Determination of Ambient Dose Equivalent at INFLPR 7 MeV Linear Accelerator

Transcription

1 Determination of Ambient Dose quivalent at INFLPR 7 MeV Linear Accelerator F. Scarlat, A. Scarisoreanu, M. Oane,. Badita,. Mitru National Institute for Laser, Plasma and Radiation Physics - INFLPR, Bucharest-Magurele, Romania scarlat.f@gmail.com ABSTRACT This paper presents the dosimetric measurements performed at INFLPR 7 MeV Linear accelerator by means of the secondary standard chamber H(10), type. A secondary standard for photon radiation is based on the linear relation between the ionizing current and the conventionally true value of the ambient dose equivalent. The measurements were made for the electron beam accelerator operation mode. Determination of the ambient dose equivalent was made by means of the PTW ionization chamber, type, calibrated by PTW Freiburg. Key Words: Radiological Protection/Secondary Standard Chamber/Ambient Dose quivalent. INTRODUCTION In the lectron Accelerator Department in the National Institute for Laser, Plasma and Radiation Physics (INFLPR) a Secondary Standard Dosimetry Laboratory for high energies it was built. It is outfitted with a secondary standard dosimeter UNIDOS equipped with proper accessories for the absorbed dose and the absorbed dose rate measurement in certain energy domains (between 8 kev and 50 MeV). For the penetration powerful radiations, the operational parameter used for certain area of the field radiation monitor, is the ambient dose equivalent H(d). According to international standards recommendations, the conventional true values of radioprotection operational parameters are based on air kerma free in air, K a, by conversion coefficients application. The mono-energetic conversion coefficients, from air kerma to H(d), very much depend o photon energy. The work is starting with presenting the ambient and directional dose equivalent concepts, photon radiation characteristic parameters specification, followed by STARDOOR laboratory secondary standard ionization chamber description with its main characteristics, the ambient dose equivalent H(d) determination methods, where d is the depth under a specific point on the body or in Ω direction inside the ICRU Sphere. Using TN ionization chamber, the ambient dose equivalent in one of STARDOOR laboratory chamber having a common wall with one wall of the irradiation hall were INFLPR 7 MeV linear accelerator is placed, is determined. The work concludes with the results and conclusions. AMBINT AND DIRCTIONAL DOS QUIVALNT To monitor the environment and area, two concepts which connect the external radiation field, to the effective dose and equivalent dose in skin are introduced. The first one, the 297

2 ambient dose equivalent, H(d), is adequate to the powerful penetration radiation, and the second, the directional dose equivalent H (d,ω), is proper for soft penetration radiation. Ambient dose equivalent, H(d), at a point in a radiation field is the dose equivalent that would be produced by the corresponding expanded and aligned field in the ICRU Sphere at a depth d on the radius opposing the direction of the aligned field. In SI, H(d) is in J/kg, and it has been given the special name Sievert (symbol Sv). In CGS, its unit is the rem (1 Sievert = 100 rem) [ICRU 85]. xpanded Radiation Field is a theoretical construct by which the influence and its angular and energy distribution have the same values throughout the volume of interest as in the actual field at the point of reference. In an aligned and expanded field the fluence and its energy distribution are the same as in the expended field but the fluence is unidirectional [ICRU85]. The directional dose equivalent, H (d, Ω,) at a point in a radiation field is the dose equivalent that would be produced by the corresponding expanded field in the ICRU Sphere at depth d on a radius in a specified direction Ω. The SI unit is J/kg or sievert [ICRU 85]. The CGS unit is the rem. 1 Sv =100 rem. Reference radiation Point of test Reference axis Fig. 1 - General view of radiation geometry for in phantom mesurements: cross section through ICRU Sphere with 300 mm diameter. Fig. 1 presents ICRU Sphere used for ambient equivalent dose measurements. ICRU Sphere is a tissue equivalent Sphere prescribed by the ICRU to have a diameter of 30 cm, a composition by mass of 76.2 % O, 10.1 % H, 11.1% C, 2.6 % N and a density of 1 g/cm 3. PHOTON RADIATION PARAMTRS In accordance with international standards [ISO ], photon radiation is characterized by following parameters: a) mean photon beam, Ē; b) spectral resolution, R ; c) half-value layer, HVL; d) homogeneity coefficient, h. Mean energy photon, Ē, expressed in kev is defined by the formula (1): max Φ d 0 =, (1) max Φ d 0 where Φ is the derivative of the fluency Φ of the primary photons of energy with respect to energies between and +d, defined as (2): 298

3 dφ Φ =. d (2) Table 1 presents some mean energies, corresponding to radiations characterized by certain parameters shows in ISO/FDIS Table 1 Radiation quality according to ISO/FDIS Ē ph (kev) according to ISO/FDIS N-10 8 N N N S-Cs 662 S-Co 1250 Spectral resolution, R, is expressed in percent and is defined by the ratio (3): Δ R = 100, (3) were Δ, is the spectrum width corresponding to half the maximum ordinate of the spectrum. Half value layer, HVL, represents the thinness of the specified material which attenuates the radiation beam to an extent such that the air kerma rate is reduced to half of its original value. Table 2 Homogeneity Name of series Resolution, R coefficient, h Low air-kerma rate 18 to Narrow spectrum 27 to to 1.0 Wide spectrum 48 to to 0.98 High air-kerma rate Not specified 0.64 to 0.86 Fig. 2 shows the theoretic spectral series for soft air kerma rate, corresponding to energies between 10 kev and 240 kev. Counts kev Counts kev , kev Fig. 2 - Theoretic spectra for low air-kerma series [4]. 299

4 Kerma, K = d tr /dm, where d tr is the sum of the initial kinetic energies of all the changed ionizing particles liberated by uncharged ionizing particle in a material of mass dm. In SI, K is in J/kg or Gy. Homogeneity coefficient, h, is given by the ratio of the first half value layer to the second half value layer (4). st 1 HVL h =, (4) nd 2 HVL Table 2 renders the spectral resolution and homogeneity coefficient corresponding to photon radiation versus their spectral series. SCONDARY STANDARD CHAMBR For ambient dose equivalent determination, a standard ionization chamber H(10) TN type, was used. The H(10) chamber characteristics having the calibration factor N H = 2,863x10 4 Sv/C (1), are the followings: - sensible volume: 1l; - the reference point: in the chamber center; nominal answer - 30 μc/sv; - chamber voltage: 400 V; - answer in energy: ± 4 %; - leakage current: ± 10 fa; - central electrode diameter: 50 mm; - exterior diameter of the chamber mm and radiation energy that can be measured is in 25 kev 1.3 MeV domain. The environment conditions in which must be done the measurements with this chamber are: - temperature: ( ) ºC, - humidity: ( ) % and air pressure: ( ) hpa. Fig. 3 - General view of secondary standard ionization chamber used for ambient dose equivalent H (d) and directional dose equivalent, H (d). The homogeneity of the answer for all chamber orientations with Makrolon surface inside the 4π solid angle was investigated by Aukerten for soft photon energies (N-15 and N-20). The leakage current is A. For the ambient dose equivalent with 1 msv/h value, the leakage current was of 1 x A. Fig. 3 presents the H(10) ionization chamber. The answer of H(10) chamber shows a minimal dependence for domain kev and it is in accordance with ISO specifications. The answer is varying under 1% for variations of the photons with energies of ± 1keV. The Spherical TN32002 ionization chamber was connected to a UNIDOS secondary standard dosimeter delivered by PTW, accompanied by its Test Certificate. 300

5 MASURMNT MTHOD Conventional true value (2) for ambient dose equivalent for R radiation quality is obtained with the relation H(10) = N 60 k(r) Q, (5) where: - N 60 - is the calibration factor at the reference radiation quality N-60; - k(r)- is the correction factor at the reference radiation quality N 60, given by expression: K a N 60 = k K (10, N 60), (6) Q where: - K a - the conventionally true value of the air kerma; - k R (10, N-60) - is the conversion coefficient from K a to H(10) for the quality N-60. For the determination of the calibration factor N 60 and the correction factors k(r), the measurements were carried out for energies ranging kev using the radiation qualities of the ISO, narrow-spectrum series (N 10 N 300 ) and the ISO gamma qualities S-Cs and S-Co. The chamber H(10) answer shows the energetic dependence for all domain kev. The answer r(r) of the chamber for R radiation quality is given by (7): Q Q r( R) = =, (7) H (10) K a hk (10; R) were Q is the measured charge made by the chamber H(10) and represents the conventional true value for ambient dose equivalent H(10), K a is the conventional true value for in air kerma free in air and h K (10; R) is the conversion coefficient from Ka to H(10) for R radiation quality. Conversion coefficient values h K (10; R) for X ray quality was taken from catalogue with characteristics of photon radiation spectra (3). The answer values for each quality of R 3 3 radiation r(r), were corrected by reference conditions (air density ρ0 = g cm, air pressure po= kpa, air temperature T o = K, air relative humidity r o = 0.65). Fig. 4 shows the energy dependence on the relative answer r rel (R) given by (8): r( R) r rel ( R) =, (8) r( N 60) were r(r) is the answer for R quality radiation and r(n-60) is the answer for reference radiation quality N-60 ( = 47.9 kev) which is presented. ph Relative answer r rel (R) for ionization chamber H(10) indicates a variation to 10 % versus photon mean energy from 20 kev si 1250 kev domain for the narrow spectrum N-10 to N-300 and the reference field for S-Cs and S-Co. The big difficulty for a ionization chamber realization is represented by the exact production of the same wall densities in each point of the Sphere and particularly of an thin level (width ~ 44 μm) made of an mixture of different metals on the Sphere ring surface, needed for a large answer for 40 kev energies. Small differences are in the width resulted from different answers, which differ more and more with the decrease of photon energy. The answer homogeneity for all spherical chamber orientations to a 4π solid angle was investigated using all the qualities from N-15 to N-120. Answer displacement except for the domain of stem chamber and open chamber is maxim ~ 2.6 %. Inside the 40 and 1250 kev 301

6 energy domain, (for example for energies for which the chamber H(10) was perfected) the ratio between the maximum and the minimum of the answer is For energy qualities from N-15 (Ē ph =12.7 kev) to N-40 (Ē ph =33.3 kev) this ratio is 2.9. Only the first value is complying with the standard ISO (4) requirements, concerning the energy dependence of secondary standard chamber. 10 r rel (R) kev 1000 Fig. 4 - The relative answer r rel (R) for ionization chamber H(10), Ē is the mean energy for photon beam [2]. Previously taking about the 30 kev mean energy, the maximum - minimum ratio for the instrument answer shall not be grater more than 1.1 over the energy domain for which standard instrument is used. For mean energies between 8 kev and 30 kev, the limit for this ratio shall not be more than 1.2. Any time accessible, the reference radiations used for secondary standard instrument calibration will be the same, like the ones used for the calibrations of radiation protection instruments. RSULTS The room in which the experiments were made, has a common wall with the hall in which the linear electron accelerator is placed and it includes a little room with lead walls in which radioactive checking devices of 20 MBq, respectively 33.3 MBq are stored. Main parameters of 7 MeV linear accelerator of are the followings: Maximum energy of accelerated electros, = hν, 7 MeV Mean beam current, I, 10 μa lectron macro-pulse duration, 2.5 μs lectron fluency at 1Gy dose, 2, e/cm 2 s Macro-pulse frequency, Hz Photon dose rate, 2 Gy/ min lectron dose rate, 100 Gy/min 302

7 Calibration certificate for secondary standard chamber, type TN H(10), specifies the following parameters: calibration factor N H = 2,863x10 4 Sv/C, obtained at a Co source with equivalent dose rate H (10) = (0,5 1000) msv/h and dose (1 50) msv, at DSC = 300 cm, isodose diameter of 97% for 28 cm, incidence angle α = 0 0 and correction factor k Q = Further Table 3 shows the results obtained after the measurements conducted for the absorbed dose equivalent determination, with the accelerator off and on generating an electron radiation with 350 R/min dose rate. The Spherical ionization chamber was oriented to the wall of the hall in the accelerator is placed, at 40 cm distance. Table 3 Measure mode Accelerator on Accelerator off Ambient dose equivalents H(10) [nsv/min] Mean ambient dose equivalent H(10) [nsv/min] / [μsv/h] / / 0.66 From Table 3 one can see that when the accelerator is on, the ambient dose equivalent is with 1.83 nsv/min and 0.11 µsv/h respectively, higher than when the accelerator is off. The preliminary measurements for ambient dose equivalent determination were made with radioactive checking devices for Sr-90 with activity 20 MBq. Table 4 Measure mode Ambient dose equivalent H(10) Without source 0.7 [μsv/h] Open container 0.7 [μsv/h] ; SSD = 100 cm Open container 1.7 [μsv/h] ; SSD = 10 cm Open container 3.2 [μsv/h] ; SSD = 0 cm Four measurements were conducted, in which the radiation Sr-90 source was placed at different distances to the Spherical ionization chamber: first measurement was made without the source followed by other three more with the ionization chamber successively placed at 100, 10 and 0 cm distance the source (Table 4). Throughout the process of obtaining the results, the accelerator was off. CONCLUSIONS Ambient dose equivalent, H(d), in one point in the radiation field, is the dose equivalent that could be produced by the aligned and expanded corresponding field, in ICRU Sphere [5], at an d depth, on a opposite ray with the alienated field. The measurements of ambient dose equivalent made with spherical ionization chamber PTW TN32002 with calibration factor N H = 2,863 x 10 4 Sv/C, lead to the following results: 1) in the room which has a common wall with the hall were the electron accelerator is placed, the ambient dose equivalent was by 1.2 times smaller than 1μSv/h value imposed by radiological standards and 2) near the radioactive checking device container, the dose equivalent was equal to 1 μsv/h at 10 cm distance to the source, value that meets the previsions in the radiological standards. 303

8 RFRNCS (1) Calibration Certificate, no for Ionization Chamber type TN , PTW- Freiburg, Physikalisch-Technische Werkstatten, Germany (2008). (2) Ankerhold, U.; Optimisation of a secondary standard chamber for the measurement of the ambient dose equivalent, H(10), for low photon energies, Radiat. Prot. Dosim. 118 (1), (2006). (3) International Standard ISO , X and Gamma Reference Radiations for Calibrating Dosemeters and Dose rate Meters and for Determining their Response as a Function of Photon nergy Part 3: Calibration of Area and Personal Dosemeters and the Measurement of their Response as a Function of nergy and Angle of Incidence (1997). (4) International Standard ISO , X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy - Part 2: Dosimetry for radiation protection over the energy ranges from 8 kev to 1,3 MeV and 4 MeV to 9 MeV (1997). 304