellulose film, although increases m O.D.(280) and 0.D (354) for cellulose film were much slower than those for tlie CTA film dosimeter. Such behavior would result from reaction of NO: with CTA or cellulose molecules, not with TPP contained in the CTA doslmeter.

The band separations m spectrum X as shown m Fig. 10 are from 910 to 1070 cm-’ which agree with the N=O stretching frequency m an excited stated (King and Moule, 1962). This fact suggests that spectrum X may arise from either NO; (Sidman, 1957: Strickler and Kasha. 1963), HNO, (King and Moule, 1962), or alkyd nitrite (Tarte. 1952) Of these, NO? can be excluded as the species for spectrum X by taking into account the fact that the U.V. spectrum 1s quite complex m the crystal (Sldman, 1957) and a structureless single band in aqueous solution (Stnck- ler and Kasha, 1963). On the other hand, the SIX wave lengths of the bands in spectrum X agree well with those of the U.V. spetra of HNOz (King and Moule, 1962) and ethyl nitrite (Tarte, 1952). as shown in Table I. Therefore, spectrum X may be attributed to either HNO, or alkyd nitrite RNO!.

It is expected that the molar extinction coefficient 6 at 280 nm would be much smaller than t at 354 nm

for HNO? and RNO:, as is found experimentally for

0.4

I\i

,0.374 (260 nm)

03 t\

I \ s

1220 KOJE MATSUDA and SIRO NAGAI

s 0.4 - ,” 0.’

,*

;; 0.3 -

0 a 0.2 -

0.1 -

0 ’ I I

3 10 30 100 300 1000 3000

0.6

0.5 c

Exposure time (min)

Fig. I I Optical density change at 280 and 353 nm for the CTA film doslmeter and addltwe-free CTA film as a func- tlon of time of contact wth 19 torr NO, Closed symbols ~0 D.(280). open symbols a0 D (353). l and ~3, CTA

film doslmeter, A and A. addltwe-free CT.4 film

NO: (Strlckler and Kdsha, 1963). Therefore. if these molecules would be rcsponslble for the mcrcasc in O.D.(280) for the CTA film dosimeter on exposure to NO,. the Increment m O.D.(280) should be smaller than that m O.D.(354), which IS not compatible with the result shown in Fig 11 Accordmgly, the increase m O.D.(280) may be due to species other than HNO, or RNOz which gives rise to spectrum X.

The I r. spectrum of additive-free C’TA film after exposure to NO, showed the presence of absorption bands due to N=O and N-~0 groups which corrc- spond to spectrum X m the u v spectrum On the other hand, cellulose film exposed to NO, gave an I r spectrum which revealed absorptlons due to C==O and N=O groups. The formatlon of C--O groups suggests that NO: induces sc~sslon of the glucose rings by oxidation of the OH groups m the cellulose molecules It seems hkely that C=O groups would be similarly produced m addltlve-free CTA film and the CTA film doslmeter, because these films undergo Identical changes m the U.V. spectra with cellulose film by exposure to NOI The formatlon of C=O groups may result m Increase in 0 D.(280). as. m fact, it has already been shown that the radlolysls products contammg C=O groups from CTA arc the likely species responsible for the stable component of m stfu coloration

On the basis of the results described above. II can be concluded that NO, react\ ulth the CTA film doslmeter to produce HNO, or alkyd mtrlte. and C=O groups, and that the latter I> marnl! respon-

Soectrum x HNO,* C‘,H,NO,i

Wavelength (nm)

Fig IO UltravIolet absorption spectrum of the CTA film dosimeter Immediately after contact wth 19 torr NO,

*Data by Kmg and Moule (I9621

Coloration of the CTA dosimeter 1221

24O(r-5000 A. J. Chem. Phys. 20, 1745. Jaffe H. H. and Orchin M. (1962) Theory and Applicaflons

of Ultraviolet Spectroscopy, pp. 397 and 477. Wiley, New York.

King G. W. and Moule D. (1962) The ultraviolet absorption spectrum of nitrous acid in the vapor state. Can. J. Chem. 40, 2057.

Laizier J. and Oproui C. (1977) Use of the CTA dosimetnc film for minute investigation of absorbed doses in com- plex materials. Radial. Phys. Chem. 9, 72 1.

Levine H., McLaughlin W. L. and Miller A. (1979) Tem- perature and humidity effects on the gamma-ray response and stability of plastic and dyed plastic dosimeters. Radial. Phys. Chem. 14, 551.

Lind S. C. (1961) Radratlon Chemrstry of Gases, Chap. 13. Reinhold, New York.

McLaughlin W. L., Boyd A. W.. Chadwick K. H., McDonald J. C. and Miller A. (1989) Dosimetry for Radiation Processmg, p. 162. Taylor & Francis, London; see also McLaughlin W. L., Humphreys J C., Radak B. B., Miller A. and Olejmk T. A. (1979) The response of plastic dosimeters to gamma rays and electrons at high absorbed dose rates. Radial. Phys Chem. 14, 535.

Ogihara T. (1963) Oxidative degradation of polyethylene in nitrogen dioxide. Bull. Chem. Sot. Jap. 36, 58.

Phillips G. 0. and Criddle W. J (1960) Radiation chemistry of carbohydrates. J. Chem. Sot Part 3, 3404.

Pmg J. R., Laizier J. and Sundardi F. (1974) Le film TAC, dosimetre plastique pour la mesure pratique des doses d’irradiation recues en sterihsation. In Radiosteriluation of Medical Products. Q. 113. Proc. Symp., Bombay, 1974. IAEA Publication STI/PUB/383, IAEA, Vienna.

Rabek J J. and Ranby (1973) ESR Apphcatlons to Polymer Research (Eds Kmell P. 0.. Ranby B. and Runnstrom- Reio), Q. 201. Almgvist & Wiksell, Stockholm.

Sidman J W. (1957) Electronic and vibrational states of the nitrite ion. J. Am. Chem. Sot. 79, 2669.

Simic B M.. Neta P. and Hayon E. (1969) Pulse radiolysis study of alcohols m aqueous solution. J. Phys. Chem. 73, 3794 and references cited therein.

Strickler S. J. and Kasha M. (1963) Solvent effects on the electromc absorption spectrum of nitrite ion. J. Am. Chem Sot 85, 2899.

Tamura N., Tanaka R., Mitomo S., Matsuda K. and Nagai S. (1981) Properties of cellulose triacetate dose meter. In Transactrons of 3rd Intern. Meeting on Radiation Processing, Tokyo (Ed Silverman J.). Radlat Phys. Chem. 18, 941.

Tanaka R., Mitomo S. and Tamura N. (1984) Effects of temperature, relative humidity, and dose rate on the sensitivity of cellulose triacetate dosimeters to electrons and gamma rays. Int. J. Appl. Radlat. Isot. 35, 875.

Tarte P (1952) Rotational isomerism as a general property of alkyl mtrites. J. Chem. Phys. 20, 1570.

Wilkinson R. W. and Wilhams T. F. (1961) The radiolysis of tri-n-alkyl phosphates. J. Chem. Sot. Part 3, 4098.

sible for the increase in O.D.(280). Although the concentration of NO, employed in these experiments would be much greater than that to be produced by irradiation of air (Lind, 1961), the above results prove that NO, indeed causes an increase in O.D.(280) for the CTA film dosimeter. The increase in O.D.(280) is a slow process, as seen in Fig. 11, in qualitative agreement with that observed by irradiation.

The above conclusion conflicts with that drawn by Gehringer et aI. (1985), which was based on the observation that the post-irradiation coloration takes place in an argon atmosphere as well as in air. This discrepancy needs to be studied further, but there is a possibility that their sample in Ar still contained a small amount of air which could induce the post- irradiation coloration.

Acknowledgements-This work was made as a Joint Pro- gramme of the Agreement between JAERI and CEA for Cooperation in Radiation Chemistry. The authors are grate- ful to Fuji Photo Film Co. for kindly supplying the CTA film dosimeter and CTA powder employed m the present study. Thanks are also due to late Professor 1. Sakurada for encouragement during the work and Dr M. Hatada for helpful discussions.

References

Blouin F. A. and Arthur J. C. Jr (1958) The effects of gamma radiation on cotton Textile Res J. 28, 198.

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Hall T. C. Jr and Blaset F. E. (1952) Separation of the absorption spectra of NO, and N,O, in the range of