The effects of squalene on the PMMA dosimeter: Focusing on a "chronodosimeter"

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Pergamon 0969-806X(94)00142-1

THE E F F E C T S OF S Q U A L E N E O N THE P M M A D O S I M E T E R : F O C U S I N G O N A " C H R O N O D O S I M E T E R "

J. L. DUROUX, M. TRIHI, M. J. HYVERNAUD and M. BERNARD Facult6 de Pharmacie de Limoges, Laboratoire de Biophysique, 87025 Limoges Cedex, France

(Received 9 October 1993; accepted 8 October 1994)

Abstract--A polymethylmethacrylate-based plastic material doped with squalene according to our laboratory protocol, has been tested by spectrophotometric measurements as a dosimeter of 7-radiation- The spectrophotometric characterization shows responses changing in time of which the modelization allows us to find the duration separating the irradiation and the measurement.

INTRODUCTION

In the radiation processing of materials by ionizing radiation ( X , ) , - r a y s and electron beams), polymethylmethacrylate and dyed polymethylmethacrylate [Red Perspex produced by Harwell (G.B.)], are well established for determining absorbed doses (Amin et al., 1991; McLaughlin, 1977; McLaughlin et al., 1979; Orton, 1970; Whittaker, 1970).

This kind of dosimeter allows, in the range of 1-50 kGy (Chadwick, 1969), a measurement essentially dependent on the absorbed dose.

In addition, the published literature data showed that the normal conditions of storing for the dosimeters after irradiation do not notably affect the response obtained just after irradiation (Khan et al., 1988; Levine et al., 1979). This fact is a quality when we want to control the absorbed dose.

The study of various responses as a function of time for doped polymethylmethacrylate (PMMA) manufactured in our laboratory allows us to find the absorbed dose and the duration separating the irradiation and the measurement.

We arrive at a "chronodosimeter" of which the first application will be the control of food irradiation.

E X P E R I M E N T A L M E T H O D S

In order to make the PMMA unstable in time, we have added to the monomer (methyl methacrylate) before polymerization, 2.5% of squalene (Fig. 1).

The steps of polymerization are the same as for PMMA (Barrett, 1982). Dosimeter sheets of 1.8 mm thickness were cut to a size of 11 x 45 mm to fit the holder of the spectrophotometer.

The thickness of samples is measured before irradiation with a precision better than 1/~m.

Irradiation of samples was carried out using a 6°Co source (IBL 460, Faculty of Pharmacy, Limoges,

France) with a nominal activity of 8000 Ci and with absorbed dose rate of the order of 3 kGy.h ~. The dosimetry of the source was carried out using alanine/

ESR dosimetry by the constructor and in routine using the Fricke method (Sehested, 1970). During the irradiation, at ambient temperature, the dosimeters for each ~,,-ray irradiation were held in a PMMA cylinder, which provided electron equilibrium conditions.

The absorption spectra and optical densities (O.D.) before and after irradiation were measured using an UVIKON 930 double beam spectrophotometer of KONTRON.

The results presented in this study were obtained with 10 series manufactured separately, a good reproducibility of O.D. measurements was registered for irradiated and unirradiated samples. The error limits are < 1.5%.

The O.D. values higher than 2.3 of our samples, which have a thickness close to 2 ram, and did not permit us to irradiate the samples with doses superior to 10.64 kGy. All series were divided into 6 batches of 5 samples irradiated at doses of: 0.66, 1.33, 2.66, 5.32 and 10.64 kGy. The sixth batch was reserved for controlling the behaviour in time of unirradiated material.

For every tenth series, the measurements of O.D.

were carried out at ambient temperature and at the following times:

- - j u s t before irradiation - - 1 h after irradiation (D + 0)

- - t h e n in days: D + 1, D-+-2, D + 5, D + 7, D + 9 , D + 1 2 , D + 1 6 , D + 2 2 .

The samples were stored, between the optical density measurements in a controlled enclosure at 20°C, sheltered from light (Chadwick, 1972).

311

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312 J.L. Duroux et al.

H 3 c ~ C H 3

H3C CH 3

Fig. 1. The chemical structure of squalene.

E X P E R I M E N T A L R E S U L T S

Stud)' o f spectra

UV-visible absorption spectra of unirradiated and irradiated sample for an absorbed dose of 10.64 kGy are shown in Fig. 2. This figure also shows the difference between these two curves which indicates the change in optical density (AO.D.) due to irradiation. This latter curve indicates that the wavelength of maximum absorption is around 294nm. One should note that there is a similar behaviour between these spectra and those obtained from PMMA without additives (maximum absorption is around 290 nm) (Khan et al., 1988).

Figure 3 shows the evolution of change in O.D. as a function of wavelength at different days after irradiation. We note an increase of maximum absorption, this phenomenon is analysed in more detail in the following paragraph. In addition, the wavelength of this maximum has not varied with time and is always around 294 nm.

After a detailed study of these spectra we have been induced to make our optical density measurements at 299 nm. These conditions allow us to attain a good compromise between an important variation of optical density as a function of absorbed dose and a good reproducibility of measurements. As a matter of fact, the wavelengths of 305 and 314 nm indicated in the literature (Berry and Marshall, 1969; Chadwick, 1973; Khan et al., 1988) for the use of classical PMMA did not give us better results.

3 . 0 't

2.5 ~,2

2 . 0 ',

1.0 0.5

' - . . . . t . . . . ~ . . .

0

2 9 0 3 1 0 3 3 0 3 5 0 3 7 0 3 9 0

Wavelength (nm)

Fig. 2. Absorption spectra for unirradiated (curve 1) and irradiated (curve 2) dosimeter (thickness 1.8 mm, absorbed dose 10.64 kGy). Curve 3 is the difference between curves 1

and 2 (~O.D.).

- - D + 1 6

/ . ~ . . . D + 9

I. 5 % - - - 2" ."N . . . . D + 5

./. . . . ,,,.

,:j:~ <'-',,X, ... D+2

/ \ ' : ' g ~ - - - - D + I

o , - - _ D + 0

1.0

0.5 ^!

2 9 0 3 0 0 3 1 0 3 2 0 3 3 0

Wavelength (nm)

Fig. 3. Evolution of spectra AO.D. measured at different days after irradiation. Absorbed dose = 10.64 kGy.

Variation o f optical density as a function o f time The final result presented in Table 1 gives the change in optical density per unit thickness (AO.D. mm ~), the average measurement registered for the 10 series. AO.D. corresponds to the difference between the optical density measured at D + i (i = number of days after irradiation) and that measured before irradiation, the parentheses indicate the corresponding standard deviation in percentage.

The reading of these standard deviations allows us to note a very good reproducibility in the behaviour of our dosimeters after irradiation, it is however less satisfactory when the absorbed dose increases and the day of measurement is distant from the irradiation.

In order to check the evolution after different doses, we represent (Fig. 4) the relative response evolution with time A ' = f ( t ) where A ' = (AO.D. mm-1)D +j - (AO.D. mm-I)D+0

Several comments may be drawn from these re- suits:

(1) The curves A' = f ( t ) show a permanent and regular evolution of the doped irradiated and unirradiated material.

However two phenomena may be distinguished:

(a) A transformation of the material itself, inde- pendently of treatment with ionizing radiations, represented by the quasilinear variation, observed in Fig. 4, of the unirradiated sample. This evolution, linked to the presence of additive in the dosimeters-- the classical PMMA being stable with time under the same conditions (Khan et al., 1988)--is due to the oxidation of squalene in the material. The intrinsic transformation of material also appears in the irradiated samples in the second part of the curves: these tend to become quasilinear and all the more rapidly as the absorbed dose is lower.

(b) An evolution due to the effect of absorbed dose. It is characterized in the first part of the curves, by a logarithmic form which is particularly accentu- ated when the absorbed dose is important. Following the first phase, we find the linear variation already observed on the control batch. This change to a behaviour similar to that of an unirradiated sample

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Effects of squalene on the PMMA dosimeter

Table I. Values of AO.D. mm -~ obtained as a function of time (in days) separating the irradiation and the measurement for different doses. The standard deviation in percentage obtained for l0 series is shown in parentheses

313

Time (days)

Dose D + 0 D + I D + 2 D + 5 D + 7 D + 9 D + I 2 D + I 6 D + 2 2

Control 0.0000 0.0041 0.0086 0.0197 0.0277 0.0347 0.0494 0.0635 0.0889

(0.0) (0.1) (0.2) (0.2) (0.3) (0.3) (0.4) (0.5) (0.5)

0.66 kGy 0.0372 0.0481 0.0538 0.0669 0.0746 0.0813 0.0939 0.1065 0.1274

(0.1) (0.1) (0.1) (0.1) (0.2) (0.2) (0.3) (0.3) (0.3)

1.33 kGy 0.0712 0.0860 0.0930 0.1067 0.1146 0.1213 0.1316 0.1424 0.1585

(0.2) (0.2) (0.2) (0.1) (0.2) (0.2) (0.3) (0.2) (0.2)

2.66kGy 0.1366 0.1623 0.1730 0.1904 0.1980 0.2050 0.2131 0.2219 0.2344

(0.3) (0.3) (0.2) (0.3) (0.3) (0.3) (0.4) (0.4) (0.4)

5.32 kGy 0.2924 0.3346 0.3510 0.3783 0.3893 0.3979 0.4057 0.4154 0.4249

(0.7) (0.5) (0.4) (0.4) (0.5) (0.5) (0.6) (0.6) (0.6)

10.64 kGy 0.6411 0.7066 0.7318 0.7726 0.7899 0.8041 0.8199 0.8384 0.8504

(1.5) (1.0) (0.8) (1.0) (1.1) (1.1) (1.2) (0.9) (1.2)

occurs all the later as the absorbed dose is important.

The passage of 7-radiation through the samples seems to accelerate the intrinsic transformation of our material.

(2) The adjunction o f squalene, in addition to the effects described in the preceding paragraph, has permitted us to increase notably the sensitivity of the new dosimeter in comparison with a classical P M M A . F o r the dosimeters without additive (squalene) manufactured in our laboratory, we have observed a variation o f AO.D. m m a k G y ~ equal to 0.035, a result comparable to those obtained in the literature (Khan et al., 1988). In Table 1, we show that the doped samples give a value of AO.D. m m t k G y ~ equal to 0.06 at D + 0 . In addition, this variation increases with time, allowing a m o r e accurate reading of the absorbed dose.

(3) The classical response as a function of dose for P M M A dosimeter is a parabola (Barrett, 1982):

AO.D. m m - l = a + b (dose) + e (dose) 2. (1) We have been able to verify that the dosimeters doped with squalene have a similar behaviour at any

0.25

O Control + 0.66 kGy

× 1.33 kGy _ ~

0.20 • 2.66 kGy A 5.32 kGy v 10.64 kGy f

/

0.15

o 0.10

0.05

i

0 5 10 15 20 25

Time after irradiation (days) Fig. 4. Evolution of relative response A' with time where

A' = (AO.D. mm-l)D+~- (AO.D. mm -t )D+0.

days (D + i). Table 2 presents the coefficients a, b, c and correlation coefficients corresponding to each regression.

It should be noted that these coefficients differ according to the day of measurement: while the coefficients b and c do not vary significantly, the coefficient a increases notably with time. Conse- quently, the variation of optical density as a function o f dose shows essentially a shift in time of a parabola on the Y-axis.

In addition, the perfect correlation between the experimental and calculated points demonstrates the extreme regularity of the physical-chemical transformation processes of doped P M M A , when it has been irradiated.

Determination of the duration separating the reading and the irradiation

The properties of the dosimeter developed in the preceding paragraphs have been compared to those of a classical P M M A . The essential difference between these two dosimeters is the instability of the O.D. as a function of time. We have exploited this variation to determine the day when the irradiation has been carried out, the absorbed dose being known in the first time. This property could be used for example, in industrial irradiation to control the storage time of ionized products.

F r o m results already presented in Table 1, we have tried to fit the variations of A O . D mm -~ with time for all doses applied in this study.

M a n y regressions have been tested; the model giving the best results for the irradiated batches is a multi-exponential regression in (AO.D. mm -1) 2:

t = Aj • exp[--:q • (AO.D./mm) 2]

+ A 2 - e x p [ - - a 2 • (AO.D./mm) -2] (2) where t = number of days between the reading and the irradiation.

Coefficients A1, A 2, al, a2 are given in Table 3 with the correlation coefficients p obtained for the five doses used in this study.

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Table 2. Coefficients of the parabolic function characterizing the evolution of optical density as a function of dose for different days following irradiation: AO.D. mm ~ = a + b (dose) + c (dose) z with the correlation coefficient obtained for each regression

Time (days)

Coeff, D + 0 D + I D + 2 D + 5 D + 7 D + 9 D + 12 D + I 6 D + 2 2

a 0.0021 0.0067 0.0108 0.0214 0.0290 0.0358 0.0500 0.0638 0.0876

b 0.0488 0.0573 0.0599 0.0629 0.0630 0.0629 0 . 0 6 0 1 0.0579 0.0530

c 0.0011 0.0008 0.0007 0.0007 0.0008 0.0009 0.0012 0.0014 0.0018

Correl. coeff. 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 0.9999 0.9999 0.9999

Table 3. Coefficients of multi-exponential function giving the time as a function of AO.D. mm t for different doses: t = A 1 . e x p [ - ~ I (AOD./mm) -2] +

A 2 . e x p [ - ~ 2 (AO.D./mm) 2]

Dose (kGy)

Coeff. 0.66 1.33 2.66 5.32 10.64

A t 44.8304 72.1619 392.61 2.143 E + 7 27097.48

A 2 26.9291 6.8974 -0.0131 1729.1091 0

~ 0.0352 0.0335] 0.1581 2.7625 5.1770

~2 0.0075 0.0225 0.0105 0.8344 0

p 0.9998 0.9997 0.9999 0.9996 0.9971

p is calculated f r o m the following relationship:

[(tcal)i - - (tobs)i] 2

p 2 = 1 - i=1 (3)

[(to~),- (tobY)] 2

where

(tcal)i = calculated time;

(lobs) / = observed time;

n = n u m b e r of o b s e r v e d times;

tob s = m e a n o f o b s e r v e d times.

A linear regression is quite sufficient to characterize the b e h a v i o u r o f a n u n i r r a d i a t e d d o s i m e t e r (correlation coefficient = 0.9996)

t = 248.4441 (AO.D. m m l). (4)

50

40 " ' - I ) / 1 . 5 . . ~

4 D/3 ~ "

o

= 30 ../,

"~ ,/ ~'~

~.~ 20 '~//

I0

I I I I

0 5 10 15 20 25

Time after irradiation (days)

Fig. 5. Evolution percentage of relative response with time for different dose rates. Absorbed dose = 2 kGy.

The essential fact to emphasize in Table 3 is the very good c o r r e l a t i o n o b t a i n e d , especially for doses lower t h a n or equal to 5 . 3 2 k G y , which are the m o r e frequently used ones in t r e a t m e n t o f food by ionizing radiation. It is n o w possible, with this fit, to determine the d u r a t i o n s e p a r a t i n g the reading a n d the i r r a d i a t i o n , with a n interval o f 22 days a n d using the doses usually r e c o m m e n d e d in the food i r r a d i a t i o n d o m a i n . The results o b t a i n e d f r o m m e a s u r e m e n t s m a d e after the 22nd day do n o t exhibit a satisfactory reproducibility between each series: in this case, the fit o f the f u n c t i o n (2) does not allow the determi- n a t i o n o f the i r r a d i a t i o n date with a n e r r o r less t h a n 2 days.

Influence o f the dose rate

In order to d e t e r m i n e the effect o f the dose rate o n the relative response, the dosimeters were irradiated at a dose o f 2 k G y with 3 dose rates: /), 2 / ) / 3 , D / 3 (/) = 1930.7 G y / h ) .

In Fig. 5 is represented the percentage of the evolution as a f u n c t i o n o f time for the 3 dose rates used.

W e notice t h a t the m o s t i m p o r t a n t difference between the dose rates is a b o u t 2 % after the 22nd day;

in a d d i t i o n , at any days (D + i), the m e a s u r e m e n t p o i n t s do n o t represent a n increasing or decreasing f u n c t i o n o f the dose rate.

So, we can conclude t h a t the dose rates do not interfere in the d e t e r m i n a t i o n o f the i r r a d i a t i o n date.

CONCLUSION

In this study, we have d e m o n s t r a t e d a new dosi- metric principle. T h e originality a n d a d v a n t a g e s o f this d o s i m e t e r are:

Its sensitivity is practically twice t h a t o f the c o n v e n t i o n a l P M M A .

Its p o s t - i r r a d i a t i o n e v o l u t i o n o f optical density which allows us to use it as a " c h r o n o d o s i m e - ter".

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Effects of squalene on the PMMA dosimeter 315 The correlation coefficients o b t a i n e d f r o m the

m o d e l i z a t i o n are very satisfactory since they take into account, n o t only the classical interaction o f radiation with matter, but also the o x i d a t i o n o f squalene which c o n t r i b u t e d to the instability o f the material.

M o r e data must be available, however, before its value will be proven. We have to study the instabili- ties o f the response o f this d o s i m e t e r which is likely to d e p e n d on storage and irradiation temperature, humidity, like the conventional P M M A .

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