Radiat Prot Dosimetry 2007 Delgado 327 30

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ON THE USE OF LIF TLD-600 IN NEUTRON-GAMMAMIXED FIELDSA. Delgado, J. L. Muniz, J. M. Gomez Ros, A. M. Romero and R. Rodrguez

Radiation Dosimetry Unit, CIEMAT, Avda Complutense 22, 28040 Madrid, Spain

A new procedure allowing the separate estimation of neutron and gamma dose in mixed radiation fields has been developedin our laboratory. In this communication, a description of the main features of the discrimination procedure and somepreliminary results obtained by its use are presented. The procedure is based on the significantly different structure of theglow curve of LiF TLD-600 produced by neutron and gamma radiation. The use of peak resolving numerical methods,sometimes called deconvolution, for the analysis of the glow curves from controlled irradiations at absorbed doses in therange 10300 mGy with different neutron and gamma proportions, permits to quantify the differences peak by peak, alsocharacterising the well-known neutron quasi-exclusive contribution to the high temperature region, above peak 5. From thisstudy, it was possible to propose a n/cTL factor by which the respective doses can be estimated through a simplified analysis,not peak resolving, of the particular features of the glow curves obtained in field measurements. A first set of rathersatisfactory results have been obtained by irradiating TLD-600 together with TLD-700 chips using Am-Be sources withdifferent degree of moderation and using lead absorbers to change the gamma component. This component is directlymeasured by the TLD-700 detectors, allowing the testing of the gamma estimation reached by the discrimination procedure

applied to the TLD-600 glow curve.

INTRODUCTION

6LiF-based thermoluminescent dosemeters (TLDs)are good thermal neutron detectors and can be aconvenient alternative for the development of dose-meters based in the moderation method. For thoseapplications in which integrating dosemeters aresuitable, the comparatively small but sensitive TLDscan offer good performance when employed insuch moderated systems. Especially, adapted TLDprocedures for mixed neutron-gamma dosimetry

have been developed allowing a reasonable discrimi-nation capability for the two dose components(1).Some of them are based on the different sensitivityto photons and neutrons of the low and high tem-perature regions (HTRs) of the LiF TLD-600 glowcurve. Nevertheless, the pair method, combiningthe neutron sensitive TLD-600 and the insensitiveTLD-700 forming a paired detector, is perhapsthe more common TL procedure employed for thedosimetry of mixed fields.

CIEMAT is engaged in the construction of amultidetector moderated dosemeter for area moni-toring, based on a single sphere allocating insideTLDs in various positions with different moderationpermitting to estimate a spectral correction factorto improve the accuracy of ambient dose determina-tions. A first prototype has been recently assembledand some preliminary and encouraging results havebeen obtained with pairs of TLD-600/700 as ther-mal detectors(2). Attempts to use in our laboratorythe low/high temperature areas ratio to estimate theneutron and gamma contributions from only the

TLD-600 glow curve were not successful, mainlybecause of the difficulty in estimating the area ofthe HTR properly due to neutrons. The gammasensitivity in that region is not negligible and canbe different among different batches of detectors.In addition, the HTR is prone to present signals notcaused by radiation, that are also depending on theparticular heating system employed for readout. Allthese factors complicate the use of the HTR areafor dosimetry, particularly for the estimation oflow neutron doses. Nevertheless, the attractive of amethod for mixed field dosimetry based exclusivelyon a single type of detector is high, as compared tothe pair method only half of the detectors arerequired.

For the CIEMAT multidetector system, the reduc-tion in the number of detectors is a considerableadvantage, and for this reason, a new approach hasbeen developed in our laboratory trying to extractseparately the neutron and the gamma doses fromthe TLD-600 glow curve, and not relaying exclu-sively in the problematic HTR. The new approachis based on the different glow curve structure pro-duced by gamma and by neutron radiation, which

can be well appreciated with the use of peak resolv-ing glow curve analysis methods. The two referencestructures for pure gamma and for nearly pureneutron fields can be employed to estimate, througha minimisation process, the relative contribution ofeach kind of radiation in an n/c mixed field ofunknown composition.

This communication describes the main featuresof the new procedure, also presenting some prelim-inary results obtained in an experiment using n/cfields in controlled proportions. The aim was toCorresponding author: [email protected]

Radiation Protection Dosimetry (2007), Vol. 125, No. 14, pp. 327330 doi:10.1093/rpd/ncm210Advance Access publication 19 June 2007

The Author 2007. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]


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check the validity of the new procedure and its rangeof application in terms of gamma and neutronabsorbed doses.

EXPERIMENT

LiF TLD-600 chips (33 0.9 mm3) were employedin the experiment. Occasionally, also TLD-700 chipsof the same dimensions were employed to allow anindependent estimation of the gamma component ina mixed field. Linear readout at 5C/s up to 350Cwas always employed. Detectors were reused simplyafter readout without any external annealing, show-ing very good stability and reproducibility.

TLD detectors were irradiated in terms of absorbeddose for gamma rays in the range between 1 and200 mGy, and for neutrons between 10 mGy andup to 2.5 Gy. For neutron irradiation, a highly mod-erated (paraffin) Am-Be source was employed andthe thermal neutron doses estimated by comparison

with the readings of an instrument LB 6411 calibratedin terms of Ambient Dose at PTB. The Am-Be sourceproduced the irradiation position a weak gamma doserate of the order of 1.5 mGy/h together with a neu-tron dose rate estimated as 100 mGy/h. Additionalgamma irradiations were imparted exposing the neu-tron irradiated detectors to one of the 137 Cs beamsof the CIEMAT Secondary Standard laboratory.In this way, mixed fields of different n/c proportionswere simulated, presenting ratios for the neutron togamma-induced TL signals within the range 1/50to 10. Both neutron and gamma irradiations in thisdemonstration exercise, have been made in very spe-cific and somewhat simplified conditions respect topractical situations in which the backscattered radia-tion may vary depending on the geometry. This pointshould be taken into account when applying themethod to routine dosimetry.

A peak resolving glow curve analysis code usingfirst order kinetics expression and a Levenberg-Marquadt minimisation algorithm has been employedfor the analysis of the TL curves obtained in theexperiment. This method allows study the evolutionof the peak distribution as a function of the n/crelative contributions and is the basis of the discri-mination procedure. The glow curve analysis codewas developed in our laboratories and has been in

very satisfactory use for more than a decade. Its per-formance was checked in an intercomparison ofglow curve analysis methods with excellent results(3).

As indicated above, the n/cdiscrimination is basedon the different and characteristic glow curve struc-ture produced by pure gamma and pure (really nearlypure) neutron irradiation. The glow curve obtainedin a mixed field is then analysed in terms of thesetwo distributions through a minimisation procedurein which the free parameter is just the proportion ofthe two reference curves. The minimisation is on the

differences between the experimentally determinedglow curve and the synthetically produced, addingthe two reference glow curves and varying their rela-tive proportion in an iterative way. In all the cases, theiteration process has found convergence and produ-cing rather satisfactory estimation of the neutron andgamma contributions, and this in a broad range ofthe relative proportions in different mixed radiationfields.

RESULTS AND DISCUSSION

The first step in the experiment consisted in thedetermination and the analysis of the referenceglow curves to determine their respective structure.The determination of the peak distribution forthe gamma curve was straightforwardly achieved.It was obtained simply irradiating the TLD-600detectors in the 137 Cs beams and analysing the cor-responding glow curves. For neutrons, the process is

somewhat more complicated due to the gamma con-tamination of the AmBe source. In order to estimatethe gamma dose contribution, some TLD-700 detec-tors were exposed to the AmBe at the irradiationposition, a fixed position employed for the wholeexperiment. The gamma contribution was estimatedas 1.5 mGy/h, i.e. a weak contribution nearly 1/10of the neutron induced TL signals. It was accountedfor when analysing the glow curves of the TLD 600detectors irradiated with the AmBe source to obtainthe peak distribution of the reference glow curve forneutrons.

Figure 1 presents the reference glow curves after20 mGy (137 Cs), curve (a); and after 20 min expo-sure to the AmBe source (approx. 20 mGy), curve(b). The glow curve corresponding to the weakgamma contribution in curve (b) (0.5 mGy) is alsopresented for comparison. The already known dif-ferences between the gamma and neutron inducedglow curves(4) can be appreciated in Figure 1: Anincreased sensitivity of the HT region to neutronsand a substantially decreased peak 4 in the neutroncurve compared to the gamma curve. In addition,the peak 3 relative intensity is systematically higherfor neutrons than for gamma irradiation.

The dose estimator employed in the experimentwas the added area of the fitted peaks 3, 4 and 5,

P345, and for every curve, each peak, i, is charac-terised by the proportion of its area, Ri, in relation tothe P345 area of that curve: RiPi/P345, wherei represents either peak 3, 4 or 5. The stability ofthe Ri values for the reference curves, gamma andneutrons, were checked by repeated irradiation andTL measurements, and found satisfactory and con-venient for the purpose of our work. For gammacurves, ten successive measurements at 20 mGy pro-duce consistent Ri values within 12%, while forneutrons they lay within 35%. The reproducibility

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for peaks 3 and 5 is better than for peak 4, forneutrons particularly. Similar figures were obtainedfor different groups of detectors. For the experiment,batches of 25 detectors, selected according to batchhomogeneity (within 5%), were used.

Once the reference glow curves were establishedand the corresponding Rni and Rci values were pro-perly determined, they were introduced in theprogramme performing the analysis of the curvesproduced by mixed fields in terms of the two refer-ence curves. A first test of the performance of theprogramme was made irradiating groups of five

detectors to a constant neutron dose (35 mGy) anddifferent gamma doses between 10 and 100 mGy.The test was designed to check: (1) the linearity ofthe estimated mixed field gamma dose, (2) the coher-ence between these gamma estimation and the resultsobtained from pure gamma irradiations at the samedose values, and (3) the due constancy of the neutroncontribution estimated for the groups with differentgamma doses.

Figure 2 presents the results of the test. The goodlinearity of the estimated gamma component, the

good agreement between these estimations and thepure gamma data for each dose value and the goodconstancy of the neutron component estimation forthe five groups of detectors, can be appreciated.Similar tests at different neutron doses (up to

200 mGy) produced results well in line with thosein Figure 2a.

Figure 3 presents the results of a different testperformed irradiating with Am-Be source for differ-ent durations (from 20 min to 24 h) with the detec-tors always placed at the same point respect to thesource. As the gamma dose rate produced by thesource at this point was independently determinedby LiF TLD-700 (1.5 mGy/h), the gamma contri-bution for the different exposure times was knownand can be compared to those estimated by the

Figure 1. TLD-600 glow curves produced by (a) 20 mGygamma irradiation, (b) 35 mGy thermal neutron irra-diation. The individual peaks resolved by the analysis arepresented. In curve (b) the weak gamma componentproduced by the neutron source is also shown. In theinset, the ratios of area of the individual peaks and thetotal area (added area of peaks 3, 4 and 5) are indicated.

Figure 2. Variation of the estimated gamma (~) andneutron (&) contributions in TLD-600 glow curves as afunction of the additional gamma dose. For comparison,the measured gamma contribution by TLD-700 is also

indicated (!).

Figure 3. Dependence of the estimated gamma (~) andneutron (&) TLD-600 components with the duration of theneutron irradiation. The gamma contribution measured by

TLD-700 is also shown (!).

ON THE USE OF LIF TLD-600 IN NEUTRON-GAMMA MIXED FIELDS

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analysis procedure. Within contrast to the previoustests, now the neutron contribution is not constantbut is directly proportional to the exposure time, andthis proportion should be observed in the estimatedneutron contribution. In the Figure 2b, it can beobserved that all these predictions are well fulfilled.The agreement between the true (TLD-700) gammadoses and the estimated ones for the different expo-sure times are always acceptable (within 1020%),and at the same time, the estimated neutron contri-bution exhibit a very good linearity with the time ofexposure, with regression coefficient of R 0.9996and a very small intercept. To be noted, the linearresponse of TLD-600 for neutron doses from 35 mGyup to high doses, which in this test reached values ofthe order of 2.5 Gy.

CONCLUSION

The satisfactory results obtained in the different tests

applied so far, support the correctness of the ideabehind the method herein described for the ana-lysis of the TLD-600 glow curves from mixed fields.That is, these glow curves are additive with respectto the neutron and gamma components and theirrespective structure, taken as reference, can be

employed for the analysis of the mixed field glowcurves, separating the two contributions and per-mitting to quantify separately the correspondingabsorbed doses. New tests are under way and someothers are being planned for the near future, includ-ing the irradiation in other facilities with differentn/c fields, looking for the improvement of the neu-tron dose estimations and also the refinement of theanalysis procedure for mixed fields TLD-600 glowcurves.

REFERENCES

1. Burgkhardt, B. and Schwartz, W.Evaluation techniquesfor different TL albedo dosemeters using automated read-out. Radiat. Prot. Dosim. 17, 131134 (1986).

2. Muniz, J. L., Vicente, M. C., Gonzalez, E. M.,Romero, A. M., Embid, M. and Delgado, A. A newarea multidetector dosemeter for mixed n-gamma fields.Radiat. Prot. Dosim. 110, 243248 (2004).

3. Bos, A. J. J., Piters, T. M., Gomez Ros J. M. andDelgado, A. An intercomparison of glow curve analysis

programs: I. Synthetic Glow Curves. Radiat. Prot.Dosim.47, 473477 (1993).

4. Youssian, D. and Horowitz, Y. S.Estimation of gammadose in neutron dosimetry using peak 4 to peak 5 ratios inLiF:Mg,Ti (TLD-100/600). Radiat. Prot. Dosim. 77,151158 (1998).

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Radiat Prot Dosimetry 2007 Delgado 327 30