Measurement of the neutron fields produced by a 62 MeV proton beam on a PMMA phantom using extended range Bonner sphere spectrometers K. Amgarou a , R. Bedogni b,n , C. Domingo a , A. Esposito b , A. Gentile b , G. Carinci b , S. Russo c a Grup de Recerca en Radiacions Ionitzants, Departament de Fı ´sica, Universitat Aut  onoma de Barcelona, E-08193 Bellaterra, Spain b INFN—Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati, Via E. Fermi n. 40, 00044 Frascati, Italy c INFN—Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali del Sud, via S. Sofia 44, 95123 Catania, Italy article info Article history: Received 18 June 2011 Received in revised form 14 July 2011 Accepted 16 July 2011 Available online 26 July 2011 Keywords: Proton therapy beam Secondary radiation Neutron spectrometry Bonner spheres abstract The experimental characterization of the neutron fields produced as parasitic effect in medical accelerators is assuming an increased importance for either the patient protection or the facility design aspects. Medical accelerators are diverse in terms of particle type (electrons or hadrons) and energy, but the radiation fields around them have in common (provided that a given threshold energy is reached) the presence of neutrons with energy span over several orders of magnitude. Due to the large variability of neutron energy, field or dosimetry measurements in these workplaces are very complex, and in general, cannot be performed with ready-to-use commercial instruments. In spite of its poor energy resolution, the Bonner Sphere Spectrometer (BSS) is the only instrument able to simultaneously determine all spectral components in such workplaces. The energy range of this instrument is limited to E o20 MeV if only polyethylene spheres are used, but can be extended to hundreds of MeV by including metal-loaded spheres (extended range BSS, indicated with ERBSS). With the aim of providing useful data to the scientific community involved in neutron measure- ments at hadron therapy facilities, an ERBSS experiment was carried out at the Centro di AdroTerapia e Applicazioni Nucleari Avanzate (CATANA) of INFN—LNS (Laboratori Nazionali del Sud), where a proton beam routinely used for ophthalmic cancer treatments is available. The 62 MeV beam was directed towards a PMMA phantom, simulating the patient, and two neutron measurement points were established at 01 and 901 with respect to the beam-line. Here the ERBSS of UAB (Universidad Auto ´ noma de Barcelona—Grup de Fı ´sica de les Radiacions) and INFN (Istituto Nazionale di Fisica Nucleare—Laboratori Nazionali di Frascati) were exposed to characterize the ‘‘forward’’ and ‘‘sideward’’ proton-induced neutron fields. The use of two ERBSS characterized by different set of spheres, central detectors, and independently established and calibrated, is important for guaranteeing the robustness of the measured spectra and estimating their overall uncertainties. & 2011 Elsevier B.V. All rights reserved. 1. Introduction Parasitic neutron fields are of practical importance for both electron (for energy higher than 10 MeV) and hadron medical accelerators. From the point of view of the occupation radiation protection, the neutron component must be carefully considered when designing lateral shields, ducts, doors, and labyrinths. In addition, dosemeters with adequate neutron response must be adopted for the surveillance of areas and personnel. From the point of view of the patient protection, the exposure to neutrons is a major concern, because the in-room neutron field produces a whole-body exposure of the patient, while the clinical beam selectively irradiates the treatment volume. As a consequence, the risk of long-term secondary cancer due to neutrons may be higher than that associated to the clinical beam and its scattered components [1,2]. Experimental studies evidenced that values of ambient dose equivalent ranging from about 0.1 to 1 mSv per prescribed Gy at the isocenter may be found in treatment rooms of medical electron LINACs, mainly depending on the field size and electron energy [3]. Larger values, up to about 20 mSv/Gy, can be observed in passively scattered proton-therapy units using large treatment fields [4]. For proton energies relevant to the medical field, i.e. below 250 MeV, the main neutron producing mechanisms are the (p,n) reactions in the accelerator materials and in the patient. The carbon is responsible for most of the neutron emission in tissue-like materials. The (p,n) reaction in carbon is characterized by a threshold (approximately. 13 MeV) and the neutron yield approximately increases as E p 2 (E p ¼ proton energy) in the mentioned energy range [5]. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A 0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.07.027 n Corresponding author. Tel.: þ39 0694032608; fax: þ39 0694032364. E-mail address: roberto.bedogni@lnf.infn.it (R. Bedogni). Nuclear Instruments and Methods in Physics Research A 654 (2011) 399–405