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Dependence of TLD thermoluminescence yield on absorbed dose in a thermal neutron field
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Pergamon PIh
Appl. Radiat. lsot. Vol. 48, No. 10-12 pp. 1467-1475, 1997 © 1997 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain S0969-8043(97)00142-5 0969-8043/97 $17.00 + 0.00
Dependence of TLD Thermoluminescence Yield on Absorbed Dose in a Thermal Neutron Field G. GAMBARIN1.1"2 a n d M. S I N H A R O Y ~ ~Dipartimento di Fisica dell'Universit/t, via Celoria 16, 20133 Milano, Italy and qstituto Nazionale di Fisica Nucleare, Sezione di Milano, via Celoria 16, 20133 Milano, Italy The emission from 6LiF and 7LiF thermoluminescencedosimeters (TLDs) exposed to the mixed field of thermal neutrons and y-rays of the thermal facility of a TRIGA MARK II nuclear reactor has been investigated for various thermal neutron fluences of the order of magnitude of those utilised in radiotherapy, with the purpose of investigating the reliability of TLD readouts in such radiation fields and of giving some information for better obtainment of the absorbed dose values. The emission after exposure in this mixed field is compared with the emission after y-rays only. The glow curves have been deconvoluted into gaussian peaks, and the differencesin the characteristics of the peaks observed for the two radiation fields, having different linear energy transfers, and for different doses are shown. Irreversible radiation damage in dosimeters having high sensitivity to thermal neutrons is also reported, showing a memory effect of the previous thermal neutron irradiation history which is not restored by anneal treatment. © 1997 Published by Elsevier Science Ltd. All rights reserved
Introduction Thermoluminescent dosimeters (TLDs) are currently used in industrial and medical applications to determine the absorbed dose in any radiation field. In fact, both their small dimensions and their tissueequivalence for most radiation allow mapping of the absorbed dose distribution without significantly modifying the radiation field. Some care has to be taken in order to obtain reliable results, but in personal dosimetry, i.e. for low absorbed doses, the protocol for TLD utilisation is satisfactorily settled. In contrast, when TL dosimeters are used for high dose measurements, such as those required in radiotherapy, a lack of precision and many ambiguities arise. Moreover, many unresolved problems may appear in radiation fields different from 7-rays, especially if the linear energy transfer (LET) of the radiation is high. In fact, the thermoluminescent yield of materials may depend on the LET of the ionising particles. Thus, after exposure to radiation of various LETs, the shape of the glow curves obtained from these dosimeters is expected to change with the LET as well as with the dose. In mixed radiation fields, often met in radiotherapy, the difference in the emission due to the different radiation components of the field may be used to relate the contributions of these components to the absorbed dose; but a good knowledge, for each type *To whom correspondence should be addressed.
of radiation, of the characteristics of the peaks in the glow curve is sometimes needed to make such an attribution correctly. In the present work, the shapes of glow curves obtained from 6LiF chips after exposure to high doses of thermal neutrons are analysed. The isotope 6Li has a high cross-section (945 barn) for the reaction with thermal neutrons: 6Li (n,~t) 3H The particles emitted in this reaction, i.e. s-particles (2.07 MeV) and tritons (2.24 MeV), have a higher LET than 7-radiation. Therefore, the glow curve after exposure in a thermal neutron field is different from that after gamma radiation. Moreover, in TL dosimeters containing a great amount of 6Li and exposed to thermal neutrons, the shape of the glow curve depends on dose, that is on neutron fluence. In this work, some 6LiF and 7LiF dosimeters were exposed in the thermal column of a nuclear reactor and then analysed. A comparative study has been made to find a correlation between the glow curve shapes for LiF dosimeters and thermal neutron fluence.
ExperimentalProcedures Dosimeters analysed The TLDs that were analysed are commercial 6LiF and 7LiF dosimeters doped with Mg, Ti or with
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G. Gambarini and M. Sinha Roy
Mg, Cu, P. LiF:Mg, Ti dosimeters, from the Harshaw Chemical Co., are TLD-600 (with 96.5% 6LiF) and TLD-700 (with 99.99% 7LiF) in the form of chips measuring 3.1 x 3.1 x 0.9 mm. LiF:Mg, Cu, P dosimeters, from the Beijing Radiation Detector Work, People's Republic of China, are Gr-206A (6LiF:Mg, Cu, P) and GR-207A (TLiF:Mg, Cu, P) in the form of circular chips, 4.5 mm diameter and 0.8 mm thick.
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Annealing procedures The dosimeters were all annealed together before each irradiation. All dosimeters compared were from the same batch and had undergone the same number of annealing cycles from the time of purchase. It is known, in fact, that the sensitivity of TL dosimeters is strongly dependent on their thermal history. In particular, the cooling rate after heating is critical (Marshall, 1984). For all dosimeters, the annealing procedure recommended by the manufacturing company has been followed. LiF:Mg, Ti chips were heated at 40ff'C for 1 h, quenched, with gradual cooling, down to room temperature and then annealed at 100°C for 2 h. No post-irradiation low temperature annealing was done, but all readouts were performed 48 h after irradiation to allow the decay of low temperature peaks. LiF:Mg, Cu, P dosimeters were annealed at 240 ~C for 10min, and then rapidly cooled to room temperature. Readouts were performed 3 days after exposure.
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For TLD-600 and TLD-700 chips a Harshaw/ Filtrol system has been utilised, composed of a Model 2000A detector interfaced to a Model 2080 Picoprocessor. The readout was performed using a constant heating rate of &C s- ~ up to 330 ~'C. The
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SEC Fig. 2. Glow curves from TLDs-600 and TLDs-700exposed to various thermal neutron fluences. software of the instrument provides glow curve representation and area evaluation in the regions of interest. Glow curve deconvolutions into gauss/an peaks have been executed with laboratory-written programs which supply the parameters of gauss/an peaks and a graphical representation of the experimental glow curve, deconvoluted peaks and peak addition. For the GR-206 and GR-207 chips, a Model 3500 TLD reader from Harshaw/Bicron was used. The readout was performed using a constant heating rate of 4°C s-~. The software of the instrument allows glow curve representation and area determination in regions of interest.
Irradiation facilities For "/-irradiation, a '37Cs irradiator with a rate of about 0.14 Gy s - ' has been utilised. Exposures to thermal neutrons were performed at the TRIGA MARK II nuclear reactor in Pavia (Italy). Most samples were exposed in the swimmingpool-type facility, with a thermal neutron fluence rate of 1.44 x l0 s neutrons cm - 2 s- ' and a cadmium ratio
Dependence of TLD thermoluminescenceon absorbed dose equal to ~ 90. A few samples were irradiated in the thermal column, with a fluence rate of 4 x 10 9 neutrons c m - 2 s - t and a cadmium ratio of about 30.
ExperimentalResults TL dosimetry in thermal neutron fields is characterised by a particular feature: such particles are not directly ionising; thus the thermal neutron response of TLDs basically depends upon the capture cross-section of the constituent elements of the dosimeter and upon the TL response to the secondary particles produced. Many investigations have been made into the thermal neutron response of TL materials, but principally aimed at developing thermal neutron sensitive and also neutron insensitive materials, on the basis of the specific requirements of personal dosimetry. In contrast, little work can be found in the literature regarding TL dosimeters exposed to high thermal neutron fluences (Ayyangar et al., 1974; Carrillo et al., 1987; Mukherjee et al., 1987; Piesch et al., 1978; Reddy et al., 1969). In the thermal neutron facilities of a nuclear reactor, besides the n~h flux, a non-negligible 7-ray background is always present, coming from the reactor fission products as well as from the activation of the materials of containers, holders and dosimeters themselves. In the response of 6LiF phosphors, or in general of TL dosimeters having high sensitivity to thermal neutrons, exposed to the (n, 7) mixed field of a reactor thermal facility, the contribution from 7-rays is generally negligible. In contrast, in the emission from 7LiF dosimeters the contributions from 7-rays and from thermal neutrons may be of the same order, and some care has to be taken to make correct determinations. In order to obtain information about the contribution of 7-background in the glow curves for TL dosimeters exposed to the (n, 7) mixed field of a nuclear reactor, small boxes with 6LiF walls 0.7 g c m - 2 thick were prepared. This 6LiF cover has been found to screen thermal neutrons by a factor of about 500; the attenuation of ~,-rays is less than 1%. The glow curves for dosimeters irradiated with and without cover have been studied comparatively. In Fig. 1 the glow curves of dosimeters exposed to the same neutron fluence in the swimming-pool-type facility, with and without 6LiF cover, are reported. Curves (a) and (b) are relative to TLD-700 exposed with and without cover respectively. Curve (c) is from a TLD-600 exposed in the cover. Glow curves of TLD-600 exposed without cover cannot be reported in the figure, which is created using the software of the instrument, because such curves are three orders of magnitude higher than those in the figure. From the comparison of glow curves, we deduce that about 47% of the emission from a TLD-700, in this thermal neutron field, comes from gamma background, and 53% from thermal neutrons.
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Thus, the situation is very different from that faced at radiation protection dose levels, where a couple of 6LiF-TLiF TLDs correctly gives the possibility of determining the doses absorbed from thermal neutrons and from 7-rays. In fact, in that case the low thermal neutron component gives a negligible contribution to the response of 7LiF dosimeters. Therefore, since the gamma response for 6LiF and 7LiF are identical, the 6LiF response, after subtracting the response from 7LiF, provides a good value for thermal neutron dose determination. On the other hand, when the thermal neutron component of the radiation field is very high, as happens in radiotherapy treatments, the discrimination of contributions from the different components has to be made with a different choice of dosimeter, and the problem of finding the best choice is still open. For high fluences of thermal neutrons, the shapes of LiF dosimeter glow curves are dependent on fluence. Moreover, 6LiF dosimeters have revealed irreversible radiation damage. 7LiF have not shown radiation damage in the fluence range investigated, i.e. up to about 1013 neutrons cm -~. The results obtained with the commercial LiF chips investigated, doped with Mg, Ti or Mg, Cu, P, are reported here. L i F : M g , Ti
TLD-600 and TLD-700 chips were first selected by comparing the glow curves after uniform 7irradiation, in order to have a response uniformity within 5%. The dosimeters were then exposed, in the swimming-pool-type facility of the reactor, to the same flux but for different times so as to have different fluences, up to about 10~2 neutrons cm -2. In correspondence with each experiment some dosimeters were irradiated with 7-rays instead of neutrons, in order to control whether dosimeter response undergoes variations due to successive annealing cycles.
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In Fig. 2, some glow curves are reported, obtained from (a) TLDs-600 and (b) TLDs-700 exposed, in the same reactor facility, to different fluences. The temperature profiles during the readout are also shown. Glow curves have been deconvoluted into gaussian peaks. From this analysis, six peaks have been obtained for every dosimeter. Hereafter, the peaks will be indicated with progressive numbers beginning
from number 2. The temperature values are, however, attributed with an error because, after the experiment, a fault was discovered in the internal scale factor of the temperature output of the instrument. The real temperature values are very likely lower. We observe a shift of the peaks towards lower temperatures after thermal neutron irradiation, as shown in Fig. 3. This fact is in accordance with the results reported by Horowitz and Yossian (1995) for
Table 1. Peak areas in deconvoluted glow curves
TLD-600
TLD-700
Fluence (10" n cm -~)
A2
A3
A4
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Dependence of TLD thermoluminescence on absorbed dose
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a comparison of glow curves after y-ray and after a-particle irradiation. In Fig. 4, some deconvoluted curves are shown. In Table 1, the mean values of the areas (indicated as Ai) of the deconvoluted peaks for TLDs-600 and TLDs-700 are reported. In Fig. 5, peak areas are shown for both groups of dosimeters. In Fig. 6, some ratios between peak areas for TLD-600 and TLD-700 are reported.
Considering the results, following observations:
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1. Peak 2 is not reliable. In TLD-600, this peak is considerably higher after thermal neutron exposure than after y-rays, and for high fluences it undergoes saturation. 2. In TLD-600, peaks lose linearity after a fluence of about 0.5 × 10 ~2 neutrons cm -2. This is in
1472
G. Gambarini and M. Sinha Roy
TLD-700
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accordance with the results of Reddy et al. (1969) and Piesch et al. (1978). 3. In TLD-700, peaks show supralinearity after a fluence of about 0.5 × 10~2 n cm "~. 4. After y-irradiation, the main structure in the glow curves for both TLD-600 and TLD-700 consists of peak 4 and peak 5. Peak 4 is much lower than peak 5; in fact, it looks like a swelling in the left side of peak 5. In TLDs-600, after thermal neutron irradiation, peak 4 becomes considerably higher than peak 5. For TLD-700 chips, whose sensitivity to thermal neutrons is much lower, peak 4 is not so high.
The ratio between the heights of peak 4 and peak 5 is therefore dependent on the relative contributions of thermal neutrons and y-rays to the absorbed dose in the mixed field. In the exposure conditions of all dosimeters considered for Table 1, the y-background was nearly the same, proportional to the thermal neutron fluence; and we can see that, for the TLD-700 glow curves, this ratio is nearly constant. Vice versa, for TLDs-600 this ratio slowly decreases with increasing fluence, as is evident in Fig. 6. This fact may be correlated to the dose dependence of TL dosimeters for high LET radiation. This effect is not evident in the TLD-700 glow curves owing to the low sensitivity of such dosimeters to the secondary particles emitted after thermal neutron capture. In general, in the various dosimetry determinations, the thermal neutron and y-ray contributions to the absorbed dose are different, and consequently in the glow curves the ratio between the areas, or between the heights, of peak 4 and peak 5 differs from one situation to another, and this difference appears in the shape of the total (peak 4 + peak 5) structure. Therefore, this ratio may be an important parameter to be taken into account in discriminating contributions from the two components of the (n, 7) mixed field. 5. Peak 6 and peak 7 increase after thermal neutron irradiation more than after gamma irradiation. On the whole, the structure (peak 6 + peak 7) stands out for thermal neutron exposure more than for 7-irradiation. This peculiarity has been taken into consideration by some authors, after the first
73011330 TLD 600
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Dependence of TLD thermoluminescence on absorbed dose n06/12/1995 16.44:13 I B5irr2Gy
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Fig. 8. Glow curves from LiF:Mg, Cu, P dosimeters irradiated with y-rays or exposed in the thermal neutron and y-ray mixed field.
suggestion of Attix (1968), a n d it was applied to personal n e u t r o n dosimetry (Douglas, 1981). The t e m p e r a t u r e corresponding to peak 7 is p r o b a b l y higher t h a n the m a x i m u m t e m p e r a t u r e utilised in this experiment. In fact, from successive reading of the same chip, the last peak is f o u n d n o t to be emptied in the first reading. Therefore, the values reported here for peak 7 area, a n d hence the area ratios, are
a consequence of the chosen temperature profile. Thus, quantitative discussion of this peak is related to our situation, but, this notwithstanding, we underline the interesting peculiarity. F o r TLDs-600 as well as for TLDs-700, the ratio of the area o f ( p e a k 4 + peak 5) to the area of (peak 6 + peak 7), which gives a value near 25 for y-irradiation, shows a noticeably lower value after thermal n e u t r o n
1474
G. Gambarini and M. Sinha Roy
irradiation. For TLDs-700 this value remains almost constant, but for TLD-600s it appears to be slowly decreasing with increasing fluence, as shown in Fig. 6. Thus, although this ratio is not a good parameter for dose measurements in high fluences, it needs consideration to avoid incorrect dose determination. A better choice of the maximum temperature and the temperature profile during dosimeter reading, however, may achieve more usable glow curves. The characteristics observed for both kinds of dosimeter investigated when exposed to high fluences of thermal neutrons have to be taken into account for correct dose determination, with whatever technique, such as dual-peak analysis, glow curve superposition, or certain other algorithms for glow curve analysis. However, in the radiation fields considered here, absorbed dose determination may present serious difficulties, because the shapes of glow curves for dosimeters exposed in the mixed field with different ratios between )'- and n,~-contributions to the absorbed dose may be very ambiguous and difficult to interpret. In Fig. 7 an example of such a situation that has been encountered is shown. LiF:Mg, Cu, P
LiF:Mg, Cu, P dosimeters have aroused great interest in recent years, in particular for their higher sensitivity to 6LiF than those doped with Mg, Ti. This characteristic makes a pair of GR-206 and GR-207 dosimeters particularly advantageous for discriminating between contributions from thermal neutrons and y-rays when neutron fluences are very low, i.e. in personal dosimetry. The same characteristic, on the other hand, makes these dosimeters more disadvantageous in therapeutic thermal neutron fluences. In Fig. 8, some glow curves from GR-206 and GR-207 dosimeters irradiated with ),-rays or exposed in the (nth,)') mixed field of the swimming-pool-type reactor are shown. After exposure to thermal neutrons, an increase in the width of the whole structure is evident, but the glow curves do not appear promising for discriminating between dose contributions. Moreover, from the results reported in another paper (Gambarini et al., 1997), it is evident that in GR-206 dosimeters the lack of linearity starts at lower fluences, more drastically than in TLD-600 chips. Moreover, as reported in the subsequent section, the radiation damage, and consequently the memory of previous thermal neutron history not restored by annealing treatment, is considerably higher in LiF:Mg, Cu, P doped dosimeters than in Mg, Ti doped. Radiation damage
In all the experiments described above, the dosimeters were receiving their first exposure to thermal neutrons: before that, they had received only )'-irradiation and annealing treatment. After thermal
neutron irradiation, dosimeters were annealed and then all exposed to the same y-dose. All 7LiF dosimeters (i.e. both TLD-700 and GR-207) showed a response unaffected by the thermal neutron irradiation. In contrast, 6LiF dosimeters suffer radiation damage from high fluences of thermal neutrons (Gambarini, 1995); in fact they show a response as much low as the fluence of the previous thermal neutron exposure had been higher. Radiation damage is higher for the Mg, Cu, P doped dosimeters than for the Mg, Ti doped ones (Gambarini et al., 1997). The dosimeters were all annealed and then exposed to the same fluence of thermal neutrons, and a similar reduction in the response was found too. It is noticeable that in the TLD-600 glow curves this memory effect is present even if the fluence in the previous exposure was in the linearity range of the dosimeter response.
Conclusion From the above results, we conclude that if thermal neutron sensitive dosimeters such as TLD-600 are used for measurements of absorbed doses in mixed (n, y) fields with a high thermal neutron fluence, one has to take a group of dosimeters of the same batch, and then a part of them must be used for calibration, the others for measurements. Following this, the dosimeters are irreversibly damaged and cannot be re-used. Dosimeters with low sensitivity to thermal neutrons, such as TLD-700 and GR-207, do not suffer radiation damage and can then be re-used, but the contribution from y-background in their response is high and suitable algorithms have to be used to make correct determinations, taking single peak behaviour properly into account. Acknowledgements--The authors are grateful to Dr G. Catolla (Tecnologie Avanzate, Torino, Italy) for allowing them to make measurements with the Model 3500 Harshaw/Bicron reader, to Prof. G. LoBianco who provided the program for glow curve deconvolution, and to A. Bicego for his valuable aid in software procedures.One of the authors (M. Sinha Roy) undertook this work with the kind support of the ICTP Programme for Training and Research in Italian Laboratories, Trieste, Italy.The work was partially supported by the Istituto Nazionale di Fisica Nucleare (Italy).
References Attix, F. H. (1968) Thermoluminescent dosimeters. US Patent No. 3484 605. Ayyangar K., Lakshamanan A. R., Chandra B. and Ramadas K. (1974) A comparison of thermal neutron and gamma ray sensitivity of common TLD materials. Phys. Med. Biol. 19, 665--676. Carrillo R. E., Uribe R. M., Woodruff G. L. and Stoebe T. G. (1987) Lithium fluoride (TLD-700) response to a mixed thermal neutron and gamma field. Radiat. Prof. Oosim. 19, 55-57. Douglas, J. A. (1981) Applications of TL materials in neutron dosimetry. In Applications of TL materials in neutron dosimetry, ed. Oberhofer and Scharmann, pp. 229-258. Adam Hilger, Bristol.
Dependence of TLD thermoluminescence on absorbed dose Gambarini, G. (1995) Problems in determining the absorbed dose in thermal neutron and gamma ray mixed fields. A tissue equivalent phantom-dosimeter for B.N.C.T. In: Neutrons and their Applications, ed. G. Vourvopoulos and T. Paradellis, pp. 271-275. Proc. SP1E 2339. Gambarini G., Martini M., Scacco A., Raffaglio C. and Sichirollo A. E. (1997) TL dosimetry in high fluxes of thermal neutrons using variously doped LiF and KMgF3. Radiat. Prot. Dosim. 70, 175-180. Horowitz Y. S. and Yossian D. (1995) Computerised glow curve deconvolution: application to thermoluminescence dosimetry. Radiat. Prot. Dosim. 60, 1-111.
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Marshall, T. O. (1984) Practical Aspects of Thermoluminescence Dosimetry, ed. A.P. Hufton, p. 12. HPA, London. Mukherjee B., B6ck H. and Vana N. (1987) Application of LiF thermoluminescence dosimeter powders in neutron gamma mixed field dosimetry and dose mapping in the thermal column of a Triga Mk II reactor. Nucl. Instr. & Methods A254, 182-185. Piesch E., Burgkhardt B. and Sayed A. M. (1978) Activation and damage effects in TLD600 after neutron irradiation. Nucl. Instr. and Meth. 157, 179-184. Reddy A. R., Ayyangar K. and Brownell G. L. (1969) Thermoluminescence response of LiF to reactor neutrons. Radial Res. 40, 552-562.
Pergamon PIh
Appl. Radiat. lsot. Vol. 48, No. 10-12 pp. 1467-1475, 1997 © 1997 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain S0969-8043(97)00142-5 0969-8043/97 $17.00 + 0.00
Dependence of TLD Thermoluminescence Yield on Absorbed Dose in a Thermal Neutron Field G. GAMBARIN1.1"2 a n d M. S I N H A R O Y ~ ~Dipartimento di Fisica dell'Universit/t, via Celoria 16, 20133 Milano, Italy and qstituto Nazionale di Fisica Nucleare, Sezione di Milano, via Celoria 16, 20133 Milano, Italy The emission from 6LiF and 7LiF thermoluminescencedosimeters (TLDs) exposed to the mixed field of thermal neutrons and y-rays of the thermal facility of a TRIGA MARK II nuclear reactor has been investigated for various thermal neutron fluences of the order of magnitude of those utilised in radiotherapy, with the purpose of investigating the reliability of TLD readouts in such radiation fields and of giving some information for better obtainment of the absorbed dose values. The emission after exposure in this mixed field is compared with the emission after y-rays only. The glow curves have been deconvoluted into gaussian peaks, and the differencesin the characteristics of the peaks observed for the two radiation fields, having different linear energy transfers, and for different doses are shown. Irreversible radiation damage in dosimeters having high sensitivity to thermal neutrons is also reported, showing a memory effect of the previous thermal neutron irradiation history which is not restored by anneal treatment. © 1997 Published by Elsevier Science Ltd. All rights reserved
Introduction Thermoluminescent dosimeters (TLDs) are currently used in industrial and medical applications to determine the absorbed dose in any radiation field. In fact, both their small dimensions and their tissueequivalence for most radiation allow mapping of the absorbed dose distribution without significantly modifying the radiation field. Some care has to be taken in order to obtain reliable results, but in personal dosimetry, i.e. for low absorbed doses, the protocol for TLD utilisation is satisfactorily settled. In contrast, when TL dosimeters are used for high dose measurements, such as those required in radiotherapy, a lack of precision and many ambiguities arise. Moreover, many unresolved problems may appear in radiation fields different from 7-rays, especially if the linear energy transfer (LET) of the radiation is high. In fact, the thermoluminescent yield of materials may depend on the LET of the ionising particles. Thus, after exposure to radiation of various LETs, the shape of the glow curves obtained from these dosimeters is expected to change with the LET as well as with the dose. In mixed radiation fields, often met in radiotherapy, the difference in the emission due to the different radiation components of the field may be used to relate the contributions of these components to the absorbed dose; but a good knowledge, for each type *To whom correspondence should be addressed.
of radiation, of the characteristics of the peaks in the glow curve is sometimes needed to make such an attribution correctly. In the present work, the shapes of glow curves obtained from 6LiF chips after exposure to high doses of thermal neutrons are analysed. The isotope 6Li has a high cross-section (945 barn) for the reaction with thermal neutrons: 6Li (n,~t) 3H The particles emitted in this reaction, i.e. s-particles (2.07 MeV) and tritons (2.24 MeV), have a higher LET than 7-radiation. Therefore, the glow curve after exposure in a thermal neutron field is different from that after gamma radiation. Moreover, in TL dosimeters containing a great amount of 6Li and exposed to thermal neutrons, the shape of the glow curve depends on dose, that is on neutron fluence. In this work, some 6LiF and 7LiF dosimeters were exposed in the thermal column of a nuclear reactor and then analysed. A comparative study has been made to find a correlation between the glow curve shapes for LiF dosimeters and thermal neutron fluence.
ExperimentalProcedures Dosimeters analysed The TLDs that were analysed are commercial 6LiF and 7LiF dosimeters doped with Mg, Ti or with
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G. Gambarini and M. Sinha Roy
Mg, Cu, P. LiF:Mg, Ti dosimeters, from the Harshaw Chemical Co., are TLD-600 (with 96.5% 6LiF) and TLD-700 (with 99.99% 7LiF) in the form of chips measuring 3.1 x 3.1 x 0.9 mm. LiF:Mg, Cu, P dosimeters, from the Beijing Radiation Detector Work, People's Republic of China, are Gr-206A (6LiF:Mg, Cu, P) and GR-207A (TLiF:Mg, Cu, P) in the form of circular chips, 4.5 mm diameter and 0.8 mm thick.
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Annealing procedures The dosimeters were all annealed together before each irradiation. All dosimeters compared were from the same batch and had undergone the same number of annealing cycles from the time of purchase. It is known, in fact, that the sensitivity of TL dosimeters is strongly dependent on their thermal history. In particular, the cooling rate after heating is critical (Marshall, 1984). For all dosimeters, the annealing procedure recommended by the manufacturing company has been followed. LiF:Mg, Ti chips were heated at 40ff'C for 1 h, quenched, with gradual cooling, down to room temperature and then annealed at 100°C for 2 h. No post-irradiation low temperature annealing was done, but all readouts were performed 48 h after irradiation to allow the decay of low temperature peaks. LiF:Mg, Cu, P dosimeters were annealed at 240 ~C for 10min, and then rapidly cooled to room temperature. Readouts were performed 3 days after exposure.
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For TLD-600 and TLD-700 chips a Harshaw/ Filtrol system has been utilised, composed of a Model 2000A detector interfaced to a Model 2080 Picoprocessor. The readout was performed using a constant heating rate of &C s- ~ up to 330 ~'C. The
'
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SEC Fig. 2. Glow curves from TLDs-600 and TLDs-700exposed to various thermal neutron fluences. software of the instrument provides glow curve representation and area evaluation in the regions of interest. Glow curve deconvolutions into gauss/an peaks have been executed with laboratory-written programs which supply the parameters of gauss/an peaks and a graphical representation of the experimental glow curve, deconvoluted peaks and peak addition. For the GR-206 and GR-207 chips, a Model 3500 TLD reader from Harshaw/Bicron was used. The readout was performed using a constant heating rate of 4°C s-~. The software of the instrument allows glow curve representation and area determination in regions of interest.
Irradiation facilities For "/-irradiation, a '37Cs irradiator with a rate of about 0.14 Gy s - ' has been utilised. Exposures to thermal neutrons were performed at the TRIGA MARK II nuclear reactor in Pavia (Italy). Most samples were exposed in the swimmingpool-type facility, with a thermal neutron fluence rate of 1.44 x l0 s neutrons cm - 2 s- ' and a cadmium ratio
Dependence of TLD thermoluminescenceon absorbed dose equal to ~ 90. A few samples were irradiated in the thermal column, with a fluence rate of 4 x 10 9 neutrons c m - 2 s - t and a cadmium ratio of about 30.
ExperimentalResults TL dosimetry in thermal neutron fields is characterised by a particular feature: such particles are not directly ionising; thus the thermal neutron response of TLDs basically depends upon the capture cross-section of the constituent elements of the dosimeter and upon the TL response to the secondary particles produced. Many investigations have been made into the thermal neutron response of TL materials, but principally aimed at developing thermal neutron sensitive and also neutron insensitive materials, on the basis of the specific requirements of personal dosimetry. In contrast, little work can be found in the literature regarding TL dosimeters exposed to high thermal neutron fluences (Ayyangar et al., 1974; Carrillo et al., 1987; Mukherjee et al., 1987; Piesch et al., 1978; Reddy et al., 1969). In the thermal neutron facilities of a nuclear reactor, besides the n~h flux, a non-negligible 7-ray background is always present, coming from the reactor fission products as well as from the activation of the materials of containers, holders and dosimeters themselves. In the response of 6LiF phosphors, or in general of TL dosimeters having high sensitivity to thermal neutrons, exposed to the (n, 7) mixed field of a reactor thermal facility, the contribution from 7-rays is generally negligible. In contrast, in the emission from 7LiF dosimeters the contributions from 7-rays and from thermal neutrons may be of the same order, and some care has to be taken to make correct determinations. In order to obtain information about the contribution of 7-background in the glow curves for TL dosimeters exposed to the (n, 7) mixed field of a nuclear reactor, small boxes with 6LiF walls 0.7 g c m - 2 thick were prepared. This 6LiF cover has been found to screen thermal neutrons by a factor of about 500; the attenuation of ~,-rays is less than 1%. The glow curves for dosimeters irradiated with and without cover have been studied comparatively. In Fig. 1 the glow curves of dosimeters exposed to the same neutron fluence in the swimming-pool-type facility, with and without 6LiF cover, are reported. Curves (a) and (b) are relative to TLD-700 exposed with and without cover respectively. Curve (c) is from a TLD-600 exposed in the cover. Glow curves of TLD-600 exposed without cover cannot be reported in the figure, which is created using the software of the instrument, because such curves are three orders of magnitude higher than those in the figure. From the comparison of glow curves, we deduce that about 47% of the emission from a TLD-700, in this thermal neutron field, comes from gamma background, and 53% from thermal neutrons.
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Thus, the situation is very different from that faced at radiation protection dose levels, where a couple of 6LiF-TLiF TLDs correctly gives the possibility of determining the doses absorbed from thermal neutrons and from 7-rays. In fact, in that case the low thermal neutron component gives a negligible contribution to the response of 7LiF dosimeters. Therefore, since the gamma response for 6LiF and 7LiF are identical, the 6LiF response, after subtracting the response from 7LiF, provides a good value for thermal neutron dose determination. On the other hand, when the thermal neutron component of the radiation field is very high, as happens in radiotherapy treatments, the discrimination of contributions from the different components has to be made with a different choice of dosimeter, and the problem of finding the best choice is still open. For high fluences of thermal neutrons, the shapes of LiF dosimeter glow curves are dependent on fluence. Moreover, 6LiF dosimeters have revealed irreversible radiation damage. 7LiF have not shown radiation damage in the fluence range investigated, i.e. up to about 1013 neutrons cm -~. The results obtained with the commercial LiF chips investigated, doped with Mg, Ti or Mg, Cu, P, are reported here. L i F : M g , Ti
TLD-600 and TLD-700 chips were first selected by comparing the glow curves after uniform 7irradiation, in order to have a response uniformity within 5%. The dosimeters were then exposed, in the swimming-pool-type facility of the reactor, to the same flux but for different times so as to have different fluences, up to about 10~2 neutrons cm -2. In correspondence with each experiment some dosimeters were irradiated with 7-rays instead of neutrons, in order to control whether dosimeter response undergoes variations due to successive annealing cycles.
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In Fig. 2, some glow curves are reported, obtained from (a) TLDs-600 and (b) TLDs-700 exposed, in the same reactor facility, to different fluences. The temperature profiles during the readout are also shown. Glow curves have been deconvoluted into gaussian peaks. From this analysis, six peaks have been obtained for every dosimeter. Hereafter, the peaks will be indicated with progressive numbers beginning
from number 2. The temperature values are, however, attributed with an error because, after the experiment, a fault was discovered in the internal scale factor of the temperature output of the instrument. The real temperature values are very likely lower. We observe a shift of the peaks towards lower temperatures after thermal neutron irradiation, as shown in Fig. 3. This fact is in accordance with the results reported by Horowitz and Yossian (1995) for
Table 1. Peak areas in deconvoluted glow curves
TLD-600
TLD-700
Fluence (10" n cm -~)
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a comparison of glow curves after y-ray and after a-particle irradiation. In Fig. 4, some deconvoluted curves are shown. In Table 1, the mean values of the areas (indicated as Ai) of the deconvoluted peaks for TLDs-600 and TLDs-700 are reported. In Fig. 5, peak areas are shown for both groups of dosimeters. In Fig. 6, some ratios between peak areas for TLD-600 and TLD-700 are reported.
Considering the results, following observations:
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1. Peak 2 is not reliable. In TLD-600, this peak is considerably higher after thermal neutron exposure than after y-rays, and for high fluences it undergoes saturation. 2. In TLD-600, peaks lose linearity after a fluence of about 0.5 × 10 ~2 neutrons cm -2. This is in
1472
G. Gambarini and M. Sinha Roy
TLD-700
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accordance with the results of Reddy et al. (1969) and Piesch et al. (1978). 3. In TLD-700, peaks show supralinearity after a fluence of about 0.5 × 10~2 n cm "~. 4. After y-irradiation, the main structure in the glow curves for both TLD-600 and TLD-700 consists of peak 4 and peak 5. Peak 4 is much lower than peak 5; in fact, it looks like a swelling in the left side of peak 5. In TLDs-600, after thermal neutron irradiation, peak 4 becomes considerably higher than peak 5. For TLD-700 chips, whose sensitivity to thermal neutrons is much lower, peak 4 is not so high.
The ratio between the heights of peak 4 and peak 5 is therefore dependent on the relative contributions of thermal neutrons and y-rays to the absorbed dose in the mixed field. In the exposure conditions of all dosimeters considered for Table 1, the y-background was nearly the same, proportional to the thermal neutron fluence; and we can see that, for the TLD-700 glow curves, this ratio is nearly constant. Vice versa, for TLDs-600 this ratio slowly decreases with increasing fluence, as is evident in Fig. 6. This fact may be correlated to the dose dependence of TL dosimeters for high LET radiation. This effect is not evident in the TLD-700 glow curves owing to the low sensitivity of such dosimeters to the secondary particles emitted after thermal neutron capture. In general, in the various dosimetry determinations, the thermal neutron and y-ray contributions to the absorbed dose are different, and consequently in the glow curves the ratio between the areas, or between the heights, of peak 4 and peak 5 differs from one situation to another, and this difference appears in the shape of the total (peak 4 + peak 5) structure. Therefore, this ratio may be an important parameter to be taken into account in discriminating contributions from the two components of the (n, 7) mixed field. 5. Peak 6 and peak 7 increase after thermal neutron irradiation more than after gamma irradiation. On the whole, the structure (peak 6 + peak 7) stands out for thermal neutron exposure more than for 7-irradiation. This peculiarity has been taken into consideration by some authors, after the first
73011330 TLD 600
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Dependence of TLD thermoluminescence on absorbed dose n06/12/1995 16.44:13 I B5irr2Gy
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suggestion of Attix (1968), a n d it was applied to personal n e u t r o n dosimetry (Douglas, 1981). The t e m p e r a t u r e corresponding to peak 7 is p r o b a b l y higher t h a n the m a x i m u m t e m p e r a t u r e utilised in this experiment. In fact, from successive reading of the same chip, the last peak is f o u n d n o t to be emptied in the first reading. Therefore, the values reported here for peak 7 area, a n d hence the area ratios, are
a consequence of the chosen temperature profile. Thus, quantitative discussion of this peak is related to our situation, but, this notwithstanding, we underline the interesting peculiarity. F o r TLDs-600 as well as for TLDs-700, the ratio of the area o f ( p e a k 4 + peak 5) to the area of (peak 6 + peak 7), which gives a value near 25 for y-irradiation, shows a noticeably lower value after thermal n e u t r o n
1474
G. Gambarini and M. Sinha Roy
irradiation. For TLDs-700 this value remains almost constant, but for TLD-600s it appears to be slowly decreasing with increasing fluence, as shown in Fig. 6. Thus, although this ratio is not a good parameter for dose measurements in high fluences, it needs consideration to avoid incorrect dose determination. A better choice of the maximum temperature and the temperature profile during dosimeter reading, however, may achieve more usable glow curves. The characteristics observed for both kinds of dosimeter investigated when exposed to high fluences of thermal neutrons have to be taken into account for correct dose determination, with whatever technique, such as dual-peak analysis, glow curve superposition, or certain other algorithms for glow curve analysis. However, in the radiation fields considered here, absorbed dose determination may present serious difficulties, because the shapes of glow curves for dosimeters exposed in the mixed field with different ratios between )'- and n,~-contributions to the absorbed dose may be very ambiguous and difficult to interpret. In Fig. 7 an example of such a situation that has been encountered is shown. LiF:Mg, Cu, P
LiF:Mg, Cu, P dosimeters have aroused great interest in recent years, in particular for their higher sensitivity to 6LiF than those doped with Mg, Ti. This characteristic makes a pair of GR-206 and GR-207 dosimeters particularly advantageous for discriminating between contributions from thermal neutrons and y-rays when neutron fluences are very low, i.e. in personal dosimetry. The same characteristic, on the other hand, makes these dosimeters more disadvantageous in therapeutic thermal neutron fluences. In Fig. 8, some glow curves from GR-206 and GR-207 dosimeters irradiated with ),-rays or exposed in the (nth,)') mixed field of the swimming-pool-type reactor are shown. After exposure to thermal neutrons, an increase in the width of the whole structure is evident, but the glow curves do not appear promising for discriminating between dose contributions. Moreover, from the results reported in another paper (Gambarini et al., 1997), it is evident that in GR-206 dosimeters the lack of linearity starts at lower fluences, more drastically than in TLD-600 chips. Moreover, as reported in the subsequent section, the radiation damage, and consequently the memory of previous thermal neutron history not restored by annealing treatment, is considerably higher in LiF:Mg, Cu, P doped dosimeters than in Mg, Ti doped. Radiation damage
In all the experiments described above, the dosimeters were receiving their first exposure to thermal neutrons: before that, they had received only )'-irradiation and annealing treatment. After thermal
neutron irradiation, dosimeters were annealed and then all exposed to the same y-dose. All 7LiF dosimeters (i.e. both TLD-700 and GR-207) showed a response unaffected by the thermal neutron irradiation. In contrast, 6LiF dosimeters suffer radiation damage from high fluences of thermal neutrons (Gambarini, 1995); in fact they show a response as much low as the fluence of the previous thermal neutron exposure had been higher. Radiation damage is higher for the Mg, Cu, P doped dosimeters than for the Mg, Ti doped ones (Gambarini et al., 1997). The dosimeters were all annealed and then exposed to the same fluence of thermal neutrons, and a similar reduction in the response was found too. It is noticeable that in the TLD-600 glow curves this memory effect is present even if the fluence in the previous exposure was in the linearity range of the dosimeter response.
Conclusion From the above results, we conclude that if thermal neutron sensitive dosimeters such as TLD-600 are used for measurements of absorbed doses in mixed (n, y) fields with a high thermal neutron fluence, one has to take a group of dosimeters of the same batch, and then a part of them must be used for calibration, the others for measurements. Following this, the dosimeters are irreversibly damaged and cannot be re-used. Dosimeters with low sensitivity to thermal neutrons, such as TLD-700 and GR-207, do not suffer radiation damage and can then be re-used, but the contribution from y-background in their response is high and suitable algorithms have to be used to make correct determinations, taking single peak behaviour properly into account. Acknowledgements--The authors are grateful to Dr G. Catolla (Tecnologie Avanzate, Torino, Italy) for allowing them to make measurements with the Model 3500 Harshaw/Bicron reader, to Prof. G. LoBianco who provided the program for glow curve deconvolution, and to A. Bicego for his valuable aid in software procedures.One of the authors (M. Sinha Roy) undertook this work with the kind support of the ICTP Programme for Training and Research in Italian Laboratories, Trieste, Italy.The work was partially supported by the Istituto Nazionale di Fisica Nucleare (Italy).
References Attix, F. H. (1968) Thermoluminescent dosimeters. US Patent No. 3484 605. Ayyangar K., Lakshamanan A. R., Chandra B. and Ramadas K. (1974) A comparison of thermal neutron and gamma ray sensitivity of common TLD materials. Phys. Med. Biol. 19, 665--676. Carrillo R. E., Uribe R. M., Woodruff G. L. and Stoebe T. G. (1987) Lithium fluoride (TLD-700) response to a mixed thermal neutron and gamma field. Radiat. Prof. Oosim. 19, 55-57. Douglas, J. A. (1981) Applications of TL materials in neutron dosimetry. In Applications of TL materials in neutron dosimetry, ed. Oberhofer and Scharmann, pp. 229-258. Adam Hilger, Bristol.
Dependence of TLD thermoluminescence on absorbed dose Gambarini, G. (1995) Problems in determining the absorbed dose in thermal neutron and gamma ray mixed fields. A tissue equivalent phantom-dosimeter for B.N.C.T. In: Neutrons and their Applications, ed. G. Vourvopoulos and T. Paradellis, pp. 271-275. Proc. SP1E 2339. Gambarini G., Martini M., Scacco A., Raffaglio C. and Sichirollo A. E. (1997) TL dosimetry in high fluxes of thermal neutrons using variously doped LiF and KMgF3. Radiat. Prot. Dosim. 70, 175-180. Horowitz Y. S. and Yossian D. (1995) Computerised glow curve deconvolution: application to thermoluminescence dosimetry. Radiat. Prot. Dosim. 60, 1-111.
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Marshall, T. O. (1984) Practical Aspects of Thermoluminescence Dosimetry, ed. A.P. Hufton, p. 12. HPA, London. Mukherjee B., B6ck H. and Vana N. (1987) Application of LiF thermoluminescence dosimeter powders in neutron gamma mixed field dosimetry and dose mapping in the thermal column of a Triga Mk II reactor. Nucl. Instr. & Methods A254, 182-185. Piesch E., Burgkhardt B. and Sayed A. M. (1978) Activation and damage effects in TLD600 after neutron irradiation. Nucl. Instr. and Meth. 157, 179-184. Reddy A. R., Ayyangar K. and Brownell G. L. (1969) Thermoluminescence response of LiF to reactor neutrons. Radial Res. 40, 552-562.