Rhodium self-powered neutron detector as a suitable on-line thermal neutron flux monitor in BNCT treatments

• Published in: Medical physics (Lancaster) 2011-12, Vol.38 (12), p.6502-6512
• Main Authors: Miller, Marcelo E., Sztejnberg, Manuel L., González, Sara J., Thorp, Silvia I., Longhino, Juan M., Estryk, Guillermo
• Format: Article
• Language: English
• Subjects: 07 ISOTOPES AND RADIATION SOURCES; BNCT; BOLTZMANN STATISTICS; Boron; Boron Neutron Capture Therapy - instrumentation; CALIBRATION; CLINICAL TRIALS; CYLINDRICAL CONFIGURATION; Electric measurements; EPITHERMAL NEUTRONS; Equipment Design; Equipment Failure Analysis; Gold; Maxwell equations; Medical treatment planning; NEUTRON CAPTURE THERAPY; NEUTRON DETECTION; NEUTRON FLUX; Neutron radiation effects; NEUTRON SOURCES; NEUTRON SPECTRA; Neutrons; on-line flux measurement; PATIENTS; PLANNING; Position sensitive detectors; QUALITY CONTROL; RA-3 REACTOR; RA-6 REACTOR; Radiation Dosage; RADIOLOGY AND NUCLEAR MEDICINE; Radiometry - instrumentation; Reproducibility of Results; RHODIUM; Rhodium - radiation effects; SELF-POWERED NEUTRON DETECTORS; Sensitivity and Specificity; SKIN; SPND; Standards and calibration; Therapeutic applications, including brachytherapy; THERMAL NEUTRONS
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.3660204

Purpose: A rhodiumself-powered neutron detector (Rh SPND) has been specifically developed by the Comisión Nacional de Energía Atómica (CNEA) of Argentina to measure locally and in real time thermal neutron fluxes in patients treated with boron neutron capture therapy (BNCT). In this work, the thermal and epithermal neutron response of the Rh SPND was evaluated by studying the detector response to two different reactor spectra. In addition, during clinical trials of the BNCT Project of the CNEA, on-line neutron flux measurements using the specially designed detector were assessed. Methods: The first calibration of the detector was done with the well-thermalized neutron spectrum of the CNEA RA-3 reactor thermal column. For this purpose, the reactor spectrum was approximated by a Maxwell–Boltzmann distribution in the thermal energy range. The second calibration was done at different positions along the central axis of a water-filled cylindrical phantom, placed in the mixed thermal–epithermal neutron beam of CNEA RA-6 reactor. In this latter case, the RA-6 neutron spectrum had been well characterized by both calculation and measurement, and it presented some marked differences with the ideal spectrum considered for SPND calibrations at RA-3. In addition, the RA-6 neutron spectrum varied with depth in the water phantom and thus the percentage of the epithermal contribution to the total neutron flux changed at each measurement location. Local (one point-position) and global (several points-positions) and thermal and mixed-field thermal neutron sensitivities were determined from these measurements. Thermal neutron flux was also measured during BNCT clinical trials within the irradiation fields incident on the patients. In order to achieve this, the detector was placed on patient’s skin at dosimetric reference points for each one of the fields. System stability was adequate for this kind of measurement. Results: Local mixed-field thermal neutron sensitivities and global thermal and mixed-field thermal neutron sensitivities derived from measurements performed at the RA-6 were compared and no significant differences were found. Global RA-6-based thermal neutron sensitivity showed agreement with pure thermal neutron sensitivity measurements performed in the RA-3 spectrum. Additionally, the detector response proved nearly unchanged by differences in neutron spectra from real (RA-6 BNCT beam) and ideal (considered for calibration calculations at RA-3) neutron source de