A Novel Twin-TLD Radiation Dosimeter for Astronauts during LEO Missions B. Mukherjee 1, C. S. Llina-Fuentes 1, J. Lambert 1, B. Timmermann 1 C. Sunil 2, S. Tripathy 2, P. K. Sarkar 2 1 Westdeutsches Protonentherapiezentrum Essen (WPE), Germany 2 Bhabha Atomic Research Centre, Mumbai, India F

Introduction During low earth orbiting (LEO) missions space vehicles are predominantly bombarded with energetic (trapped) protons from the sun, during the solar flare events (SFE), the proton flux sporadically increases by many orders of magnitudes. The solar protons interact with the containment wall of the vehicle producing high- energy neutrons with a broad energy distribution as well as gamma rays, which result in a high radiation exposure to astronauts. By implementing pairs of TLD700 ( 7 LiF:Ti, Mg) and BeO (Thermolux 995) chips we have developed a personal dosimeter for an accurate assessment of average LET and Quality Factor ( ~ biological dose) of high-energy mixed radiation field. The dosimeters were irradiated using high-energy neutrons and secondary particles produced by bombarding a 25  25  35 cm 3 polystyrene plate phantom with therapeutic protons up to 230 MeV at West German Proton Therapy Center Essen (WPE). The dosimeters were calibrated in-situ with a tissue equivalent proportional counter (TEPC), also known as Rossi Counter. The operation principle of the novel twin-TLD personal dosimeter for astronauts during LEO mission as well as in proton therapy will be highlighted in our presentation.

Solar Flare Spectra and EVA Solar Flare : Protons (~88%), Alpha Particles (~10%), Electrons (~2%) Historical Solar Flare Event (SFE) spctra adopted from the reference (NASA/TP ). The spectra have been binnedfor the purpose of proton dose calculations. Historical Solar Flare Event (SFE) spctra adopted from the reference (NASA/TP ). The spectra have been binned for the purpose of proton dose calculations. Photograph showing a typical extravehicular actvity (EVA) during low earth orbit (LEO) mission by the astronauts at International Space Station (ISS). The picture is taken from NASA outreach publications database.

Space Radiation Dosimeter (single TLD) High-Temperature-Peak Ratio (HTR) Method : Schöner, W. et al. Rad.Prot.Doim. 85(1999)263 Following radiation exposure the well known 6 LiF:Mg,Ti (TLD600) and 7 LiF:Mg,Ti (TLD700) thermoluminescence dosimeters (TLD) show five TL- glow peaks. Ratio of the high-temperature (~280 o C) peak (HTP) areas of a mixed radiation field and that of 60 Co-gamma irradiated TLD-chip is a function of the average LET. Area of low temp. peak (LTP) is proportional to dose. Hence, a single TLD could be used to estmate the dose and average LET (Quality Factor) of a complex mixed- radiation field, usually prevelant in space. Pitfalls of HTR Method : Horowitz, Y. et al. Rad.Prot.Dosim. 106(2003)7 The area of HTP (peak 5) is NOT an explicit function of LET but of mass, charge and energy of the impinging particles. There are severe discripancies in the relationship between LET and HTR of different types of particles as highlighted in the Figure on RHS.

Space Radiation Dosimeter (twin TLD) We have circumvent the pitfalls of single-TLD method as folllows In stead of a single TLD chip we have used two chips with dissimilar senstivites (LET response) to high-energy protons and heavy charged particles We have selected pairs of 7 LiF:Mg,Ti (TLD700) and Al 2 O 3 :C (TLD500), later replaced by BeO (Thermolux 950) thermoluminescence dosimeters The TLD pairs were irradiated with: a)Gamma rays from a 60 Co-Source b) Simulated radiation fields generated by bombarding a polystyrene plate phantiom with high-energy protons ( MeV) a)Microdosimetric spectra, including dose equivalent, average LET and QF of the simulated radiation fields were evaluated using a Tissue Equivalent Proportional Counter (TEPC) b) TLD chips were evaluated at a heating rate of 5 o C/s and the glow curves analysed a)The ratio of the glow curve areas of TLD700 and BeO chips were cross-calibrated for average LET and QF estimated using the TEPC data b) Glow curve areas of TLD700 chips calibrated for dose equivalent using the TEPC data a)Higher sensitivity as we have used the entire glow curve area, not just the HTP region b) Discripancies of the estimated LET and QF eliminated (calibration using a TEPC). b) Discripancies of the estimated LET and QF eliminated (calibration using a TEPC).

Characteristics of BeO and LiF TLD The relative TL-response of BeO dosi- meter increases with LET, on the other hand, it drops substantially in the case of LiF (K. Becker, CONF ) We took the above data as a guideline. We have irradiated the pairs of TLD700 and BeO chips with the simulated radiation field (119 MeV protons impinging on Polystyrene plate phantom) and 60 Co gamma rays. The TL-glow curves are shown in Figures a and b respectively. The ratio of glow curve areas (150 o C – 250 o C) of chip pairs irradiated with simulated radiation field (proton) and gamma rays are shown. The ratio (A BeO /A LiF ) rises with increasing LET of the impinging radiation field.

TLD Calibration using TEPC The REM 500 TEPC used in our Studies (chamber vol cm 3 ). A polystyrene plate phantom 25x25x35 cm3 of variable thickness was irradiated with therapeutic proton beams. The TEPC (TLD pairs attached to its tip) was placed next to phantom. Microdosimetric spectra of the radiation field produced in polystyrene plates bombarded with high-energy protons. Proton beams of energy 81, 119, 150, 177, 201and 231 MeV were used. The TLD chips (attached to TEPC) were adjusted at the plane of the corrsponding Bragg-Peak, i.e. end of the proton range. A proton dose of 2 Gy was delivered to a single layer (Bragg-Peak) in all cases.

TLD Calibration using TEPC (Results) Figure a: Average LET as a function of TL glow-curve area ratio. Figure b: ICRP 60 Quality Factor as a function of average LET Figure c: Dose Equiv. Calib. factor as function of TL glow-curve area ratio. Figure d: Average LET as a function of proton energy.

Example 1: Flight Dose Calculation Date: ; Route: Paris-Seattle; Max. Cruising Altitude: m; Flight time: 11h 30min Date: ; Route: Seattle-Atlanta; Max. Cruising Altitude: m; Flight time: 4h 15min Date: ; Route: Atlanta-Paris; Max. Cruising Altitude: m; Flight time: 14h 0min A LiF = 2982 nC; A BeO = 775 nC; A BeO /A LiF = 0.26 Average LET was calculated to be 15.3 keV/  (Figures a and d) Quality factor was calculated to be 3.2 (Figure b) Integrated flight dose equivalent : H = kA LiF (1) Value of k was estimated to be : (Figure c) By substituting the values of k and A LiF in equation 1, the integrated (29h 45min) flight dose equivalent H was calculated to be 11.9  Sv

Example 2: Dosimetry- Proton Therapy The Treatment Planning System (TPS) image of a skull-base tumour of a paediatric case. The treatment (proton irradiation) volume of the tumour is highlighted. A paediatric phantom (5y old) was placed on the dedicated couch, Patient Positioning System (PPS). Unifrom Scanning (US) and Double Scattering (DS) modes for proton irradiation were used. Dosimetry points (out of the field) are highlighted by red dots (. ). Out-of-the-field dose equivalents at criti- cal organs per unit primary proton dose were estimated. High Lav and QF values are compatible to that of astronauts receive during typical LEO missions.

Summary and Conclusions We have developed a novel radiation dosimeter for space and particle therapy applications based on pairs of LiF (TLD700) and BeO chips. The dosimeters were calibrated in a simulated radiation field generated using high- energy protons at West German Proton Therapy Centre Essen (WPE). A Tissue equivalent propotional counter (TEPC) of type REM 500 was used. By analysing the TL-glow curves we were able to estimate an average LET and quality factor (QF) of impinging particles up to 80 keV/  and 15 respectively. High detection sensitivity was achieved as we have used the entire glow-curve areas of the chips instead of only the high-temperature regions. Flight dose equivalent, average LET and QF during a civil passenger flight: Paris- Seattle-Paris was estimated.

Summary and Conclusions (contd.) Out-of-the-field radiation dose equivalents, LET and QF during proton therapy (paedi- atric patient) were evaluated using the twin chip dosimeter. Further research at our centre is ongoing aiming to estimate the LET, QF and DE of secondary radiations in critical internal organs of cancer patient during proton therapy using a modified version of this twin-chip dosimeter. The data will be vitally important to asses the risk of secondary cancer incidence, in particular for paediatric patients. Evidently, similar situation arises when astronauts are in LEO missions, which makes the major goal of the „MATROSHKA“ project currently operated by the ESA. Implementation of this simple, user friendly, bug free, twin-chip dosimeter (micro dosimeter) in future LEO missions could be intersting. Thank You For Your Attention Bhaskar Mukherjee

Epilogue (Matroshka of ESA) Matroshka Anthropomorphic Phantom of European Space Agency (ESA) Deployed at International Space Station (ISS) Deployed at International Space Station (ISS) Predict the proton dose in the critical organs of the astronauts during LEO MissionsPredict the proton dose in the critical organs of the astronauts during LEO Missions Various types of TLD chips used for the assessment of Physical Dose (Rad)Various types of TLD chips used for the assessment of Physical Dose (Rad) Solid State Nuclear Track Detector (SSNTD) used for the evaluation of LET distribution, biological dose (Dose Equivalent)Solid State Nuclear Track Detector (SSNTD) used for the evaluation of LET distribution, biological dose (Dose Equivalent) Highly concentrated caustic soda (NaOH) is used to assess the SSNTD => user un-friendly and hazardousHighly concentrated caustic soda (NaOH) is used to assess the SSNTD => user un-friendly and hazardous The twin TLD-Chip method for the assessment of biological dose and associated microdosimetric parameters found to be a practical solution. The upper limit of estimated LET is ~ 80 keV/ , hence, unsuitable for application in deep space missions, where the heavy charged particles of LET > 80 keV/  are prevalent.

Epilogue (COSPAR 2012 Participants)