ABSOLUTE AIR-KERMA MEASUREMENT IN A SYNCHROTRON LIGHT BEAM BY IONIZATION FREE-AIR CHAMBER

Transcription

1 ABSOLUTE AIR-KERMA MEASUREMENT IN A SYNCHROTRON LIGHT BEAM BY IONIZATION FREE-AIR CHAMBER M. Bovi (1), R.F. Laitano (1), M. Pimpinella (1), M. P. Toni (1), K. Casarin(2), E. Quai(2), G. Tromba(2), A. Vascotto(2), D. Dreossi(3) (1) Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti, C R Casaccia, cp 2400 Roma, Italy. (2) Società Elettra, impianto sincrotrone, Trieste (3) Università di Trieste, Trieste 1

2 INTRODUCTION Monochromatic synchrotron light provides better resolution and sharpness of the radiological images and the advantages of using such beams for mammography have been widely assessed since the past years. One of the research programmes of the Synchrotron ELETTRA operating in Trieste, Italy, is to dedicate a beam line of the accelerator for high-definition mammography in the framework of the SYRMA project (SYnchrotron Radiation for MAmmography). Accurate air-kerma measurements in this beam are then necessary in order to optimize the patient absorbed dose with respect to the definition of the radiographic image. A collaboration between the INMRI- and Synchrotron ELETTRA was started aiming at characterizing monochromatic synchrotron light beams in terms of air kerma. 2

3 OVERVIEW SYRMA beam line Radiation detectors Method Results: absolute air kerma measurements SYRMA monitors chambers calibration secondary standard free-air chamber calibration Conclusions 3

4 THE SYRMA BEAM LINE SCHEMATIC DIAGRAM OF THE LIGHT BEAM bending magnet of ELETTRA polychromatic laminar beam Light source Safety and control elements vacuum slits Si(111) double crystal monochromator Vacuum slits OPTICS HUTCH beam preparation (energy, flux, geometry) Monochromator Beam stopper air slits monochromatic laminar beam fast shutter Al filters Air Slits SETUP SYRMEP Fast shutters Slits, dose monitors EXPERIMENTAL HUTCH beam monitoring (dose monitors, exposure times, safety system) ENERGY SELECTION FLUX AND GEOMETRY SELECTION CCD beam monitors detector reference point low energy free-air chamber PATIENT ROOM Patient exposure X-RAY BEAM Schematic layout of the ELETTRA medical beam line for high definition mammography in the framework of the SYRMA project (SYnchrotron Radiation for MAmmography) at the Synchrotron ELETTRA in Trieste, Italy. 4

5 photons flux / mm -2 s -1 Absolute air-kerma measurement in a synchrotron light beam by ionization free-air chamber THE SYRMA BEAM LINE BEAM PHOTON FLUX AND CHARACTERISTICS GeV ; 300 ma 2.4 GeV ; 180 ma E / kev Photon flux values - as function of the photons energy - at the reference point for patient exposure, when synchrotron Elettra is operated at 2 GeV and 2.4 GeV with stored the maximum electron beam currents of 300 ma and 180 ma, respectively. Light source CHARACTERISTIC Relevant characteristics of the light source and x-ray beams for high definition mammography in the framework of the SYRMA project at the Synchrotron ELETTRA in Trieste, Italy. 5 VALUE Type Bending magnet Critical energy (at 2.0 GeV) 3.21 kev Critical energy (at 2.4 GeV) 5.59 kev Source size 100 µm x 1100 µm Horizontal angular acceptance (beam divergence) 7 mrad X-ray beam at reference point Energy range 8 kev - 35 kev Energy range for patient exposure 16 kev 25 kev Energy Resolution ΔE/E = 2*10-3 Typical photons fluxes at 15 kev 2 x 10 8 / mm -2 s -1 (at 2 GeV, 300 ma) 7 x 10 8 phot./mm -2 s -1 (at 2.4 GeV, 180 ma) Beam Size 120 mm x 4 mm Source-to-sample distance about 30 m (26.5 m in vacuum)

6 THE SYRMA BEAM LINE REFERENCE MEASUREMENT POINT (a) Speed controlled movable support (a) for patient exposure to obtain high definition mammography by scanning the patient through the beam itself at the Synchrotron ELETTRA. Detail in figure 4b shows the position of the mild compression system used for stretching and equalizing the breast tissues thickness. (b) The standard chambers are placed in this position for air kerma measurements, after removing the mild compression system. 6

7 METHOD Because of the specific irradiation conditions in the synchrotron light beam (energy, intensity and shape), it was decided to perform absolute air kerma measurements directly in the SYRMA beam by the standard lowenergy-free-air chamber (-FAC) of INMRI-. To this end, the diaphragm of the standard chamber -FAC (4 mm in radius) was replaced by a smaller one with a radius of 1.5 mm. A re-determination of relevant correction factors was made Two SYRMA monitor chambers - in fixed positions on the beam were then calibrated by absolute air-kerma rate measurements in the energy range from 8 kev to 30 kev. To assure the periodical calibration of these beam monitors the synchrotron users were equipped by a non-standard free-air chamber (-SEC) made at INMRI- and acting as a secondary standard. The chamber -SEC was calibrated in terms of air kerma in the SYRMA beam against the standard chamber -FAC (It will be periodically calibrated at the INMRI- in the low-energy filtered x- ray beams to check for stability). 7

8 BEAM STABILITY AND UNIFORMITY Before and after each series of calibration measurements: the fine angular alignments of the monochromator crystals was controlled as the energy of the monochromatic beam depend on monochromator position. the spatial uniformity and stability of the beam was accurately verified by measurements made by the standard chamber and by a digital detector based on a silicon charge coupled detector (CCD) that presents to the incident photons a grid of 2048x2024 pixels (each with typical dimensions of 14 µm x 14 µm). the homogeneous irradiation of the chamber diaphragm was verified by the CCD that provided beam imaging after the chambers. 3 mm 3.7 mm 191 mm 8

9 STANDARD IONIZATION CHAMBERS The standard free-air chamber for low energy x-rays (-FAC) and the secondary standard free-air chamber (-SEC) at the reference measurement point (inside the red circle) under the patient support, at the Synchrotron ELETTRA in Trieste, Italy. The two chambers use the same diaphragm. 9

10 BEAM MONITOR IONIZATION CHAMBERS Designed by ELETTRA for the laminar beam geometry with an entrance window larger than the largest beam size. The electrodes and windows are perpendicular to the beam and are realized in Mylar foils - 50 mm thick - covered on each side by a 50 nm layer of Al. HV ELECTRODES HV LIGHT BEAM The sensitive volume is constituted by two regions delimited by the two HV electrodes and separated by the central collecting electrode. The chambers are inside an Al box, shielded against RF. 10

11 EFFECTIVENESS OF THE MONITORING Photons fluence in the synchrotron light beam was never constant because it varies according to the electron beam currents stored in the ring of the synchrotron. Possibility to compare measurements carried out in different period is based on the confidence in the beam monitoring system. I IOC1 /A I IOC2 /A I -SEC /A IOC1/-SEC IOC2/-SEC MEDIA E E E+14 STD E E E+11 STD% 1.27% 1.28% 1.29% 0.03% 0.02% MaxVar% 5.02% 5.04% 5.06% 0.18% 0.17% 11

12 CORRECTION FACTORS K SC, K FL AND K E Determined by the Monte Carlo code PENELOPE for the standard chamber - FAC - with the 1.5 mm in radius chamber aperture - in monochromatic photon beams at the energies in the range from 8 kev to 30 kev. Relative standard uncertainty estimated for the k sc k fl k e product: within 0.1%. No significant difference in the k sc k fl k e values obtained for the smaller 1.5 mm diaphragm and the usual 4.0 mm diaphragm.

13 MASS AIR ATTENUATION COEFFICIENTS, μ/ρ μ/ρ values measured in synchrotron monochromatic beams (triangles) and ISO 4037 filtered x-radiation (circles), in the energy range from 8 kev to 30 kev. Standard uncertainties were estimated to be within 0.5 %. The difference from the experimental values obtained for filtered x radiation having a mean energy of the spectra E and those obtained for monochromatic photons having the same energy E, resulted in the range from 0.7 % to 1 % being values referring to filtered x radiation lower. The values given by Hubbell 95 (squares) are also reported for comparison and differ of about 0.9 % from those measured, being those measured lower. Such deviations would influence the corresponding correction factors by no more than 0.1 %.

14 ION RECOMBINATION CORRECTION I(1600 V)/I(400 V Ksat determination -FAC ksat < E+0 2.0E E E E E E E E E E-10 0 I(1600 V) /A I(2000 V)/I(250 V Ksat determination -SSC ksat < E-12 1E E-11 2E E-11 3E E-11 4E E-11 I(2000 V) /A I(400 V )/ I(100 V Ksat determination SYRMA-IOC ksat < E+00 1.E+05 2.E+05 3.E+05 4.E+05 5.E+05 6.E+05 7.E+05 I(400 V) /A The effect of ion recombination in the chamber exposed to the synchrotron light beam (with a kerma rate of up to 7 mgy s -1, was corrected for by the method described by Boutillon Standard uncertainty within 0.05%.

15 CALIBRATION RESULTS FOR SYRMA MONITOR Calibration coefficients of the SYRMA monitor chambers IOC 1(blue square) and IOC2 (red triangle) at reference ambient conditions (temperature of K, pressure of Pa and 50% of relative humidity). The associated standard uncertainty is 0.75%. Due to the SYRMA beam energy resolution ΔE/E and the chamber energy dependance, the possible variation of the calibration coefficient is 0.6% at each energy, in the energy range useful for patient exposure (from 16 kev to 25 kev).

16 CALIBRATION COEFFICIENTS FOR THE - SEC CALIBRATED FREE-AIR CHAMBER The associated standard uncertainty is within 0.80%. The difference between the determinations by monitor IOC1 (blue square) or IOC2 (red triangle). is within 0.15%. Due to the SYRMA beam energy resolution DE/E and the chamber energy dependance, the possible variation of the calibration coefficient is 0.03% at each energy, in the energy range useful for patient exposure (from 16 kev to 25 kev).

17 CONCLUSION Absolute air-kerma measurements by the standard free-air chamber for low energy x-rays (-FAC) were performed in the synchrotron light beam used for high definition mammography in the framework of the SYRMA project (SYnchrotron Radiation for MAmmography) at the Synchrotron ELETTRA in Trieste. These measurements were necessary to perform directly in the synchrotron light beam the calibration of the two beam monitors. The -SEC chamber was calibrated during the present measurements directly against the -FAC as a secondary standard to assure the periodical calibration of the synchrotron beam monitors. A procedure to verify the -SEC stability by measurements in lowenergy filtered x-ray beams, was established.