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Fast neutron response of coumarin in water and heavy water
0020-708X/79/0501-0289102.00/0
,nrprnarionol .four~l of Applied Radiarion and Isotope.% Vol. 30. pp. 289 to 292 $i pergamon Press Ltd 1979. Printed in Great Britain
Fast Neutron Response of Coumarin Water and Heavy Water
in
D. KRISHNAN, R. K. KHER, KAMALA GOPAKUMAR and NIRMALA S. BHANDARI Division of Radiological Protection, Bhabha Atomic Research Centre, Trombay, Bombay-400085 (AS), India (Received 25
August 1978; in revised form 25 September 1978)
Response of coumarin in aqueous solution has been studied earlier for gamma rays and fast neutrons by fluorescence measurement. For further fast neutron studies, two systems viz coumarin in Hz0 and coumarin in D20, were irradiated with fast neutrons in SNIP facility in the swimming pool type APSAI~A reactor at Trombay. Neutron fluence was estimated by measuring induced activity in sulphur pellet and associated gamma radiation was estimated using C&O4 :Dy TLD powder. The KERMA values were calculated for Hz0 and DsO, assuming modified fission spectrum for fast neutron in SN~F position, and they were in the ratio of 2: 1. Response of a chemical dosimetric system is expected to be proportional to the absorbed dose in the respective system for the same neutron fluence. This was experimentally found to be the case for coumarin in H,O or DsO. These results are likely to be true in general for any aqueous chemical system. The limitations of using such a dual system for dosimetry in a mixed field is discussed.
1. IN~ODU~~ON RESPONSE of coumarin (C9Hs02, cl-benzopyrone) in aqueous solution has been studied for gamma radiation by fluorescence measurement.“) Fast neutron response of this system was also studied at SNF position in the Trombay APSARA reactor.“’ The response of this system to fast neutrons was found to be 40% of that of 6oCo gamma radiation, on a rad to rad basis. In continuation of the above studies, we report here our results on the response of coumarin in heavy water (D,O) solution to neutron and gamma radiation. Though the results indicate a method of dosimetry in a mixed field of gamma and fast neutrons, experimental limitations exist. These are also discussed.
2. MATERIAL
AND METHODS
2.1 Chemicals Aqueous solution of coumarin, 10e4 M, (Analar grade) was prepared with triple distilled water, Heavy water was obtained from the Heavy Water Division of Bhabha Atomic Research Centre, Trombay (greater than 99.5% purity) and the chemical was directly dissolved in it. This ~oncentation of coumarin was used to enable measurements after irradiation without any further treatment or dilution. Control measurements (in the absence of any irradiation) in Hz0 or D20 were always made to check if there was any chemical interference with the measurements. 2.2 Irradiation
’
Gamma irradiations were done in a “Co gamma cell (AECL-2t)o) having a dose rate of 2l~r~/rn~, All neutron irradiations were done in SNIP(Standard 289
Neutron irradiation Facility) in APSARA reactor operating at 200 kW power level. This is a specially designed facility where gamma radiation and thermal neutrons are preferentially shielded as described earlier.(2) 2.3 Neutron fruence and gamma estimation Sample tubes containing coumarin in Hz0 or in D20 were irradiated along with a sulphur pellet and CatSO.+: Dy thermolumin~~nt dosimeter, all sealed in a polythene bag. Activation of sulphur pellet by fast neutrons is a measure of fast neutron fluence. CaSO, :Dy was used to estimate the gamma component, as this phosphor is relatively insensitive to both thermal and fast neutrons.(3) Thermal neutron response of this thermoluminescent phosphor with 0.05% Dy has been stud&P3) and found to be equivalent to 0.38r of 6oCo gammas, for a fluence of 1O’O thermal neutrons per cm2. Thermal neutron fluence at the position of irradiation was measured using gold foil activation and found to be 1.55 x 10” n*cme2 for 1 h irradiation. Contribution of this thermal neutron fluence to CaSO, : Dy response amount to 6 r of 6oCo gamma rays which is about 2% of the gamma background at the same position and this mounts to 0.5% of total dose. This correction has been neglected. The details of gamma and neutron dosimetry along with the calibration procedure for the TLD phosphor were described elsewhere.‘2’ The fast neutron flux at the position of irradiation was 6 x log n.cm-2min-’ and the gamma background was 3rad/min. 2.4 Fluorescence measurement The response of the chemical system was measured by measuring the fluorescent products using an Amin-
290
D. Krishnan, R. K. Kher, K. Gopakumar and N. S. Bhandari
co-Bowman spectrophotofluorometer. The instrument was always standardized using a fluorescence standard and using the same slit programme and instrument sensitivity. Uranium Glass Reference Rodsc4) made of glass fused with various concentrations of uranyl nitrate were used for day to day instrument standardization. The fluorescence of unirradiated neutral aqueous 10e4 M coumarin was found to be negligible: i.e. the background fluorescence was less than 1% of measurement levels. Neutral aqueous solution of coumarin on irradiation gave fluorescent products having an excitation peak at 332 nm and an emission peak at 460~1. The excitation and emission spectra of gamma irradiated coumarin and that of 7-hydroxycoumarin, both in water, were found to be matching.“) The spectra of 7-hydroxycoumarin in water and heavy water, as well as gamma irradiated coumarin in water and heavy water and neutron irradiated coumarin in water were seen to be matching with the above spectra. It was previously shown that 7-hydroxycoumarin is the main product giving the fluorescence at the measurement wavelengths. Using a calibration graph of concentrations of pure 7-hydroxycoumarin vs fluorescence, the measured fluorescence in all our experiments is expressed in terms of fluorescence equivalent concentration of 7-hydroxycoumarin”) (referred to hereafter as FEC of HC). The necessary quenching corrections for water solutions are always applied. It was observed that when a pure sample of 7-hydroxycoumarin, used as a calibration standard, was dissolved in H20 and D,O, the resulting fluorescence in the case of the D20 sample was on the average 8% less as compared to the Hz0 sample. This could be attributed to solvent effect such as scattering, quenching etc. Presence of 10m4M coumarin in these samples is found not to alter this difference in fluorescence response. By accurately estimating this correction, use of only one calibration standard either in Hz0 or in D20 is necessary. However, using separate calibration standards for samples in Hz0 and in DsO this correction could be eliminated. The graphs plotted for D,O incorporate this correction. In all cases of reactor irradiation the fluorescence due to the gamma background was estimated from the gamma calibration graphs and subtracted from
5oL
0
2
24
28
Ga”:a doZ I&&l) x:,9,4 FIG. 1. Fluorescence of coumarin (10e4 M) in water or heavy water, due to gamma irradiation. (A) O---O, solution in light water; (B) O-O, solution in heavy water.
the total measured fluorescence, to estimate the FEC of HC due to neutrons alone.
3. RESULTS 3.1
KERMA
AND
DISCUSSION
CdCUhiO?I
KERMArelease of charged particle kinetic energy by neutrons interacting with the medium at a particular location was calculated for D20 by a procedure similar to the one used for Hz0 by BACHand CASWELL.‘~) The appropriate cross sections for D were taken from JOANOU,GCK~DJOHN and WIKNER.(@The dose conversion factor for a normalised spectrum 4 (E) is given by 14MeV D, =
s 0.1 MeV
4(E).
K(E). dE
(1)
where K(E) is KERMAfor neutron of energy E in the medium in rad.n-’ cm2 and DF is the first collision dose for the spectrum, in rad.n-’ cm*.K(E) values for H20 were taken from BACH and CASWELL.(‘)For
TABLE1. Dose conversion factors for Hz0 and D,O for fast neutron spectra
Material H D 0
H20 D,O
Dose conversion factor, rad n- ’ cm’, for 1 g of material Fission spectrum? SNIF spectrum* 30.12 9.99 0.183 3.51 2.15
x x x x x
1O-9 1o-9 lO-9 1o-9 1O-9
28.54 x 1O-9 0.277 x 1O-9 3.43 x 1o-9
* Q(E) dE = 0.63 E* exp(-0.68E) dE (Shinde, 1975).“’ t Uranium converter plate fission spectrum (Watt, 19527s) qb(E)dE = 0.484 sinh (2E)* exp( - E) dE.
FIG. 2. Fast neutron response of coumarin (lo-“ M) in water or heavy water, plotted against KERMA of fast neutrons. 0, solution in light water; 0, solution in heavy water.
Fast neutron response ofcoumarin
Fast Newtron
in water and heavy water
Fluence
(n/cm’)
291
x IO-”
FIG. 3. Fast neutron response of coumarin (10e4 M) in water or heavy water, plotted against fluence of fast neutrons. (C) O-O, solution in light water; (D) O-O, solution in heavy water.
this, the modified fission spectrum earlier determined in this laboratory by a spectral index method using four pairs of activation detectors, viz. In-Al, In-Mg, S-Al and S-Mg, was used. This spectrum is given by (SHINDE, 1975)“) 4(E) dE = 0.63 E* exp( - 0.68 E) dE.
(2) The dose conversion factors for the above spectrum are given in Table 1. Values for fission spectrum@’ for water are also given for comparison. It can be seen that dose conversion factors for HZ0 for both the spectra are nearly the same, indicatbg that any small errors in spectrum determination may not affect the KERMA calculation significantly. 3.2 Gamma response of coumarin in Hz0
and D20
To see whether substitution of HZ0 by D,O drastically alters the radiation chemistry of the system a control experiment with gamma rays was done. The results are shown in Fig. 1. Results of the figure show that the response in DzO is slightly higher than in Hz0 for the same absorbed energy (eV/ml). Similar difference on substitution of HZ0 by D20 for gamma irradiation of chemical systems has been reported by many workers. (9-iz) The observed increase in gamma response in D20 compared to that in Hz0 can be attributed to the difference in Go, and GOH values for gamma irradiation.“1*13) 3.3 Neutron response of coumarin in Hz0
and D20
Figure 2 gives the response of coumarin in Hz0 or D20 to fast neutron absorbed dose alone, as calculated using Table 1. The fluorescence due to gamma background, though small, has been subtracted using the gamma dose estimated by CaSO, :Dy TLD. The figure shows that the neutron response of coumarin in Hz0 or DzO is proportional to their respective absorbed doses in Hz0 or D1O, thereby giving experimental evidence for the fundamental assumption in chemical dosimetry.
Figure 3 gives the same data as in Fig. 2 but plotted against neutron fluence. The response of these two systems viz. coumarin in HZ0 and D20 for simultaneous neutron irradiation for the same duration (say, in a reactor) will be as shown in Fig. 3, i.e. the response of the systems will differ considerably (i.e. by a factor of approximately 2) for the same fluence. In principle this large difference in response (factor of 2) of these two systems with respect to neutron fluence and nearly equal response for gamma dose suggests a method for measurement of neutron fluence in a mixed field. However, in practice, due to experimental limitations, the gamma-neutron ratio under which this dual system could be useful is limited. This limitation is applicable even for a dual chemical dosimeter in Hz0 and D20 whose response is independent of LET, as discussed in the Appendix. Acknowledgements-We wish to thank R. C. BHATT, U. MADHVANATH, S. J. SUPE and K. G. VOHRAfor their interest, and the operating crew of APSARA reactor for their kind co-operation.
REFERENCES 1. GOPAKUMARK., KINI U. R., ASHAWA,S. C., BHANDARI N. S., KRISHNANG. U. and KRISHNAND. Radiation E@cts 32, 199 (1977). 2. GOPAKUMARK., BHANDARIN. S., KHER R. K. and KRI~HNAND. Int. J. appl. Radiat. Isotopes 29, 9 (1978). 3. AYYANGAR K., BHUWAN CHANDRA and LAKSHMANAN A. R. Physics Med. Biol. 19, 656 (1974). 4. JOYCE R. J., GALLAWASG. E. and MCCALLUM,J. D. Fluorometric Procedures Manual 1447 p.9, 15. Beckman Instruments Inc., (1965). 5. BACH R. L. and CASWELL R. S. Radiat. Res. 35, 1 (1968). 6. JOANOUG. D., G~~DJOHN A. J. and WIKNER N. F. Nucl. Sci. Engng 13. 171 (1962). 7. SHINDES. S.-Unpublished results (1975). 8. WATT B. E. Phvs. Rev. 81. 1037 (1952). 9. ARMSTRONGd., COLL&N k., ~AINTON F. S., DONALDSON D. M., HAYON E., MILLERN. and WEM
292
10. 11. 12. 13.
14. 15.
D. Krishnan,
R. K. Kher, K. Gopakumar
J. International Conference on Peaceful Uses of Atomic Energy 29, 80 (1958). HAYON E. J. phys. Chem. 69, 2628 (1965). FIELDEN E. M. and HART E. J. Radiat. Res. 33, 426 (1968). KUMAR S. S. and FAW R. E. Nucl. Instrum. Meth. 51, 175 (1967). SPINKS J. W. T. and WOODS R. J. An Introduction to Radiation Chemistry 2nd edn, p. 254. Wiley, New York (1976). MARKOVIC V. and DRAGANIC I. Radiat. Res. 36, 588 (1968). DRAGANIC I. G., RADAK B. B. and MARKOVIC V. M. Int. .I. appl. Radiat. Isotopes 16, 145 (1965).
APPENDIX The response per gram for a double chemical dosimetric system in Hz0 or D20, used in a mixed field of gamma and fast neutrons will be given by: R, - = AG, P1
(Al)
DG1 + AN1. DN,
- = AG2.DG2 P2
642)
+ AN2.DNZ
where Rt = observed R2 = observed AG1 = response gamma, AGI = response gamma, AN1 = response neutron, ANI = response neutron, DG1 = gamma DG2 = gamma DN1 = neutron DNZ = neutron
response response of Hz0
(per ml) of Hz0 system, (per ml) of D20 system, system per unit rad of
of D,O
system
per
unit
of
of H,O
system
per unit KERMA
of D,O
system
per unit KERMA of
energy absorbed energy absorbed energy absorbed energy absorbed
(rad) (rad) (rad) (rad)
in in in in
for same neutron
and D,O
X Y \ 0.1 0.4 0.8 1.0 4.0 8.0
0.1
0.4
0.6
1.0
22.5 9.0 5.0 4.1 1.1 0.5
36.0 22.5 15.0 12.8 4.1 2.1
38.6 27.0 19.3 16.9 5.9 3.1
40.9 32.1 25.0 22.5 9.0 5.0
For explanations
of F, Q, M, X and
Q = KERMA in ~~~~~~~~
F
=
=
Rz - R, RI rn. AG,
DG1 + AN1
DN1 [l.lQ(l
+ m) - 1.01
+ AN1. DN,
mY + X[l.lQ(l
+ m) - 1.01
(A9)
y+x
’
and
DG, p=
Y.
DN,
reTable 2 gives the typical values of F for various values of X and Y, for Q = 0.5 and m = 0. Under such conditions, for values of R, > RZ (A3) 1 -(l.lQ) IFI = ~ 1 + (Y/X) (A4)
in H20.
AG2 = (1 + m)AG,.
(A5)
of any certain experimental evidence for absence of this effect in the case of neuthe same isotopic effect of DzO for neugamma rays. Then AN2 = (1 + m)AN,
(A6)
in (Al) and (A2) we get + AN1.DN,
(A7)
R2 = (1 + m)AG,.DG, + l.l(l
=
AG,
The gamma response per rad of a dosimeter in Hz0 and D20 may differ by a small factor, probably due to isotopic effect of D20 as seen in Section 3.2. Taking this into account
R, = AG,.DG1
Y see text.
From the knowledge of neutron spectrum at the place of irradiation the KERMA ratio Q can be calculated. If the response of the two dosimetric systems to gamma and neutrons is known (i.e. AGI, AN1, and m) experimentally these two simultaneous equations can be solved to get DG, and DN,. The difference in observed response of the two systems will be limited due to experimental errors. Fluorescence or spectral absorption measurements usually employed in chemical dosimetric systems have accuracy of about 5%. To analyse the limitations due to experimental errors under various conditions of neutron and gamma doses and response ratio we define the parameter F, given by
AN, -=X
where
where p, = 1 g/cm3
of F for Q = 0.5 and m = 0.0
where
fluence
DN2 = Q.DN,
Substituting
values
of
H20, DzO, H20, D20,
DG1 = p2 DG*.
In the absence the presence or trons, we assume trons as that for
TABLE 2. Percentage
AG1. DGI
and pI and pt are density (g/cm3) of Hz0 spectively. For the same gamma exposure
Similarly,
rad
and N. S. Bhandari
+ m).Q.ANI-DN,
and pz = 1.1 gjcmj,
(A8)
have been used.
(AlO)
From Table 2 it can be seen that for X = 1 i.e. an LETindependent chemical system, for a 5% accuracy in difference measurement of R, and R,, the gamma-neutron dose ratio in which this dual system of Hz0 and D,O will be useful is 8. However, for systems like FeS04, coumarin, calcium benzoate, terephthallic acid etc. X is in the range of 0.40.5. This fact limits the range of usefulness of a dual system using one of the above chemical dosimeters in Hz0 or DzO. From Table 2, for X = 0.4 and m = 0, the upper limit of Y is around 3 for 5% accuracy in difference measurement of R, and RZ If m = 0.1 this goes down to 0.9. However, for the same m = 0.1 but X = 1, a Y value of 2.3 is in measureable range. For values of m higher than 0.1, the maximum measurable value of Y becomes still less. Any improvement in measurable range of Y is possible by improving the accuracy of measurement to better than 3%. A dual system using oxalic acid in H,O or in D20 which has been suggested for reactor dosimetry”4*15’ was successful because their measurements were done with a maximum gamma-neutron ratio of 2: 1 (i.e. within the limitations discussed here).
,nrprnarionol .four~l of Applied Radiarion and Isotope.% Vol. 30. pp. 289 to 292 $i pergamon Press Ltd 1979. Printed in Great Britain
Fast Neutron Response of Coumarin Water and Heavy Water
in
D. KRISHNAN, R. K. KHER, KAMALA GOPAKUMAR and NIRMALA S. BHANDARI Division of Radiological Protection, Bhabha Atomic Research Centre, Trombay, Bombay-400085 (AS), India (Received 25
August 1978; in revised form 25 September 1978)
Response of coumarin in aqueous solution has been studied earlier for gamma rays and fast neutrons by fluorescence measurement. For further fast neutron studies, two systems viz coumarin in Hz0 and coumarin in D20, were irradiated with fast neutrons in SNIP facility in the swimming pool type APSAI~A reactor at Trombay. Neutron fluence was estimated by measuring induced activity in sulphur pellet and associated gamma radiation was estimated using C&O4 :Dy TLD powder. The KERMA values were calculated for Hz0 and DsO, assuming modified fission spectrum for fast neutron in SN~F position, and they were in the ratio of 2: 1. Response of a chemical dosimetric system is expected to be proportional to the absorbed dose in the respective system for the same neutron fluence. This was experimentally found to be the case for coumarin in H,O or DsO. These results are likely to be true in general for any aqueous chemical system. The limitations of using such a dual system for dosimetry in a mixed field is discussed.
1. IN~ODU~~ON RESPONSE of coumarin (C9Hs02, cl-benzopyrone) in aqueous solution has been studied for gamma radiation by fluorescence measurement.“) Fast neutron response of this system was also studied at SNF position in the Trombay APSARA reactor.“’ The response of this system to fast neutrons was found to be 40% of that of 6oCo gamma radiation, on a rad to rad basis. In continuation of the above studies, we report here our results on the response of coumarin in heavy water (D,O) solution to neutron and gamma radiation. Though the results indicate a method of dosimetry in a mixed field of gamma and fast neutrons, experimental limitations exist. These are also discussed.
2. MATERIAL
AND METHODS
2.1 Chemicals Aqueous solution of coumarin, 10e4 M, (Analar grade) was prepared with triple distilled water, Heavy water was obtained from the Heavy Water Division of Bhabha Atomic Research Centre, Trombay (greater than 99.5% purity) and the chemical was directly dissolved in it. This ~oncentation of coumarin was used to enable measurements after irradiation without any further treatment or dilution. Control measurements (in the absence of any irradiation) in Hz0 or D20 were always made to check if there was any chemical interference with the measurements. 2.2 Irradiation
’
Gamma irradiations were done in a “Co gamma cell (AECL-2t)o) having a dose rate of 2l~r~/rn~, All neutron irradiations were done in SNIP(Standard 289
Neutron irradiation Facility) in APSARA reactor operating at 200 kW power level. This is a specially designed facility where gamma radiation and thermal neutrons are preferentially shielded as described earlier.(2) 2.3 Neutron fruence and gamma estimation Sample tubes containing coumarin in Hz0 or in D20 were irradiated along with a sulphur pellet and CatSO.+: Dy thermolumin~~nt dosimeter, all sealed in a polythene bag. Activation of sulphur pellet by fast neutrons is a measure of fast neutron fluence. CaSO, :Dy was used to estimate the gamma component, as this phosphor is relatively insensitive to both thermal and fast neutrons.(3) Thermal neutron response of this thermoluminescent phosphor with 0.05% Dy has been stud&P3) and found to be equivalent to 0.38r of 6oCo gammas, for a fluence of 1O’O thermal neutrons per cm2. Thermal neutron fluence at the position of irradiation was measured using gold foil activation and found to be 1.55 x 10” n*cme2 for 1 h irradiation. Contribution of this thermal neutron fluence to CaSO, : Dy response amount to 6 r of 6oCo gamma rays which is about 2% of the gamma background at the same position and this mounts to 0.5% of total dose. This correction has been neglected. The details of gamma and neutron dosimetry along with the calibration procedure for the TLD phosphor were described elsewhere.‘2’ The fast neutron flux at the position of irradiation was 6 x log n.cm-2min-’ and the gamma background was 3rad/min. 2.4 Fluorescence measurement The response of the chemical system was measured by measuring the fluorescent products using an Amin-
290
D. Krishnan, R. K. Kher, K. Gopakumar and N. S. Bhandari
co-Bowman spectrophotofluorometer. The instrument was always standardized using a fluorescence standard and using the same slit programme and instrument sensitivity. Uranium Glass Reference Rodsc4) made of glass fused with various concentrations of uranyl nitrate were used for day to day instrument standardization. The fluorescence of unirradiated neutral aqueous 10e4 M coumarin was found to be negligible: i.e. the background fluorescence was less than 1% of measurement levels. Neutral aqueous solution of coumarin on irradiation gave fluorescent products having an excitation peak at 332 nm and an emission peak at 460~1. The excitation and emission spectra of gamma irradiated coumarin and that of 7-hydroxycoumarin, both in water, were found to be matching.“) The spectra of 7-hydroxycoumarin in water and heavy water, as well as gamma irradiated coumarin in water and heavy water and neutron irradiated coumarin in water were seen to be matching with the above spectra. It was previously shown that 7-hydroxycoumarin is the main product giving the fluorescence at the measurement wavelengths. Using a calibration graph of concentrations of pure 7-hydroxycoumarin vs fluorescence, the measured fluorescence in all our experiments is expressed in terms of fluorescence equivalent concentration of 7-hydroxycoumarin”) (referred to hereafter as FEC of HC). The necessary quenching corrections for water solutions are always applied. It was observed that when a pure sample of 7-hydroxycoumarin, used as a calibration standard, was dissolved in H20 and D,O, the resulting fluorescence in the case of the D20 sample was on the average 8% less as compared to the Hz0 sample. This could be attributed to solvent effect such as scattering, quenching etc. Presence of 10m4M coumarin in these samples is found not to alter this difference in fluorescence response. By accurately estimating this correction, use of only one calibration standard either in Hz0 or in D20 is necessary. However, using separate calibration standards for samples in Hz0 and in DsO this correction could be eliminated. The graphs plotted for D,O incorporate this correction. In all cases of reactor irradiation the fluorescence due to the gamma background was estimated from the gamma calibration graphs and subtracted from
5oL
0
2
24
28
Ga”:a doZ I&&l) x:,9,4 FIG. 1. Fluorescence of coumarin (10e4 M) in water or heavy water, due to gamma irradiation. (A) O---O, solution in light water; (B) O-O, solution in heavy water.
the total measured fluorescence, to estimate the FEC of HC due to neutrons alone.
3. RESULTS 3.1
KERMA
AND
DISCUSSION
CdCUhiO?I
KERMArelease of charged particle kinetic energy by neutrons interacting with the medium at a particular location was calculated for D20 by a procedure similar to the one used for Hz0 by BACHand CASWELL.‘~) The appropriate cross sections for D were taken from JOANOU,GCK~DJOHN and WIKNER.(@The dose conversion factor for a normalised spectrum 4 (E) is given by 14MeV D, =
s 0.1 MeV
4(E).
K(E). dE
(1)
where K(E) is KERMAfor neutron of energy E in the medium in rad.n-’ cm2 and DF is the first collision dose for the spectrum, in rad.n-’ cm*.K(E) values for H20 were taken from BACH and CASWELL.(‘)For
TABLE1. Dose conversion factors for Hz0 and D,O for fast neutron spectra
Material H D 0
H20 D,O
Dose conversion factor, rad n- ’ cm’, for 1 g of material Fission spectrum? SNIF spectrum* 30.12 9.99 0.183 3.51 2.15
x x x x x
1O-9 1o-9 lO-9 1o-9 1O-9
28.54 x 1O-9 0.277 x 1O-9 3.43 x 1o-9
* Q(E) dE = 0.63 E* exp(-0.68E) dE (Shinde, 1975).“’ t Uranium converter plate fission spectrum (Watt, 19527s) qb(E)dE = 0.484 sinh (2E)* exp( - E) dE.
FIG. 2. Fast neutron response of coumarin (lo-“ M) in water or heavy water, plotted against KERMA of fast neutrons. 0, solution in light water; 0, solution in heavy water.
Fast neutron response ofcoumarin
Fast Newtron
in water and heavy water
Fluence
(n/cm’)
291
x IO-”
FIG. 3. Fast neutron response of coumarin (10e4 M) in water or heavy water, plotted against fluence of fast neutrons. (C) O-O, solution in light water; (D) O-O, solution in heavy water.
this, the modified fission spectrum earlier determined in this laboratory by a spectral index method using four pairs of activation detectors, viz. In-Al, In-Mg, S-Al and S-Mg, was used. This spectrum is given by (SHINDE, 1975)“) 4(E) dE = 0.63 E* exp( - 0.68 E) dE.
(2) The dose conversion factors for the above spectrum are given in Table 1. Values for fission spectrum@’ for water are also given for comparison. It can be seen that dose conversion factors for HZ0 for both the spectra are nearly the same, indicatbg that any small errors in spectrum determination may not affect the KERMA calculation significantly. 3.2 Gamma response of coumarin in Hz0
and D20
To see whether substitution of HZ0 by D,O drastically alters the radiation chemistry of the system a control experiment with gamma rays was done. The results are shown in Fig. 1. Results of the figure show that the response in DzO is slightly higher than in Hz0 for the same absorbed energy (eV/ml). Similar difference on substitution of HZ0 by D20 for gamma irradiation of chemical systems has been reported by many workers. (9-iz) The observed increase in gamma response in D20 compared to that in Hz0 can be attributed to the difference in Go, and GOH values for gamma irradiation.“1*13) 3.3 Neutron response of coumarin in Hz0
and D20
Figure 2 gives the response of coumarin in Hz0 or D20 to fast neutron absorbed dose alone, as calculated using Table 1. The fluorescence due to gamma background, though small, has been subtracted using the gamma dose estimated by CaSO, :Dy TLD. The figure shows that the neutron response of coumarin in Hz0 or DzO is proportional to their respective absorbed doses in Hz0 or D1O, thereby giving experimental evidence for the fundamental assumption in chemical dosimetry.
Figure 3 gives the same data as in Fig. 2 but plotted against neutron fluence. The response of these two systems viz. coumarin in HZ0 and D20 for simultaneous neutron irradiation for the same duration (say, in a reactor) will be as shown in Fig. 3, i.e. the response of the systems will differ considerably (i.e. by a factor of approximately 2) for the same fluence. In principle this large difference in response (factor of 2) of these two systems with respect to neutron fluence and nearly equal response for gamma dose suggests a method for measurement of neutron fluence in a mixed field. However, in practice, due to experimental limitations, the gamma-neutron ratio under which this dual system could be useful is limited. This limitation is applicable even for a dual chemical dosimeter in Hz0 and D20 whose response is independent of LET, as discussed in the Appendix. Acknowledgements-We wish to thank R. C. BHATT, U. MADHVANATH, S. J. SUPE and K. G. VOHRAfor their interest, and the operating crew of APSARA reactor for their kind co-operation.
REFERENCES 1. GOPAKUMARK., KINI U. R., ASHAWA,S. C., BHANDARI N. S., KRISHNANG. U. and KRISHNAND. Radiation E@cts 32, 199 (1977). 2. GOPAKUMARK., BHANDARIN. S., KHER R. K. and KRI~HNAND. Int. J. appl. Radiat. Isotopes 29, 9 (1978). 3. AYYANGAR K., BHUWAN CHANDRA and LAKSHMANAN A. R. Physics Med. Biol. 19, 656 (1974). 4. JOYCE R. J., GALLAWASG. E. and MCCALLUM,J. D. Fluorometric Procedures Manual 1447 p.9, 15. Beckman Instruments Inc., (1965). 5. BACH R. L. and CASWELL R. S. Radiat. Res. 35, 1 (1968). 6. JOANOUG. D., G~~DJOHN A. J. and WIKNER N. F. Nucl. Sci. Engng 13. 171 (1962). 7. SHINDES. S.-Unpublished results (1975). 8. WATT B. E. Phvs. Rev. 81. 1037 (1952). 9. ARMSTRONGd., COLL&N k., ~AINTON F. S., DONALDSON D. M., HAYON E., MILLERN. and WEM
292
10. 11. 12. 13.
14. 15.
D. Krishnan,
R. K. Kher, K. Gopakumar
J. International Conference on Peaceful Uses of Atomic Energy 29, 80 (1958). HAYON E. J. phys. Chem. 69, 2628 (1965). FIELDEN E. M. and HART E. J. Radiat. Res. 33, 426 (1968). KUMAR S. S. and FAW R. E. Nucl. Instrum. Meth. 51, 175 (1967). SPINKS J. W. T. and WOODS R. J. An Introduction to Radiation Chemistry 2nd edn, p. 254. Wiley, New York (1976). MARKOVIC V. and DRAGANIC I. Radiat. Res. 36, 588 (1968). DRAGANIC I. G., RADAK B. B. and MARKOVIC V. M. Int. .I. appl. Radiat. Isotopes 16, 145 (1965).
APPENDIX The response per gram for a double chemical dosimetric system in Hz0 or D20, used in a mixed field of gamma and fast neutrons will be given by: R, - = AG, P1
(Al)
DG1 + AN1. DN,
- = AG2.DG2 P2
642)
+ AN2.DNZ
where Rt = observed R2 = observed AG1 = response gamma, AGI = response gamma, AN1 = response neutron, ANI = response neutron, DG1 = gamma DG2 = gamma DN1 = neutron DNZ = neutron
response response of Hz0
(per ml) of Hz0 system, (per ml) of D20 system, system per unit rad of
of D,O
system
per
unit
of
of H,O
system
per unit KERMA
of D,O
system
per unit KERMA of
energy absorbed energy absorbed energy absorbed energy absorbed
(rad) (rad) (rad) (rad)
in in in in
for same neutron
and D,O
X Y \ 0.1 0.4 0.8 1.0 4.0 8.0
0.1
0.4
0.6
1.0
22.5 9.0 5.0 4.1 1.1 0.5
36.0 22.5 15.0 12.8 4.1 2.1
38.6 27.0 19.3 16.9 5.9 3.1
40.9 32.1 25.0 22.5 9.0 5.0
For explanations
of F, Q, M, X and
Q = KERMA in ~~~~~~~~
F
=
=
Rz - R, RI rn. AG,
DG1 + AN1
DN1 [l.lQ(l
+ m) - 1.01
+ AN1. DN,
mY + X[l.lQ(l
+ m) - 1.01
(A9)
y+x
’
and
DG, p=
Y.
DN,
reTable 2 gives the typical values of F for various values of X and Y, for Q = 0.5 and m = 0. Under such conditions, for values of R, > RZ (A3) 1 -(l.lQ) IFI = ~ 1 + (Y/X) (A4)
in H20.
AG2 = (1 + m)AG,.
(A5)
of any certain experimental evidence for absence of this effect in the case of neuthe same isotopic effect of DzO for neugamma rays. Then AN2 = (1 + m)AN,
(A6)
in (Al) and (A2) we get + AN1.DN,
(A7)
R2 = (1 + m)AG,.DG, + l.l(l
=
AG,
The gamma response per rad of a dosimeter in Hz0 and D20 may differ by a small factor, probably due to isotopic effect of D20 as seen in Section 3.2. Taking this into account
R, = AG,.DG1
Y see text.
From the knowledge of neutron spectrum at the place of irradiation the KERMA ratio Q can be calculated. If the response of the two dosimetric systems to gamma and neutrons is known (i.e. AGI, AN1, and m) experimentally these two simultaneous equations can be solved to get DG, and DN,. The difference in observed response of the two systems will be limited due to experimental errors. Fluorescence or spectral absorption measurements usually employed in chemical dosimetric systems have accuracy of about 5%. To analyse the limitations due to experimental errors under various conditions of neutron and gamma doses and response ratio we define the parameter F, given by
AN, -=X
where
where p, = 1 g/cm3
of F for Q = 0.5 and m = 0.0
where
fluence
DN2 = Q.DN,
Substituting
values
of
H20, DzO, H20, D20,
DG1 = p2 DG*.
In the absence the presence or trons, we assume trons as that for
TABLE 2. Percentage
AG1. DGI
and pI and pt are density (g/cm3) of Hz0 spectively. For the same gamma exposure
Similarly,
rad
and N. S. Bhandari
+ m).Q.ANI-DN,
and pz = 1.1 gjcmj,
(A8)
have been used.
(AlO)
From Table 2 it can be seen that for X = 1 i.e. an LETindependent chemical system, for a 5% accuracy in difference measurement of R, and R,, the gamma-neutron dose ratio in which this dual system of Hz0 and D,O will be useful is 8. However, for systems like FeS04, coumarin, calcium benzoate, terephthallic acid etc. X is in the range of 0.40.5. This fact limits the range of usefulness of a dual system using one of the above chemical dosimeters in Hz0 or DzO. From Table 2, for X = 0.4 and m = 0, the upper limit of Y is around 3 for 5% accuracy in difference measurement of R, and RZ If m = 0.1 this goes down to 0.9. However, for the same m = 0.1 but X = 1, a Y value of 2.3 is in measureable range. For values of m higher than 0.1, the maximum measurable value of Y becomes still less. Any improvement in measurable range of Y is possible by improving the accuracy of measurement to better than 3%. A dual system using oxalic acid in H,O or in D20 which has been suggested for reactor dosimetry”4*15’ was successful because their measurements were done with a maximum gamma-neutron ratio of 2: 1 (i.e. within the limitations discussed here).