Dose rate effects in the dichromate dosimeter

Radiat. Phys. Chem. Vol. 35, Nos 4-6, pp. 757-761, 1990 Int. J. Radiat. Appl. lnstrum., Part C

0146-5724/90 $3.00 + 0.00 Pergamon Press plc

Printed in Great Britain

DOSE RATE EFFECTS IN THE DICHROMATE DOSIMETER

P H G Sharpe*, A Miller** and E Bjergbakke** *National Physical Laboratory, Teddington, UK *'Riso National Laboratory, Roskilde, Denmark

ABSTRACT The response of the 10- 50 kGy range dichromate dosimeter (2.0 mM K2Cr2OT, 0.5 mM Ag2CrgOT, 0.1 M HC10,) has been studied in pulsed electron beams with doses per pulse ranging from 30 to 600 Gy (1-4 ~ts pulse's, 12.5pps). Rdative monitoring of absorbed dose was achieved by placing a glass encapsulated thermistor within the dosimeter solution. No significant effect of dose rate could be observed within the precision of the experiment (_-t2%) over this range of dose rates. Results are also presented of computer simulations of dosimeter response based on the known reaction mechanism. These calculations confirm the absence of dose rate effects up to 600 Gy per pulse. Keywords: Dichromate dosimeter, Dose rate effects, Electron beams, Gamma irradiation.

INTRODUCTION Dosimeters based on the radiolytic reduction of Cr(VI) to Cr(III) in aqueous solution have become well established for use in the kilogray dose range~l'z) and several standardising laboratories now employ dichromate solutions in their reference dosimetry services. Readout is by spectrophotometric measurement of the Cr(VI) concentration with two different initial dichromate concentrations generally being used to cover the dose ranges 2 - 10 kGy and 10- 50 kGy. Dosimeter solutions used at the National Physical Laboratory are made up in 0.1 M perchloric acid and have the following compositions:

2 - 10 kGy

0.5 mM Ag2Cr2OT, measurement wavelength 350 nm

10 - 50 kGy

0.5 mM Ag2Cr207 + 2.0 rlLMK2Cr2OT, measurement wavelength 440 nm

MECHANISM The mechanism of the dosimeter can be regarded as a competition between the oxidising and reducing species produced by radiolysis of the solvent(a). Hydrogen atoms (H) and hydrogen peroxide (H202) will both reduce Cr(VI) to Cr(IIl) whilst hydroxyl radicals (OH) will reoxidise Cr(III) back to Cr(VI). Thus the overall reduction yield of Cr(VI) can be written as:

G{ -Cr(VI) } = 1/3 {G(H) + 2G(H202) - G(OH) } The reaction of OH radicals with Cr(III) is relatively slow and the addition of silver ion was originally suggested by Matthews(4) to suppress the competing reaction

OH + H2 ---> H + H20 k = 3.4x107 dm3molJs"l which occurs if radiolytic hydrogen is allowed to build up in the solution. The effect of this reaction is to convert the oxidising OH radical into a reducing hydrogen atom with a resultant increase in the reduction yield of Cr(VI). Ag+ is known to be a very efficient scavenger of OH radicals by the reaction OH + Ag+ ---> Ag(II) + OH" k = 1.5x1010 dm3molls-1 757

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SHARPE et al.

The Ag(II) species produced are capable of oxidising Cr(III), thereby regenerating Ag ÷, but do not react with molecular hydrogen. DOSE RATE EFFECTS Solutions not containing Ag ÷ ion have been shown to suffer severe dose rate effects due to the reaction OH + OH ---> H202 k = 6x109 dm3mol-Ss"l

and, for this reason, are not recommended as dosimeters(~). The concentration of OH radicals does not build up to the same extent in solutions containing Ag ÷ ions and several studies have shown there to be no significant difference between the response of dichromate dosimeters calibrated with 6°Co radiation or in pulsed electron beams up to doses of about 10 Gy per pulse (5). The aim of the present work is to investigate the response of the dichromate dosimeter at doses above 10 Gy per pulse both by experimental measurements and by consideration of the known reaction mechanism.

EXPERIMENTAL

Although it is possible to vary the dose per pulse on the HRC LINAC at Riso by changing the pulse width and beam current, it is not possible to relate the dose received to the standard machine monitor reading with the accuracy needed to examine possible dose rate effects. It was decided, therefore, to monitor the energy absorbed in the dosimeter directly by measuring the temperature rise in the solution. The apparatus used consisted of a glass ampoule, 15 mm in diameter, filled to a depth of 15 mm with dosimeter solution (approx. 3 ml). Into the solution dipped a calibrated glass encapsulated thermistor Type VECO P32-A180. The resistance of the thermistor was measured every second by a Hewlett Packard Type 3456A DVM and the results stored to floppy disc for later analysis. The entire holder was placed inside an expanded polystyrene block for thermal insulation.

50.

o

L

40.

a) b)

2 30.

20. 0.

i

i

50.

100.

TIME

/

SEC

Figure t Temperature vs Time plots, a) 600 Gy per pulse, b) 30 Gy per pulse. Total dose approx. 36 kGy

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The dosimeter solution used was 10- 50 kGy range dichromate (see above), the total dose received during each irradiation being in the region 20 - 45 kGy leading to a temperature rise in the solution of between 8 and 15°(2. The irradiation conditions ranged from 4 I~s pulse length, 600 mA pulse current to 1 Ixs pulse length, 75 mA pulse current. Pulse repetition rate for all irradiations was 12.5pps giving total irradiation times ranging from 4 to ll2seconds. Immediately after irradiation the dosimeter solution was transferred to a clean glass ampoule in which it was stored until measurement approximately 30 minutes later. Doses were calculated from a calibration curve prepared using 6°Co radiation.

RESULTS Fig. 1 shows typical temperature vs time curves for the highest and lowest dose rates used. The overshoot is due to radiation beating of the glass encapsulation of the thermistor which dissipates its heat to the bulk of the solution with a time constant of a few seconds; all temperature measurements were taken after this equilibration was complete. The temperature rise was measured by extrapolating the ple and post irradiation temperatures to the mid-point of the irradiation time. In the case of the longer irradiations a linear cooling correction was applied. Table 1 shows the results of two sets of experiments conducted on different days; the only difference between the sets is a slightly narrower range of doses in the second set. The "Total Dose" is the measured dichromate dose derived using a e°Co calibration and the "Dose per Pulse" is derived by dividing "Total Dose" by the "Number of Pulses". The last column shows the ratio of "Total Dose" to "Temperature Rise". The mean values of the measured ratios of dose to temperature rise are 2.875 (0--0.043) for the fast set and 2.891 (0---0.047) for the second set. There is no significant difference between the ratios obtained at different dose rates for either set of data. This indicates that, within the precision of the experiment (:t2% at 95% confidence), there is no dose rate effect on dosimeter response over this range of dose rates.

MECHANISTIC STUDIES Sharpe and Sehested have recently published details of some of the underlying mechanism of the dichromate dosimetert3) and, in particular, on the reactions involving silver ions. Sufficient rate constants are now known to enable the reactions of OH radicals and silver ions to be modelled using the chemical kinetics simulation program CHEMSIMUL developed at Riso re). In order to test for possible dose rate effects the program was used to follow the concentrations of OH radicals and silver ions both during and after the application of a 4 ~s, 600 Gy pulse ie the highest dose rate used in the experimental work. In addition to the well known water radiolysis reactions~7),the following reactions were also considered:

H + Cr(VI) ---> Cr(V) + H + k = 1.6x10 w H + O 2 - - > HO z k = 2.1x10 l° OH + OH ---> H202 k = 6x109 OH + Ag + ---> Ag(II) + O H

k = 1.5x101°

OH + Cr(V) ---> Cr(VI) + OH" k = 1.Sx109 Ag(II) + Cr(V) ---> Cr(VI) + Ag + k = 5.8x107 Ag(II) + HO 2 ---> Ag + + H + + 02 k = 1.7x108 Ag(II) + H202 - - > Ag ÷ + HO 2 k = 4.5x107 All rate contants are expressed in units o f dm3molqs"1

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Table 1 Pulse Length ~s

I mA

Number Pulses

Dose/ Pulse Gy

4 4 4 4 4 4 1 1 1 1 1 1 1 1 1 1 1 1

600 600 600 160 160 160 600 600 600 300 300 300 150 150 150 75 75 75

48 48 48 188 180 160 160 180 180 360 360 360 720 720 720 1202 1202 1202

489 602 613 228 227 227 166 160 149 91 91 89 53 53 53 31 31 30

8.27 10.07 10.35 15.52 14.51 12.79 9.22 9.94 9.19 11.40 11.24 11.05 13.06 13.12 13.15 12.87 12.74 12.72

23.49 28.88 29.44 42.88 40.89 36.32 26.49 28.78 26.80 32.75 32 85 32 09 38 36 38 22 38 17 37 23 36 84 36 30

2.84 2.87 2.84 2.76 2.82 2.84 2.87 2.90 2.92 2.87 2.92 2.90 2.94 2.91 2.90 2.89 2.89 2.85

4 4 4 4 4 4 1 1 1 1 1 1 1 1 1 1 1 1 1

600 600 600 120 120 120 600 600 600 300 300 300 300 150 150 150 75 75 75

60 60 60 240 220 220 220 230 230 460 420 400 400 800 800 800 1327 1400 1400

606 611 604 161 161 162 158 156 158 92 93 91 84 47 46 47 27 26 27

13.07 13.12 12.64 13.52 12.27 12 37 ii 83 12 32 12 48 14 67 13 43 12 48 Ii 38 12 89 12 76 12 64 12.28 12.90 12.99

36.36 36.68 36.25 38.58 35.49 35.62 34.85 35.87 36.34 42.19 38.96 36.24 33.75 37.69 37.12 37.29 35.83 36.97 37.45

2.78 2.80 2.87 2.85 2.89 2.88 2.95 2.91 2.91 2.88 2.90 2.90 2.97 2.92 2.91 2.95 2.92 2.87 2.88

Temp Rise oC

Total Dose kGy

Dose/ Temp

4.

3.

z M

l---

2.

N 1.

i "~.

0. 0. TIME /

= 4.

.HSEC

Figure 2 Concentration of OH radicals vs time, 4 ~ts 600 Gy pulse

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I

I

I

I

100.

2tnZ.

3~1~1.

761

Z E

O.g tJ

z

0.8 0.

400.

TIME / ~SEC

Figure 3 Concentration of Ag + following 4 p.s 600 Gy pulse Figure 2 shows the calculated concentration of OH radicals during a 4 Its, 600 Gy pulse and Figure 3 shows the recovery of Ag + concentration following the same pulse. The OH concentration achieves a virtual steady state during the pulse, the concentration at the end of the pulse being 3.4 gM. At the end of the pulse the fraction of OH reacting to form H202, rather than Ag(II) is less than 0.2% and thus no dose rate effect would be expected during the first pulse applied to a solution. Whether or not this situation still obtains for subsequent pulses depends on the recovery of the Ag + concentration. Figure 3 shows that the Ag+ concentration has recovered to within 0.5% of its pre-pulse level after 400 ~ts which suggests no dose rate effects would be expected using 600 Gy pulses provided the spacing between pulses was greater than 1 ms or so, ie the repetition rate was lower than 1000 Hz.

CONCLUSIONS Both the experimental and mechanistic work presented strongly indicate that no dose rate effects occur in the dichromate dosimeter consisting of 0.5 mM Ag2Cr207 and 2.0 mM K2Cr207 in 0.1 M perchloric acid up to a dose per pulse of 600 Gy at a repetition rate of 12.5 Hz. In addition the mechanistic studies highlight the importance of pulse repetition rate as well as dose per pulse and care should certainly be exercised if the dosimeter is used at repetition rates above a few hundred pulses per second. REFERENCES

1)

Sharpe P H G, Barrett J H and Berkley A M Int. J. Appl. Radiat. Isot., 36, 647, (1985)

2)

Thomassen J High Dose Dosimetry, Proc. of Int. Syrup. IAEA, Vienna, (STI/PUB/671), 171, (1984)

3)

Sharpe P H G and Sehested K Radiat. Phys. Chem., In press, (1989)

4)

Matthews R W Int. J. Appl. Radiat. hot., 32, 861, (1981)

5)

McLaughlin W L, Miller A and Morris W T To be published

6)

Rasmussen O L and Bjergbakke E "CHEMSIMUL - A program package for numerical simulation of chemical reaction systems" Report Riso-R-395, Riso National Laboratory, Denmark, (1984)

7)

Bjergbakke E, Sehested K, Rasmussen O L and Christensen H "Input Files for Computer Simulation of Water Radioiysis" Riso-M-2430, Riso National Laboratory, Denmark, (1984)