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In-Vivo Calibration of a LE Ge detection system for the assessment of Americium in bone, in the Whole Body Counter of CIEMAT

M.A. Lopez, J.F. Navarro, T. Navarro, J.M. Gómez Ros, M. Moraleda

Radiation Dosimetry, CIEMAT, Avda. Complutense 22, E-28040 Madrid, Spain. E-mail: ma.lopez@ciemat.es

Abstract. The calibration of a LE Ge detector system has been performed at CIEMAT Whole Body Counter for the assessment of 241Am in skull and knee. The facility counts with a Cohen phantom simulating a human head with 1000 Bq of Americium homogeneously distributed in the bone equivalent material. Measurements of the LE Ge system with the skull phantom were carried out in an optimisation study of counting efficiency for 241Am determination. A Monte Carlo approach was also applied and the mathematical efficiencies were compared with experimental measurements, showing a bias less than 10% at a short detector-skull distance. A Spitz knee phantom simulating internal contamination of Americium in bone was counted for calibration purposes using two LE Ge detectors; the efficiencies obtained for each pair of detectors were used to calculate the Activity contained in the IAEA knee phantom received at CIEMAT during an In-Vivo Intercomparison. A new calibration was performed using the four LE Ge detectors and two knee phantoms. The efficiency obtained with the LE Ge detection system measuring Americium in two knees showed a lower value comparing with skull efficiencies for 59.5 keV emissions. As knee counting geometry was proved to be more comfortable for the subject, an optimisation study of knee counting efficiency will be considered in the future. 1. Introduction 241Am is a bone-seeking and long-lived radionuclide whose retention in the body is assessed by gamma spectrometry in lungs and skeleton. Occupational exposures of Americium are mainly related to decommissioning and decontamination procedures of nuclear facilities or handling radioactive wastes. The in-vivo evaluation of 241Am Activity in bone is recommended in case of chronic intakes or in measurements long time after the internal exposure. The calibration of direct measurement systems for low-energy photons needs of realistic anthropomorphic phantoms; further more, anatomically realistic phantoms are required when photons from the radionuclides of interest are severely attenuated by body tissues (this is the case of 59.5 keV photons of 241Am in bone). Following ICRU-69 recommendations [1] the in-vivo assessment of radionuclides that emit low-energy photons (e.g., 210Pb or 241Am) should be carried out in a region of the body that is isolated or can be shielded from interference by activity in the rest of the body, for example, the skull or the knee [2] , [3] , [4]. It is then necessary to estimate the fractional skeletal activity deposited in the region viewed by the detector in order to derive total skeletal deposits from these measurements; carefully designed calibrations phantoms are required to guarantee the accuracy in these assessments [5]. In-vivo measurements of Americium are performed in the CIEMAT Whole Body Counting facility using a Low Energy germanium detectors (LE Ge) with different available counting geometries to evaluate the Activity of 241Am in lungs and bone. A calibration methodology of the system was developed to evaluate 241Am in skull and knee. The objective is to obtain the most useful and operative calibration factor to be applied for Activity calculations related to the in-Vivo detection of 59.5 keV photons of the Americium in skeleton; realistic physical phantoms (Cohen skull and Spitz knee) with a known value of Activity homogeneously distributed in the bone-equivalent material were used to fulfil this purpose. A Monte Carlo approach was also carried out to simulate the different scenarios involved in the process of Americium in skeleton detection. MCNP code together with voxel phantoms were proved to be powerful and useful tools for numerical calculations in internal dosimetry applications.

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2. 241Am in bone calibration methodology The Whole Body Counting (WBC) facility of CIEMAT counts with a germanium system for the in-vivo detection of Uranium and Americium in the body associated to occupational and public exposures [6]. The “LE (Low Energy) Ge” detectors provide relevant operative advantages as low noise and excellent resolution at low and moderate energies, clear nuclide identification, decrease of the lower limit of detection comparing with Phoswich detectors, and faster availability of the analysis results by using Canberra Genie2000 software. The shielded room (13 cm steel walls lined with Pb, Cd and Cu) of the WBC laboratory is the unique facility in Spain with capability for in-vivo measurement of Actinides deposited in the body. The LE Ge system consists of four detectors, mounted two each as arrays in two ACTII cryostats; the active area of each detector is 3800 mm2 , with a diameter of 70 mm and a thickness of 25 mm; each device is equipped with a Carbon Epoxy window, 0.5 mm thick. The Canberra Genie2000 Gamma Spectrometry software provides a complete set of operating procedures to analyse subjects, to perform calibration functions and quality assurance operations. The LE Ge detection system is defined as four inputs or individual detectors, being each device set for 4096 channel acquisition region and amplifier gain of 0.267 keV/channel, in an analysis energy range of 10-1000 keV. The final result of the measurement is analysed from the individual detectors and from the different composed spectra. Energy and Shape (FWHM and low-tail) calibrations are performed using multiline emitters (241Am and 152Eu) covering the energy range of interest. 2.1.- Calibration of LE Ge system for in-vivo evaluation of 241Am in skull Efficiency calibration factors associated to the in-vivo detection of 59.5 keV photons (241Am) in different skull counting geometries, were obtained using the LE Ge detector system and a realistic head phantom with 1000 Bq of Activity homogeneously distributed in the bone-equivalent material. CIEMAT contacted New York University Medical Center (USA) in 1997 and an 241Am skull calibration phantom was prepared under the supervision of Dr. Norman Cohen, for the In-Vivo assessment of Americium deposited in skeleton (Figure 1). To simulate bone the Cohen phantom was constructed of Plaster-of-Paris, in the form of a human head. To simulate brain, muscle and facial soft-tissue, tissue-equivalent paraffin was used. 1000 Bq of 241Am was distributed uniformly over the skull at approximately mid-line depth within the bone surrogate, on interior mid-bone surface. The facial tissue paraffin had been applied consistently with an average human head. The thickness of the Plaster-of-Paris skull is around 6 mm, which is the typical value of a Western skull. Six layers of Plaster of Paris cast bandage were applied to the foam-mannequin head. After drying, the foam was removed from the interior and the space filled with tissue-equivalent beads. The activity was then applied to the outside of the third layer by placing 0.75” filter paper disks (in sufficient number and as evenly distributed as possible) over the skull surface, yielding a total phantom activity of 1000 Bq. After the disks had thoroughly dried, the second three of the six layers of Plaster of Paris were added. Thus, the 241Am activity is in the centre of the simulated bone material. The paraffin was then applied in layers of varying thickness depending on the section (forehead, cheeks, mouth, ears, neck, etc) until a reasonable likeness to an average human head was obtained. The simplicity of the head construction is valid for calibration purposes. A Computerized Tomography (CT) image was taken from the Cohen phantom (Figure 1) at the Hospital Puerta de Hierro (Madrid), showing that the skull simulation is not so realistic as it was expected. Some differences were found in the phantom comparing with a real human skull: the distribution of the Plaster-of-Paris (bone-equivalent material) as an uniform layer below the phantom surface confirms that some parts of the skull (especially inner bones) of a human being were not modeled in the Cohen phantom.

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FIG 1. Cohen Head Phantom used at CIEMAT-WBC laboratory for the calibration of 241Am in skull

Monte Carlo Approach WBC facilities need of complete “librarie s” of physical phantom for the in-vivo calibration of detection systems, covering different counting geometries and considering all the radionuclides of interest. This means great investments in each calibration procedure to provide the laboratory of a real capability for the assessment of internal exposures. The mathematical voxel phantoms represent an alternative to the physical phantoms, being proved to be an useful dosimetry tool in the application of Monte Carlo calculations for the counting efficiency estimation of in-vivo detection systems. A joint study WBC laboratory - Computational Dosimetry Group has been carried out at CIEMAT to deal with Monte Carlo application to internal dosimetry matters [7] [8]. For the simulation of in-vivo measurement of actinides in skull a human head has been built based on the tomographic phantom VOXELMAN developed by Zubal et al [9] which is available at the website of the Yale School of Medicine. The original phantom consists of a series of computed tomography images taken from a patient whose height is 1.75 m height and whose weight is 70.2 kg. For Americium in skull calibration purposes, only the slices corresponding to the head were processed and the resulting phantom consists of 104017 (43x59x41) cubic voxels 4 mm on each side (Figure 2).

FIG 2. Zubal Voxel Phantom used in the mathematical calibration of the LE Ge system With reference to the LE Ge system, the detectors were simulated as careful as possible in relation with the features (materials, densities and size) provided by the manufacturer (Canberra Industries). The characteristics of each LE Ge detector are summarized in Table I. A dead layer thicker than the theoretical value was considered in the entrance window of the LE Ge detector.

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Table I. Dimensions and materials composition of the Canberra LE Ge detector. Structure Composition Density Dimension

(1) Ge crystal (High purity Ge) Ge 5.32 g/cm3 Diameter: 70 mm Thickness: 25 mm

(2) Outer electrode (Doped Ge) Ge 5.32 g/cm3 Thickness: 4000 Å

(9) Crystal holder (Copper) Cu 8.96 g/cm3 1 mm in lower part 3 mm in upper part

(3) Superinsulator (PVC) H (9%), C (38%), Cl (53%) 1 g/cm3 2 mm

(6) Vacuum - 0 5 mm

(5) End cap (Stainless steel) Fe (72,85%), C (0.15 %), Mn (2 %), Si (1%), Cr (17%), Ni (7%)

7.9 g/cm3 Diameter: 82.5 mm Thickness: 1.575 mm

(4) Composite window (carbon epoxy)

C (83.4%), H (7.3%), O (9.3%) 2.3 g/cm3 Thickness: 5 mm

FIG 3. LE Ge system; Design supplied by the manufacturer (Canberra Industries) and Monte Carlo

simulation of the detector MCNP code was used to simulate the propagation of 59.5 keV photons (241Am) from the skull to the LE Ge detector system, supplying as output the mathematical efficiency (counts per second, per photon per second) which is equivalent to the peak efficiency experimentally determined in the detector with the Cohen head phantom.

FIG 4. 241Am (59.5 keV) Peak: spectrum from Genie2000 compared with Monte Carlo(MCNP) output

241Am 59.5 keV (36%)

Genie2000 Spectrum: 241Am

59.5 keV (36%)

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The Monte Carlo methodology was applied in some simple source-detector geometries to check the mathematical efficiency comparing with experimental data. Results from simulations for point sources of 241Am (59.5 keV) and 152Eu (122 and 444 keV) and cylindrical head (BOMAB phantom) filled with a solution of 57Co (122 keV) and 137Cs (662 keV), showed a good agreement with experimental values. 2.2. Calibration of LE Ge system for in-vivo evaluation of 241Am in knee Skull and knee were chosen as the most appropriate counting geometries to perform in-vivo measurements to evaluate the Activity of 241Am deposited in the skeleton. Efficiency calibration factors for photons of 59.5 keV were obtained in both cases using the LE Ge detector system. CIEMAT–WBC laboratory counts with one anthropometric knee phantom simulating the internal contamination of 34 kBq of Americium in bone for calibration purposes using LE Ge detectors. The knee was fabricated by Dr. Spitz at University of Cincinnati [10] containing a removable femur, patella, tibia and fibula; the shell of the phantom consists of a polyurethane-based substitute for 100% muscle tissue and the bones were fabricated with a substitute for trabecular bone. All the materials have the same density, attenuation coefficient and effective Z as that for human muscle and trabecular bone. During the manipulation of the skeletal components 241Am was incorporated uniformly within the bone-equivalent material. To provide the most realistic counting geometry for in-vivo knee calibration, the LLNL (Lawrence Livermore National Laboratory, USA) with the blank lungs set was located on the reclined dentist-chair together with two active knee phantoms (CIEMAT and IAEA) containing 241Am in the bone-equivalent material (Figure 5). Each cryostat cooling a pair of LE Ge detectors was placed above each Spitz phantom, with a minimum detectors-knee distance of 2 cm. A calibration factor was obtained considering the four LE Ge detectors measuring the two knee simulators.

FIG 5. LLNL phantom with 2 Spitz knee simulators for the calibration of 241Am in knee

3. Results and Discussion In case of internal exposure, when Americium is absorbed to blood the main sites of deposition are liver and bone. Estimates of the total skeleton content are obtained by measuring the Activity deposited in the skull or in the knee. In-Vivo detection of 241Am in skeleton is carried out when chronic exposures occur or long time after an acute intake as a consequence of an incident.

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3.1. In-vivo measurement of 241Am in skull: Counting Efficiency and sensitivity The decision for Peak Analysis of Americium spectrum was to use a pre-selected region of interest (ROI) for peak location and area calculation of 59.5 keV (35.9 %) emission (Figure 4). The use of pre-selected ROI for the determination of peak areas is suitable both when using semiconductor detectors and when analysing for one or a few known radionuclides. A ROI of (54.3 , 61.8) keV was defined for the main 241Am photopeak (59.5 keV), being FWHM of 0.66 keV. Counting efficiency (counts/gamma) of the LE Ge system for the detection of 59.5 keV gamma photons was obtained dividing the counting rate (counts per second) by the emission rate (gammas per second) of the calibration source (Cohen skull phantom). A set of calibration factors were calculated in different counting geometries. An experimental study was carried out in order to optimise detection efficiency at a minimum operative distance head-devices of 2 cm; the detectors were located at different counting geometries, in different relative angles: vertical position, rotated geometry towards the back, and wrapped geometry covering the back of the head with the four detectors. A preliminary estimation of the sensitivity of the LE Ge system evaluating 241Am in skull was also performed [11]. Minimum Detectable Activity (MDA) was calculated applying the methods developed by Currie and ANSI N13.30 criteria [12], with a 95% confidence factor. Two blank persons were in-vivo measured: an adult male (79 kg, 1.65 m, 35 years) and a female (60 kg, 1.56 m, 39 years). No important differences were found in the head background contributions of both volunteers in the region of interest of the 59.5 keV peak. - Vertical Geometry: Detectors on the vertical with an angle of 16º between the two pairs of detectors

Eff–vertical = 1.83E–2 ± 3% c/g Eff–vertical = 6.56E–3 ± 3% cps/Bq MDA–vertical= 8.5 Bq (t= 1200 s)

- Rotated Geometry: Detectors rotated towards the back 14º from the vertical and still 16º between

Eff–rotated = 2.11E–2 ± 3% c/g Eff–rotated = 7.57E–3 ± 3% cps/Bq MDA–vertical= 7.5 Bq (t= 1200 s)

- Wrapped Geometry: Detectors rotated 23º towards the back and with an angle of 25º between them

Eff–wrapped = 2.27E–2 ± 3% c/g Eff–wrapped = 8.15E–3 ± 3% cps/Bq MDA–wrapped = 7.0 Bq (t= 1200 s)

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An averaged value from these three calibration factors was calculated: Eff-skull= 2.07E–2 c/g, with standard deviation of 10.7%; this value in an indicator of the efficiency to be applied in case the relative position detectors-head of the person doesn’t correspond to any of the three positions (vertical, rotated, wrapped) considered in this study. LE Ge system has been constructed to perform measurements of Actinides in lungs; thus, skull counting geometry is not considered as a comfortable option. Vertical position was chosen as the most comfortable geometry for the person being monitored, and wrapped geometry as the most efficient and the most sensitive for the in-vivo evaluation of 241Am in skull. The same conclusions were obtained from the Monte Carlo approach.

FIG 6. Counting geometry for the in-vivo measurements of 241Am in skull

with LE Ge detectors, at the CIEMAT Whole Body Counter facility Counting efficiencies of the LE Ge system obtained with physical phantoms were compared with mathematical efficiencies determined by Monte Carlo technique [7] , [8]. Various distances detector-head were tried, following the same three skull geometries of the experimental study. Bias less than 10% were obtained at short distances, for the three available counting positions. Discrepancies found in the calculated versus experimental values are likely due to anatomical differences between the Zubal voxel phantom and the Cohen physical phantom. Mathematical phantoms obtained from CT images of a real subject are proved to be more realistic than physical phantoms 3.2. In-vivo measurement of 241Am in knee: Counting Efficiency and sensitivity

FIG 7. In-vivo measurements of 241Am in 2-knees with LE Ge detectors, at the CIEMAT Whole Body Counter facility

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The Spitz phantom simulating internal contamination of 33.8 kBq of Americium in knee was counted at the WBC laboratory for calibration using two LE Ge detectors. The efficiencies obtained for each pair of detectors were applied to calculate the Activity contained in the IAEA knee phantom received at CIEMAT during an In-Vivo Intercomparison Program organized by the Agency. A new calibration factor was obtained using the four LE Ge detectors on the two knee phantoms. The study of sensitivity of the LE Ge system in the in-vivo evaluation of 241Am photons (59.5 keV) in 2-knees was carried out using the results of the background measurements of the same two blank persons considered for MDA calculations of Americium in skull. Minimum Detectable Activity was calculated applying the methods developed by Currie and ANSI N13.30 criteria [12], with a 95% confidence factor. The results are presented in the Table as follows: Table II. Efficency and Sensitivity of the 4 LE Ge detectors system in knee counting geometry for the

in-vivo measurement of 241Am (59.5 keV) Counting Geometry MDA (Bq) Tc (s) Efficiency (c/g)

1-knee (Det1-2, right side) 9.0 1800 1.046e-2 ± 1% 1-knee (Det 3-4, left side) 9.5 1800 1.013e-2 ± 1%

2 knees (4 detectors) 11.0 1800 1.215e-2 ± 4%

Results of Efficiency and MDA show the measurement of Americium in skull as more efficient and more sensitive in all the available counting geometries (vertical, rotated, wrapped) than the corresponding values associated to the detection in 2-knees geometry. 3.3. In-vivo evaluation of total Activity of 241Am in skeleton In the assessment of the Americium deposited in the skeleton, the preference is to perform the measurements on bones surrounded by a thin layer of soft tissue. Skull and knee were chosen to be the most appropriate counting geometries. Estimates of the total skeleton content can be obtained by measuring the Activity in the skull or in the 2-knees and adjusting the result for the fraction of skeleton monitored. ICRU69 publication describes the case of a worker who earlier in life had an accidental intake of 241Am. Having donated his body to the US Transuranium Registry, the opportunity was taken after the death of the person, when measurements on the body had been performed before the soft tissues and one half of the skeleton were subjected to radiochemical analyses. The measurements on the body provided accurate calibration factors for 241Am in bone and confirmed the earlier recommendation that “In Vivo” measurement of the skull probably provides the best estimate of 241Am in skeleton [1], [5]. Experimental results related to the efficiency and the sensitivity of the LE Ge system in the different available bone geometries reveal the in-vivo measurement of 241Am in skull as the more convenient option from a detection point of view, to evaluate the total content of Activity in the skeleton. The bones contained in one Spitz knee phantom represents 10.7% of the total skele tal mass or 12.4% of the total skeletal surface area. Improved efficiency is achieved if in-vivo measurements are performed with the four LE Ge detectors placed on two knees, being considered in this case the 22% of the mass of the total bone content of an adult male. An average value of 15% is considered for skull bone content following ICRP publications [13]. The efficiency obtained with the LE Ge detection system measuring Americium in two knees showed a lower value comparing with skull efficiencies for 59.5 keV emissions; as the knee counting geometry was proved to be more comfortable for the subject, an optimisation study of the calibration factor will be considered in the future. The Monte Carlo technique will be also applied in the improvement of the counting efficiency for in-vivo detection of bone-seeking radionuclides in the knee.

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References 1. International Commission on Radiation Units and Measurements. Direct Determination of the Body Content of Radionuclides. ICRU Report 69. Journal of the ICRU 3 (1) Oxford Pergamon Press (2003). 2. Cohen, N., Spitz, H.B. and Wrenn, M. Estimation of skeletal burden of bone-seeking radionuclides in man from in vivo scintillation measurements of the head. Health Phys. 33, 431- 441 (1977). 3. Cohen, N., et.al . Long-Term retention of 210Pb in man : a unique case of internal contamination. Health Phys. 62, 553-555 (1992). 4. Laurer, G.R. et. Al. Skeletal 210Pb levels and lung cancer among radon-exposed tin miners in southern China. Health Phys. 64, 253-259 (1993). 5. Hickman, D.P. and Cohen, N. Reconstruction of a human skull calibration phantom using bone sections from an 241Am exposure case. Health Phys. 55, 59-65 (1988). 6. Lopez, M.A., Navarro T., Sensitivity of a Low Energy Germanium Detector system for in-vivo monitoring in the framework of ICRP78 applications. Radiat. Prot. Dosim. 105:477-482, (2003). 7. Moraleda M., Lopez M.A., Gómez Ros J.M., Navarro, T., Navarro, J.F., Calibration Human Voxel Phantoms for in-vivo measurement of 241Am in bone at the Whole Body Counter facility of CIEMAT. CIEMAT Technical Report 1005 ISSN: 1135-9420, (2002). 8. Gómez-Ros, J.M., Moraleda, M., López, M.A., Navarro, T., Navarro, J.F.A Numerical Method for the Calibration of a Whole Body Counter.Application to In Vivo Measurements of 241Am in Skull. 48th Annual meeting of the Health Physics Society, USA 2003. Health Phys. 84, S172-S173 (2003). 9.- Zubal, G., Harrell, G., Smith, E., Ratner, Z., Gindi, G., Hoffer, P. Computerized three-dimensional segmented human anatomy. Med. Phys. 1994; 21:229.302 (1994). 10. Spitz, H.B., Lodwick, J.C., A New Anthropometric Calibration Phantom for in -Vivo Measurement of Bone Seeking Radionuclides. In Proceedings of IRPA 10 Congress, Hiroshima (2000). 11.- Robredo L., Lopez, M.A., Navarro T., Sierra I., Navarro J.F. Comparison Study of in-vivo and in -vitro techniques for the assessment of internal exposures of Actinides. In Proceedings of IRPA 11 Congress, Madrid (2004). 12.- ANSI N 13.30-1996.- Perfomance Criteria for Radiobioassay. American Nationial Standards Institute Publication (1996). 13.- International Commission on Radiation Protection. Basic anatomical and physiological data for use in radiological protection: Reference values. ICRP Publication 89. Annals of the ICRP Vol. 32 No. 3-4 (2002).