IMPROVEMENT OF THE PILLE-SYSTEM FOR THE DETERMINATION OF THE BIOLOGICALLY RELEVANT DOSE IN SPACE CRAFT

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1 IMPROVEMENT OF THE PILLE-SYSTEM FOR THE DETERMINATION OF THE BIOLOGICALLY RELEVANT DOSE IN SPACE CRAFT W. Schöner 1, N. Vana 1, 2, M. Fugger 1, S. Deme 3, I. Apathy 3, I. Hejja 3, I. Feher 3 1 Atominstitute of the Austrian Universities, Stadionallee 2, A-1020 Vienna, Austria 2 Institute for Space Dosimetry, Lustkandlgasse 52/3, A-1090 Vienna, Austria 3 Atomic Research Institute KFKI, Aerospace Laboratory, H-1525 Budapest ABSTRACT The system PILLE was developed by the Hungarian group and is used for dose measurements with TLDs on board of space station MIR and during NASA missions since several years. The goal of the Hungarian Austrian co-operation is the implementation of the HTR-method in the PILLE system, in order to measure additionally to the absorbed dose the average LET of absorbed radiation. This method was developed at the Atominstitute of the Austrian Universities and was used with great success for the determination of the average LET and the assessment of the biologically relevant dose during many space missions, in aircraft and in therapeutical proton beams. In contradiction to the standard HTR-method the PILLE-system uses bulb dosemeters and therefore a new LET-calibration is necessary. Irradiations of the bulb dosemeters were carried out in a 62 MeV proton beam at the Paul Scherrer Institut and in the High Energy Reference Field at CERN. Similar to standard dosemeters (TLD-600 and TLD-700), the Hungarian LiF-bulb dosemeters show a monotone increase of the HTR with increasing LET of absorbed radiation. Besides the first steps of LET-calibration, results from measurements on board of MIR-station are presented. Based on these first results possible improvements of the PILLE-system are discussed. 1. INTRODUCTION 1.1. The PILLE system The KFKI Atomic Energy Research Institute has developed and manufactured a series of thermoluminescent dosemeter (TLD) systems for spacecraft, consisting of a set of bulb dosemeters and a small, compact, TLD reader suitable for on-board evaluation of the dosemeters. By means of such a system highly accurate measurements were carried out on board the Salyut-6 (1), -7 and Mir Space Stations (2, 3, 4) as well as on the Space Shuttle. A new implementation of the system will be placed on several segments of the ISS as the contribution of Hungary to the great international enterprise. The well proven CaSO 4 :Dy dosemeters will be used for routine dosimetry of the astronauts and in biological experiments. For the measurement of the mean LET value of cosmic radiation, necessary for the determination of the biologically relevant dose, a co-operation with the Atominstitute of the Austrian Universities was started in order to implement the HTR method for determination of the mean LET in mixed radiation fields in the PILLE-system. First tests of LiF PILLE dosemeters confirmed in principal the possibility to use this system for the on board determination of the biologically relevant dose using the HTR-method The HTR method In standard TL dosimetry with LiF dosemeters, light emission at temperatures below about 240 C is analysed. The intensity of the dominant peak (peak 5) in the glow curve appearing 1

2 at about 200 C is linear over a wide range of absorbed dose. For evaluation of absorbed dose either peak amplitude or peak integral is used. The ratios of the high temperature peaks (peak 6, 7, 8,...) to the main dosimetry peak (peak 5) are more or less constant at low doses after irradiation with low LET-radiation (below the onset of supralinearity). These peaks appear in the temperature range from 230 to 350 C. After absorption of radiation with higher LET, peak ratios change significantly, in general with the high temperature peaks enhanced (5, 6, 7, 8). Based on this effect, the HTR method was developed at the Atomic Institute, Vienna (9, 10), in order to measure not only the absorbed dose but also to get information about the average LET of the absorbed radiation. The HTR method has been used with great success for measurements of the average LET and the estimation of the radiation quality factor of space radiation. Measurements using the HTR method have been carried out during various space missions and in aircraft (11, 12). In order to use the HTR method for measurements in unknown complex mixed fields like space radiation, a relation between a LET-dependent parameter and the LET of absorbed radiation has to be established: the LET calibration. As a parameter for the average LET of absorbed radiation, the HTR method uses the ratio of the intensity of TL light in the high temperature region (225 to 300 C) to the intensity of emission in the same temperature range after 60 Co reference irradiation. In order to avoid errors caused by small deviations from the linear dose dependence of the high temperature peaks, reference irradiations were performed to the same absorbed dose. Before the calculation of the individual HTR of each dosemeter, the two glowcurves, HCP irradiation and reference 60 Co irradiation, were normalised to the maximum of peak 5. Additionally, the LET-dependent behaviour of peak 5 (decreasing efficiency with increasing LET) was measured. This relation can be used for corrections of the efficiency of the dose measurement with peak 5, if the spectrum of the radiation field is known. In previous works nonlinear relations between HTR and the LET of the absorbed radiation was established for standard TL dosemeters (TLD-100, TLD-600 and TLD-700). Therefore calibration irradiations were carried out at the Joint Institute for Nuclear Research (JINR) in Dubna with fluoride ions in the energy range from 65 to 275 MeV/amu and carbon ions with energies from 100 MeV/amu up to 3650 MeV/amu. Irradiations with protons (10 and 62 MeV) were performed at the Paul Scherrer Institute (PSI) in Switzerland. Additionally, irradiations with 241 Am alpha particles and with thermal neutrons were performed in the Atominstitute (13). In order to implement the HTR-method in the PILLE system, the LET dependence of PILLE dosemeters had to be investigated and the properties of the PILLE readout system for the detection of the high temperature TL-emission had to be optimised. 2. THE PILLE TLD-SYSTEM The main features of the new generation of the PILLE TLD system are (14): Each TLD bulb is encapsulated in a cylindrical, pen-like metal holder made of oxidised aluminium. A one-wire-port integrated electronic programmable memory chip mounted inside the holder contains the identification code and the individual calibration parameters of the dosemeter. The PILLE reader is a µp (microprocessor) controlled unit. The functions of the microprocessor are: Full control of the measurement according to the parameters (heating cycle) stored in the dosemeters memory chip. 2

3 The heating power supply controlled via a D/A (digital/analog) converter by the microprocessor is a tuneable current source working in the A range. In this way any, e.g. quasilinear or steplike, temperature profile needed for different types of dosemeters can be obtained. Preliminary evaluation of the measurement with the individual calibration factor stored in the dosemeters memory chip. Display of the measured dose, doserate since the last readout, time and date. Registration of the glow curve (light profile emitted by the TL material) in digital form (time resolution s), and storing of all measured data and numerous parameters on a removable flash memory card for later analyses. Beside manual measurements, automatic sequential readouts of one dosemeter left in the reader are possible. Especial the feature of the programmable temperature cycle is essential for the analyses of the high temperature TL-emission and the registration of the glowcurves on a flash memory card allowed the post flight analyses of glowcurves measured on board of space station MIR during the MIR23/NASA4 mission with the HTR-method. For this post flight analyses LET calibration of PILLE dosemeters were performed. 3. LET CALIBRATION OF PILLE DOSEMETERS 3.1. Irradiations with 62 MeV protons The irradiations of the PILLE dosemeters were performed at the OPTIS facility in the PSI. The primary energy of the protons was 62 MeV. The homogeneous beam diameter was 30 mm. The dosemeters (within the holder) were adjusted in the centre of the beam, with the reading window towards the beam outlet. The protection tubes of the dosemeters were closed during irradiation. The irradiation time was controlled by the internal ionisation chamber. With this chamber the depth dose distribution in acryl glass, corrected for tissue was measured (standard procedure for therapy irradiations). For all irradiations a dose of 100 mgy in the depth of 5 mm tissue was chosen. Upstream the dosemeters blocks of polyethylene were situated in order to variate the incident proton energies entering the dosemeters. The thickness of the additional absorbers was varied in steps from 5 mm to 30 mm. For comparison measurements were carried out with PILLE dosemeters type A (CaSO 4 :Dy bulbs) and type B (LiF:Mg, Ti bulbs). Additionally standard TLDs (TLD-600 and TLD-700) were used as well as Hungarian LiF chips from the same material used in the bulbs Calibration with 60 Co-Gamma radiation The irradiation for the Co-60 calibration was carried out with the calibration source (calibration theratron) at the University Clinic for Radiation Therapy, General Hospital Vienna. The doserate was measured with a calibrated ionisation chamber. Parameters of irradiation: Collimator settings: Beam section: 20 cm x 20 cm/ 80 cm Distance from source: 3 m Dosimetry: Device: ionisation chamber no series 720 Measurement in free air with build up cap Reading corrected for actual temperature and air pressure Measured dose rate: 83.7 mgy/min in H 2 O ( ) 3

4 The dosemeters (within the holder) were adjusted in free air the centre of the beam, with the reading window towards the source. The protection tubes of the dosemeters were closed during irradiation LET-calibration The results of dose measurements in the 62 MeV proton beam are summarised in table 1. Table 1: Measurement of the depth dose distribution in the 62 MeV proton beam with PILLE bulb dosemeters Absorber Bulb No. Proton dose mean Dose Error Bulb No. Proton Dose mean Dose Error meas. meas. (mm) LiF:Mg,Ti (mgy tiss.) (mgy tiss.) (1 σ) CaSO 4 - (mgy tiss.) (mgy tiss.) (1 σ) 5 B1(1) 135,5 A1(1) 121,4 5 B2(1) 135,7 A2(1) 117,2 5 B3(1) 131,8 134,3 2,2 A3(1) 117,2 118,6 2,4 20 B4(1) 204,5 A4(1) 178,0 20 B5(1) 202,5 A5(1) 179,3 20 B6(1) 203,5 203,5 1,0 A6(1) 176,3 177,9 1,5 25 B7(1) 310,9 A7(1) 277,8 25 B8(1) 294,5 A8(1) 274,4 25 B9(1) 308,1 304,5 8,8 A9(1) 255,2 269,1 12,2 26 B1(2) 358,9 A1(2) 330,5 26 B2(2) 333,6 346,2 17,9 A2(2) 318,6 324,6 8,5 27 B3(2) 439,6 A3(2) 375,9 27 B4(2) 442,2 440,9 1,9 A4(2) 401,9 388,9 18,4 28 B5(2) 390,4 A5(2) 452,4 28 B6(2) 404,0 397,2 9,7 A6(2) 450,5 451,5 1,4 29 B7(2) 57,1 A7(2) 74,9 29 B8(2) 99,0 78,1 29,6 A8(2) 73,1 74,0 1,3 30 B9(2) 0,4 A9(2) 6,0 In fig.1 the results of the measurements are plotted. For comparison the results of PILLEbulbs (LiF-type and CaSO 4 -type) are shown together with values measured with standard TLDs (TLD-600 and 700). Additionally the depth dose distribution measured with the ionisation chamber (measured behind acryl-glass, converted to tissue depth) and calculated values are shown. A commend on the calculation of the dose- and LET-distribution see later. Because of the shielding of the dosemeter material within the PILLE-bulbs (glass of the bulb and dosemeter holder) the maximum of the Bragg peak was detected in a depth of 28 mm. According to the measurement with ionisation camber and the calculation the maximum of the Bragg peak of 62 MeV protons is in a depth of about 30 mm (in tissue). Therefore the shielding of the PILLE dosemeters is equivalent to 2mm of tissue. In the figure the measured dose values are plotted versus the effective shielding (absorber thickness in front of the dosemeter plus 2 mm tissue). 4

5 Depth-dose distribution 62 MeV protons 500 Dose (mgy, in tissue) I-chamber LiF-bulbs CaSO 4 -bulbs calc. Dose TLD-600 TLD effective shielding (mm tissue) Fig. 1: Measured and calculated depth dose distribution in a 62 MeV proton beam 3.4. Calculation of Dose and LET-Distribution of 62 MeV Protons in polystyrene The energy loss was calculated in steps of 1 mm polystyrene, the dose average LET was calculated in layers of 1 mm tissue, for the calculated start energy of protons in each layer. The calculation was performed at the Atominstitute with our program LETBER (15), based on the BETHE formula and the proton range-energy tables published by J.F.Janni (16). The absorbed dose was normalised to 100 mgy in a depth of 5 mm tissue (according to the parameters of irradiation) Discussion of the calculation method The calculation was carried out iterative in layers 1 mm thickness. For the primary energy, the energy loss in 1 mm polystyrene was calculated. This energy loss was subtracted from the primary energy to get the energy of the protons entering the next layer. For protons with this actual energy the dose average LET was calculated in a layer of 1mm tissue, and so on. This iterative method causes considerable errors of the calculated values towards the Bragg peak. On the other hand the effects of scattering as energy- and range straggling were neglected. Therefore the calculated Bragg peak is too small and too high. Nevertheless, the results of the calculated LET- and dose distribution are in good agreement with the measurements, excepting the value in the peak maximum. This agreement is shown in fig. 1 (compare the calculated values (black balls) with the values measured with the ionisation chamber (black line). 5

6 4. RESULTS 4.1. Evaluation of the HTR from the PILLE dosemeters The data from the PILLE-reader were transformed to ASCII format and then imported to our evaluation software. The background glowcurves (second glow) were subtracted from the gross glowcurves in order to generate net glowcurves. After this a temperature recalibration was performed with the glowcurves. For this reason the x-axis was transformed by a linear function in order to set the maximum of peak 5 to 200 C. The integral light emission was kept constant during this transformation. All the glowcurves were processed in the same manner. For the evaluation of the HTR, the glowcurve after Co-60 calibration was normalised to the same height of peak 5 of the glowcurve after proton irradiation (both glowcurves from the same dosemeter). The integral light emission in the high temperature region from 250 to 300 C was calculated from both of the glowcurves. The high temperature ratio HTR was evaluated by calculating the ratio of the two values (high temperature integral after proton irradiation divided by high temperature integral after Co-60 calibration) Correlation of HTR with calculated average LET In Table 2 the measured HTR values are shown together with the calculated values of dose average LET for the PILLE dosemeters. Additional to the values gathered with proton irradiations a value after thermal neutron irradiation is shown (pre-flight calibration of NASA4 dosemeters). Parallel to the irradiations of PILLE dosemeters irradiations of standard dosemeters (TLD-600 and TLD-700, Atominstitute and Hungarian LiF dosemeters) were carried out in the proton beam. These results are resumed in Table 3. The results from TLD- 600, TLD-700 and the Hungarian LiF-dosemeters are in excellent agreement. All the results are plotted in figure 2. Table 2: Correlation of average LET with HTR, PILLE dosemeters LiF-bulbs PILLE Absorber eff. shielding Dose av. LET Error mean HTR Error (mm) (mm) (kev/µm tiss) (1 σ) (1 σ) 62 MeV protons therm.neutrons

7 HTR versus calculated LET LiF-bulbs TLD-600, AI TLD-700, AI LiF-KFKI 4 HTR LET in tissue (kev/µm) Fig. 2: LET response of HTR from different dosemeters Table 3: Correlation of dose average LET with HTR, standard dosemeters 62 MeV protons Atominstitute material Hung. material TLD-600 TLD-700 LiF-KFKI Absorber Dose av. LET Err. HTR Err. HTR Err. HTR Err. (mm) (kev/µm tiss) (1 σ) (1 σ) (1 σ) (1 σ) PILLE measurements on space station MIR during MIR-23/NASA-4 mission with the HTR method During the mission MIR-23/NASA-4 in the period from Feb.6 th to May 6 th, 1997 measurements were carried out on board of space station MIR with LiF PILLE dosemeters. The data was recovered on a flash memory card with space shuttle STS-84. The data was processed in the same way as described in the section LET-calibration. In order to evaluate the dose average LET of absorbed space radiation the data of the LET-calibration was fitted and the relation between HTR and LET was used to calculate the average LET. The results are shown in Table 4. The values of the dose average LET varied from 4.5 to 7.6 kev/µm depending on the period of measurement. The average quality factors were estimated using the Q(L) relation (ICRP26) in the form <Q> = Q(dose av. LET). This method does not fulfil the definition of the average quality factor (<Q> = 1/D D(L)*Q(L)dl) where the assessment of the LET spectrum is needed, but during previous investigations the results with our method have shown an excellent agreement with LET-spectrometer measurements (17, 18). 7

8 Table 4: Results of dose and average LET measurements with PILLE LiF dosemeters on board of space station MIR Dosemeter block no. Duration Dose HTR dose av. LET quality factor no.02b gross/bgnd (h) (mgy tiss.) C (kev/µm tiss) (ICRP26) / ,35 1,62 6,1 1, /35 359,2 4,57 1,72 6,9 1, /46 330,3 3,90 1,60 6,0 1, /57 384,1 4,87 1,80 7,5 2, /68 220,6 2,37 1,51 5,4 1, /81 334,2 3,73 1,59 5,9 1, /93 403,4 4,48 1,81 7,6 2, / ,4 3,84 1,38 4,5 1,29 Dosemeter block no. Duration Dose HTR dose av. LET quality factor no.03b gross/bgnd (h) (mgy tiss.) C (kev/µm tiss) (ICRP26) / to high / red/infrared / background / for evaluation / of high / temperature / TL-emission / The analyses of the data revealed, that the thermal background of the dosemeter no 03 B is about four times higher as from dosemeter no. 02 B. Therefore it was impossible to evaluate the high temperature TL-emission from dosemeter no 03 B. Nevertheless the results from measurements during 8 periods with dosemeter no 02 B are in good agreement with our former long duration measurements on board of space station MIR, where the dose average LET ranged from 5.8 ± 0.2 to 7.1 ± 0.1 kev/µm in tissue and quality factors between 1.65 and 2.1 were found (19). 5. DISCUSSION Similar to the LET effect of standard dosemeters, the LiF PILLE bulbs show a smooth relation between HTR and LET of absorbed radiation. With the LET-calibration of PILLE dosemeters and the HTR-method it was possible to evaluate the average LET of absorbed radiation on space station MIR The statistical error of the evaluated HTR values is rather high, because of the high thermal background in the glowcurves measured with the PILLE device. The further aims of our co-operation are the optimisation of the TL material used in the PILLE dosemeters for LET-measurements and the introduction of appropriate optical filters either in the PILLE reader or in the dosemeter holder in order to reduce the thermal background. All these improvements should lead to a new generation of PILLE systems suitable for on board determination of the biologically relevant dose. This would be a step forward for radiation protection measurements on the international space station ISS and in mixed radiation fields on the earth. 8

9 ACKNOWLEDGEMENT The authors wish to thank Dr. E. Egger for the irradiations with protons at the PSI. This work was supported by the Austrian Federal Ministry of Science and Traffic. REFERENCES (1) Fehér, I., Deme, S., Szabó, B., Vágvölgyi, J., Szabó, P.P., Csöke, A., Ránky, M. and Akatov, Yu. A. A new Thermo-Luminescent Dosimeter System for Space Research. Adv. Space Res (1981) (2) Reitz, G., Apáthy, I., Beaujean, R., Deme, S., Heilmann, C., Kopp, J., Leicher, M. and Strauch, K. Results of Dosimetric Measurements in Space Missions. Proceedings of Sixth European Symposium on Life Sciences Research in Space. Trondheim, ESA SP-390, (1996) (3) Deme, S., Reitz, G., Apáthy, I., Héjja, I., Láng, E. and Fehér I. Doses due to the South Atlantic Anomaly during the Euromir 95 mission Measured by an On-Board TLD System. Radiat. Prot. Dosim.. 85, Nos.1-4, (1999) (4) Deme, S., Apáthy, I., Héjja, I., Láng, E. and Fehér I. Extra Dose due to EVA during the NASA4 Mission Measured by an On-Board TLD System. Radiat. Prot. Dosim. 85, Nos.1-4, (1999) (5) Budd, T., Marshall, M., Peaple, L. H. J. and Douglas, J. A. The low and high temperature response of lithium fluoride dosemeters to X-rays, Phys. Med. Biol (1979) (6) Busuoli, G., Cavallini, A., Fasso, A. and Rimondi, O., Mixed radiation dosimetry with LiF (TLD-100), Phys. Med. Biol (1970) (7) Driscoll, C. M. H. Studies of the effect of LET on the thermoluminescent properties of thin lithium flouride layers, Phys. Med. Biol 23(4) (1978) (8) Horowitz, Y. S., Fraier, I., Kalef-Ezra, J., Pinto, H. and Goldbart, Z., Non-universality of TL-LET reponse in thermoluminescent LiF: the effect of batch composition, Phys. Med. Biol (1979) (9) Vana, N., Freiler, Ph. and Fugger, M. Radiation dose detection with TLDs in mixed radiation fields, Proceedings of 2 nd Italian-Austrian Radiat Prot. Symposium, (1991) (10) Vana, N., Schöner, W., Fugger, M., Akatov, Yu. A. DOSIMIR - Radiation measurements inside the Soviet space station MIR - first results, Proc.Intern.Space Year Conference, ESA ISY-4, 193 (1992) (11) Noll, M., Schöner, W., Fugger, M., Vana, N., Brandl, H., Dose measurements in mixed radiation fields with TLDs under consideration of the peak height ratio, Rad. Prot. Dosim. 66(2) (1996) (12) Noll, M., Vana, N., Schöner, W., Fugger, M., Determination of average LET and the equivalent dose in aircrafts using the HTR-method, Proceedings of the International Congress of IRPA9, (2) (1996) (13) Schöner, W., Vana, N., Fugger, M., The LET dependence of LiF:Mg, Ti dosemeters and ist application for LET measurements in mixed radiation fields, Radiat. Prot. Dosim. 85, Nos.1-4, (1999) (14) Aphaty, I., Deme, S., Bodnar, L., Csöke, A., Heijja, I., An on-board TLD system for dose monitoring on the international space station, Radiat. Prot. Dosim. 84, Nos.1-4, (1999) 9

10 (15) Schöner, W., Die HTR-Methode zur Messung des mittleren LET der Strahlung im Weltraum mit Thermolumineszenz-Dosimetern und ihre mikrodosimetrische Interpretation, Thesis at the University for Technology, Vienna, (1997) (16) Janni, J.F. Proton Range - Energy Tables, Part 1, ATOMIC AND NUCLEAR DATA TABLES (1982) (17) Nguyen, V.D., Bouisset, P., Akatov, J.A. et al, Measurements of quality factors and dose equivalents with CIRCE inside of the Soviet space station MIR, Radiat. Prot. Dosim., 31, 1/4, , (1990) (18) Badhwar, G.D., M. J. Golightly, A. Konradi, W. Attwell, J. W. Kern, et al, In-flight radiation measurements on STS-60, Radiat.Meas., 26, 1, (1996) (19) Vana, N., Schöner, W., Fugger, M. and Akatov, Yu. A. Absorbed dose measurements and LET-determination with TLDs in space, Radiat. Prot. Dosim. 66(2) 145 (1996) 10

APPLICATION OF 3D SILICON

SPACE DOSIMETRY WITH APPLICATION OF 3D SILICON TELESCOPE AND PILLE ONBOARD TLD DEVICE Thesis TAMÁS PÁZMÁNDI Scientific Supervisor: DR. SÁNDOR DEME (KFKI AEKI) Faculty consultant: DR. PÉTER ZAGYVAI (BME