Extended Range Bonner Sphere Spectrometer Research Articles

Design and development of the extended-range Bonner sphere spectrometer at Ciemat

The Neutron Standards Laboratory (LPN) at CIEMAT serves as the national reference for calibrating neutron measurement devices and conducts research on measuring different neutrons fields, arising the necessity of neutron spectrometry. There are different neutron fields which are attracting interest in the recent years, for example ambient neutron spectra, generated basically by cosmic rays in the atmosphere or secondary neutrons on the surroundings of accelerators, for instance in protontherapy centres. These neutrons spectra are characterized by having an important component of high energy neutrons up to hundreds MeV or even GeV. In order to do spectrometry in this kind of neutron fields, a Bonner Sphere Spectrometer extended range was designed, constructed and tested.

Cosmic neutrons at ground: new spectral measurements at 3480 m a.s.l. and benchmarking of the cascade component as a function of the elevation at around 45° geomagnetic latitude

Within the INFN-based project SAMADHA, environment neutron spectrum measurements were performed at 3480 m a.s.l. on the Italian western Alps. An extended range Bonner Sphere Spectrometer was used, that was especially designed to operate unattended and to discriminate genuine neutron events from spurious ones. In addition to the measurements of neutron fluence rate, its energy distribution and the related ambient dose equivalent, as well as the arrival time distribution of the events were also studied. The high-energy component of the neutron fluence rate was compared to literature data measured with the same technique at similar latitudes as a function of the elevation, in turn being fitted with an exponential curve and compared to the results from EXPACS code. As the cascade component is not influenced by the materials surrounding the measurement site, this curve can be regarded as “universal” at the studied latitudes.

DEVELOPMENT OF AN EXTENDED-RANGE BONNER SPHERE SPECTROMETER FOR CHINA INITIATIVE ACCELERATOR-DRIVEN SYSTEM.

To measure the extended-range neutron spectra and calibrate the extended-range neutron dosemeters of the China initiative Accelerator-Driven System (CiADS), an Extended-range Bonner Sphere Spectrometer (EBSS) has been developed. The EBSS was designed based on the PHITS codes, investigating various combinations of materials and diameters of the neutron moderators and the neutron multipliers for extended-range neutrons. Finally, seven polyethylene-only spheres and seven extended-range spheres were selected and subsequently built. The neutron multipliers of the extended-range spheres embedded concentric shells of lead, copper and tungsten. The response functions of the EBSS were analyzed and experimentally validated. It was subsequently tested with 252Cf neutron source and cosmic ray neutron source. The results demonstrate that the EBSS is capable of accurately measuring neutron spectra.

Extended range Bonner sphere spectrometer for high-elevation neutron measurements

Extended range Bonner sphere spectrometer for high-elevation neutron measurements

A Study of Stray Neutron Field Measurements for the Neutron Scattering Instruments at CSNS

Stray neutrons might cause several negative impacts. However, it is usually difficult to conduct precise stray neutron simulations using the Monte Carlo method. Therefore, in this study, a measurement technique was proposed to study the stray neutrons experimentally inside the neutron scattering instruments at China Spallation Neutron Source (CSNS). The adopted measurement instruments comprise an extended-range Bonner sphere spectrometer and a commercial neutron ambient-dose-equivalent dosimeter, which enables us to directly measure the neutron spectra and ambient-dose equivalent H*(10) values. Verification experiments were performed inside the BL06 beam line experimental area at CSNS at two exposed locations with different sample conditions. Comparison of the experimentally measured neutron spectra, integral neutron fluence, and H*(10) value with the simulations demonstrated the feasibility of using the proposed method for studying stray neutrons for the neutron instruments.

Assessment of neutrons from secondary cosmic rays at mountain altitudes – Geant4 simulations of environmental parameters including soil moisture and snow cover

Abstract. Ground-based measurements of neutrons from secondary cosmic rays are affected by environmental parameters, particularly hydrogen content in soil. To investigate the impact of these parameters, in particular snow cover, Geant4 Monte Carlo simulations were carried out. In a previous study the model used for the Geant4 Monte Carlo simulations was already validated by measurements performed with an extended-range Bonner sphere spectrometer (ERBSS) at Zugspitze, Germany, and at Jungfraujoch, Switzerland. In the present study a sensitivity analysis including different environmental parameters (i.e. slope of mountain, snow height, and soil moisture) and their influence on the flux of neutrons from secondary cosmic rays was performed with Geant4. The results are compared with ERBSS measurements performed in 2018 at the Environmental Research Station Schneefernerhaus located at the Zugspitze, Germany. It is shown that the slope of the Zugspitze mountain reduces the neutron flux from secondary cosmic rays between about 25 % and 50 % as compared to a horizontal surface, depending on neutron energy and snow cover. An increasing height of snow cover, simulated as snow water equivalent (SWE), reduces the total neutron flux exponentially down to a factor of about 2.5 as compared to soil without any snow cover, with a saturation for snow heights greater than 10 to 15 cm SWE, depending on neutron energy. Based on count rates measured with the individual spheres of the ERBSS, SWE values were deduced for the whole year 2018. Specifically, mean SWE values deduced for the winter months (January to March) are between 6.7 and 10.1 cm or more, while those for the summer months (July to September) are between 2.1 and 3.6 cm. Soil moisture of 5 % water mass fraction in limestone leads to a decrease of the total neutron flux by about 35 % compared to dry limestone. It is concluded that the measurement of neutrons from secondary cosmic radiation can be used to gain information on the height of snow cover and its seasonal changes, on soil moisture, and on local geometry such as mountain topography. Because the influence of such parameters on neutron flux from secondary cosmic rays depends on neutron energy, analysis of the whole neutron energy spectrum is beneficial.

Assessment of secondary neutrons from galactic cosmic rays at mountain altitudes – Geant4 simulations and ground-based measurements of neutron energy spectra

In this study, the Geant4 Monte Carlo toolkit was used to simulate energy spectra of neutrons from secondary cosmic radiation at mountain altitudes for the Environmental Research Station “Schneefernerhaus” at the Zugspitze mountain, Germany (2660 m a.s.l.) and for Sphinx astronomical observatory at the Jungfraujoch, Switzerland (3585 m a.s.l.). Simulations were performed with different intra-nuclear cascade models available in Geant4, and the results were compared with those of measurements that had been performed at both locations by means of an Extended-Range Bonner Sphere Spectrometer. Measurement conditions were quite different for both locations – at Schneefernerhaus the measurements had been performed on the flank of a hill in March 2018 with much snow, while at Jungfraujoch the measurements had been performed on top of a steep local hill in September 2018 with much less snow. Despite these differences, agreement between measurement and simulation was reasonable at both locations, especially at neutron energies greater than 20 MeV where the (unknown) hydrogen content of the environment did not influence the neutron fluence much (i.e., results from simulations were 6–22% lower than those from the measurements for the Schneefernerhaus, and were 22–29% lower for Jungfraujoch, depending on intra-nuclear model used in the simulations). The agreement was less favorable for lower energies, where environmental hydrogen (e.g., snow cover, soil moisture) is known to influence the shape of the neutron energy spectrum, because the real conditions of the snow accumulation close to the location of the measurements were not known and, therefore, a detailed description of the real hydrogen environment in the simulations was not possible. When the results simulated using different intra-nuclear cascade models were compared with each other, agreement was found within ±5%, ±15%, ±20%, and ±20%, for cascade, evaporation, epithermal and thermal neutrons, respectively. While the latter results are consistent with those of simulations and measurements at the CERN EU High-Energy Reference Field (CERF) facility published recently, a detailed sensitivity analysis of the influence of environmental hydrogen on neutron energy spectra is required before a final quantitative comparison of measurements and simulations can be made. This sensitivity analysis is presently under way. It is concluded that simulation of energy spectra of neutrons from secondary cosmic rays close to the atmosphere-lithosphere interface, validated by the spectrometer measurements, showed differences of less than 30%, for neutron energies greater than 20 MeV, whatever intra-nuclear cascade model was used in the simulations.

Analysis by Monte Carlo methods of the response of an extended-range Bonner Sphere Spectrometer

High-energy neutrons up to 230 MeV are generated as a consequence of (p,n) nuclear reactions in proton therapy facilities. The aim of this work is to evaluate the potential extension of the UPM Bonner Sphere Spectrometer (BSS) through the use of spallation materials, for its future application to determine neutron fluence spectra in such facilities. Monte Carlo methods have been used to model the response of the actual and modified spheres with the introduction of a spallation material layer, lead or copper, analyzing their response functions. An analysis is also made of the neutron fluence spectra over the 6LiI(Eu) scintillator volume and the uncertainty that can be associated to the response functions at high energies as a consequence of the different physics models that can be applied for their analysis.

Monte Carlo simulation of the CERN-EU High Energy Reference Field (CERF) facility

In the present paper Geant4 Monte Carlo simulations of the CERN-EU high-energy Reference Field (CERF) facility in Geneva, Switzerland, are presented. At this facility a neutron field with a broad energy distribution from thermal energies up to GeV is available. To validate the simulated neutron fluences, the CERF neutron energy distribution was also measured with an extended range Bonner Sphere Spectrometer. Both measurements and simulations are compared against the official CERF reference distributions that were calculated with the Monte Carlo code FLUKA. It turned out that the differences between the total neutron fluences simulated with Geant4 and those measured were less than 41%, depending on the measurement position. While the total fluences simulated with the Geant4 Binary INC model (QGSP_BIC_HP) and the corresponding FLUKA reference values were up to 41% lower than those measured, the total fluences simulated with the Geant4 Bertini INC model (QGSP_BERT_HP) overestimated (up to 27%) the experimental results. As compared to the measured ambient dose equivalent H*(10), similar differences were observed with H*(10) values higher by about 29% for the Geant4 Bertini INC simulations and lower by about 48% for the Geant4 Binary INC and FLUKA simulations.

Influence of environmental parameters on secondary cosmic ray neutrons at high-altitude research stations at Jungfraujoch, Switzerland, and Zugspitze, Germany

Neutrons of secondary cosmic radiation (CR) are produced when primary cosmic radiation interacts with the nuclei in the Earth's atmosphere. The secondary neutron spectra in the atmosphere do not include many thermal and epithermal neutrons at energies below several eV. In contrast, close to the Earth's surface many more of those neutrons are present, due to albedo neutrons backscattered from the ground. The number of albedo neutrons is mainly determined by the environment, i.e. the material in the environment (characteristics of the underground and the environment (building, topology of landscape)). If the environment (buildings, underground) is constant with time then the variation of the albedo neutron fluence is dependent on the soil moisture and snow cover.To investigate snow cover effect in detail, in June 2016 and September 2018 two measurement campaigns were carried out at the High Altitude Research Station Jungfraujoch, Switzerland. During these campaigns the energy distributions of secondary CR neutrons were measured by means of a mobile extended range Bonner sphere spectrometer (ERBSS) at two different positions with different environmental conditions: under the cupola of the astronomical observatory in the Sphinx building at an altitude of 3585 m a.s.l., and below the shelter roof of the research station at 3466 m a.s.l.In addition, since 2004 the energy spectra of neutrons from secondary CR have been continuously measured at the Environmental Research Station (UFS Schneefernerhaus; 2650 m a.s.l.) close to the summit of the Zugspitze mountain, Germany. To measure the neutron spectra in the energy range from a few meV up to several GeV, a stationary ERBSS has been used.The chosen measurement positions allow quantification of environmental conditions which affect the neutron spectral distribution in the whole neutron energy range up to several GeV. With this spectral information, it is also possible to derive detailed information about the neutron ambient dose equivalent (H*(10)) at these altitudes and geomagnetic latitudes.The ERBSS measurements at Jungfraujoch presented here show that snowpack in the surrounding area does not affect the fluence rate of secondary neutrons significantly. However, the influence of topography of the chosen measurement locations on secondary CR neutrons was observed.

Instrument intercomparison in the high-energy field at the CERN-EU reference field (CERF) facility and comparison with the 2017 FLUKA simulations

This paper discusses an instrument intercomparison performed in the high-energy field at the CERF facility at CERN between 2015 and 2017 (October 2015, May 2016, November 2016 and June 2017). Measurements were performed in several reference exposure locations with the CERN extended-range Bonner Sphere Spectrometer (BSS), a Berthold LB4611, the LINUS rem counter from CERN, the LUPIN rem counter from ELSE Nuclear, the FHT 762 Wide Energy Neutron Detection Instrument (WENDI-II) from Thermo Scientific, the LUDLUM MODEL 42–41 PRESCILA NEUTRON PROBE and two models of Tissue-equivalent proportional counters (TEPCs) from Far West Technologies. All results are compared with the latest FLUKA reference values from simulations performed in 2017.

Electronic neutron dosimeter in high-energy neutron fields

In neutron fields including neutron energies above 20 MeV a conventional neutron dosimeter is not suitable for measurements of neutron personal dose equivalent, Hp(10), over the whole energy range. Therefore, for such fields an electronic neutron dosimeter has been developed recently at Helmholtz Zentrum München (HMGU). In general, neutron dose measurements performed with this dosimeter at neutron energies below 2 MeV show an accuracy of about 30% [1]. Here we report the use of this dosimeter at the CERN-EU high-energy Reference Field (CERF) facility in Geneva, Switzerland. At this facility the available neutron fields include neutrons with energies below, but also above 20 MeV.In the present paper, personal dose equivalent (Hp(10)) values obtained with the ELectronic neutron DOsimeter (ELDO) are compared to neutron personal dose equivalent (Hp(10)) values obtained with the HMGU extended-range Bonner Sphere Spectrometer, and to reference values from FLUKA Monte Carlo simulations provided by CERF. It is shown that for continuous neutron spectra as those at CERF behind concrete shielding or secondary neutrons from cosmic rays, the dosimeter results are satisfactory for radiation protection purposes. However, in neutron fields including neutrons above about 7 MeV, where the major neutron dose contribution is from neutrons between 10 keV and several MeV (like those at CERF behind iron shielding), the doses provided by ELDO might be too small and care must be taken in interpreting the results.

Neutron spectral fluence and dose distribution inside a NYLON 6 phantom irradiated with pencil beam of high energy protons

Monte Carlo (MC) simulations using the MCNPX™ code supported by measurements were exploited for detailed characterization of secondary neutrons generated by high energy protons during pencil beam proton therapy. The study focused on the estimation of the distribution of secondary neutron points of origin, their average initial energy, and the distribution of absorbed dose and equivalent dose from neutrons and photons inside the NYLON 6 phantom (diameter of 25.5 cm, length of 31.0 cm) irradiated with a proton beam with the energies of 100, 150 and 200 MeV. Validation of the MC model and the used methods was done by comparison of calculated responses of the extended-range Bonner Sphere Spectrometer at several positions outside the phantom with the measured ones. The results show that high energy neutrons predominate in the direction of the proton beam and more neutrons are generated by higher energy protons. The MC simulations also demonstrated that the majority of high energy neutrons is generated at the beginning of the proton trajectory in the phantom and the neutron yield and neutron initial average energy decrease with increasing depth. Therefore, attention should be paid not only to the tissues behind the irradiated volume, but also to the preceding ones. However, the neutron spectral fluence in the vicinity of the treated tissue can only be determined by calculation, mainly due to the dimensions of the neutron spectroscopic instrumentation.The calculation of the spatial distribution of the radiation weighting factor for neutrons allowed us to estimate the distribution of the neutron equivalent dose inside the phantom. The maximal value of the neutron equivalent dose within the phantom reached 42, 24, and 15 pSv per one primary 100, 150, and 200 MeV proton, respectively, and occurred on the beam axis roughly at the half of the proton range. With respect to the proton dose, these maximal values of the neutron equivalent dose correspond to 2.1%, 1.3%, and 1.4% of the proton dose in the Bragg peak for these proton energies. Moreover, equivalent doses of secondary induced photons were more than 3 orders of magnitude lower that the neutron ones.

A neutron source for IGISOL-JYFLTRAP: Design and characterisation

.A white neutron source based on the Be(p, nx) reaction for fission studies at the IGISOL-JYFLTRAP facility has been designed and tested. 30MeV protons impinge on a 5mm thick water-cooled beryllium disc. The source was designed to produce at least 1012 fast neutrons/s on a secondary fission target, in order to reach competitive production rates of fission products far from the valley of stability. The Monte Carlo codes MCNPX and FLUKA were used in the design phase to simulate the neutron energy spectra. Two experiments to characterise the neutron field were performed: the first was carried out at The Svedberg Laboratory in Uppsala (SE), using an Extended-Range Bonner Sphere Spectrometer and a liquid scintillator which used the time-of-flight (TOF) method to determine the energy of the neutrons; the second employed Thin-Film Breakdown Counters for the measurement of the TOF, and activation foils, at the IGISOL facility in Jyväskylä (FI). Design considerations and the results of the two characterisation measurements are presented, providing benchmarks for the simulations.

Systematic out-of-field secondary neutron spectrometry and dosimetry in pencil beam scanning proton therapy.

Systematic investigation of the energy and angular dependence of secondary neutron fluence energy distributions and ambient dose equivalents values (H*(10)) inside a pencil beam scanning proton therapy treatment room using a gantry. Neutron fluence energy distributions were measured with an extended-range Bonner sphere spectrometer featuring ³He proportional counters, at four positions at 0°, 45°, 90°, and 135° with respect to beam direction and at a distance of 2 m from the isocenter. The energy distribution of secondary neutrons was investigated for initial proton beam energies of 75 MeV, 140 MeV, and 200 MeV, respectively, using a 2D scanned irradiation field of 11 × 11 cm² delivered to a 30 × 30 × 30 cm³ PMMA phantom. Additional measurements were performed at a proton energy of 118 MeV including a 5 cm range-shifter (PMMA), yielding a Bragg peak position similar to that of 75 MeV protons. Ambient dose equivalent values from 0.3 μSv/Gy (75 MeV; 90°) to 24 μSv/Gy (200 MeV; 0°) were measured inside the treatment room at a distance of 2 m from the isocenter. H*(10) values were lower (by factors of up to 7.2 (at 45°)) at 75 MeV compared to those at 118 MeV with the 5 cm range-shifter. At 0° and 45°, an evaporation peak was found in the measured neutron fluence energy distributions, at neutron energies around MeV, which contributes about 50% to total H*(10) values, for all investigated proton beam energies. This study showed a pronounced increase of secondary neutron H*(10) values inside the proton treatment room with increasing proton energy without beam modifiers. For example, in beam direction this increase was about a factor of 50 when protons of 75 MeV and 200 MeV were compared. The existence of a peak of secondary neutrons in the MeV region was demonstrated in beam direction (0°). This peak is due to evaporation neutrons produced in the existing surrounding materials such as those used for the gantry. Therefore, any simulation of the secondary neutrons within a proton treatment room must take these materials into account. In addition, the results obtained here show that the use of a range-shifter increases the production of secondary neutrons inside the treatment room. Using a range-shifter, the higher neutron doses observed mainly result from the higher incident proton energy (118 MeV instead of 75 MeV when no range-shifter was used), due to higher neutron production cross-sections.

Secondary neutrons inside a proton therapy facility: MCNPX simulations compared to measurements performed with a Bonner Sphere Spectrometer and neutron H*(10) monitors

Neutron spectrometry measurements with an extended-range Bonner Sphere Spectrometer (BSS), as well as neutron H*(10) measurements using an extended-range rem meter WENDI-2, a conventional rem meter LB 6411 and a tissue-equivalent proportional counter, were performed inside and around the Fixed-Beam Treatment Room at the proton therapy facility of Essen, in Germany. The WENDI-2 stood out as the easiest detector for making accurate neutron H*(10) measurements, since its direct measurements were equivalent to the H*(10) rates obtained with the BSS. The measurements were also compared to simulation results obtained with MCNPX 2.7.0 using two different selections of physics models for the hadron interactions above 150 MeV: the Bertini & Dresner models and the CEM03 model. For neutron H*(10) rates outside the treatment room, factors of 1.6–1.8 were obtained between the results of the two simulations, the Bertini & Dresner models yielding the largest values in all positions. The comparison of the simulation results with the WENDI-2 and BSS measurements for positions inside the treatment room showed that the Bertini & Dresner models reproduce the global neutron production in the water phantom relatively well, whereas the CEM03 model underestimates it by a factor of ∼1.3. At the most-forward angle, however, the Bertini model (unlike the CEM03 model) seemed to overestimate the production of neutrons with energies above 100 MeV. Outside the shielding, the simulated H*(10) overestimated the WENDI-2 measurements by factors of 2–3 with the Bertini & Dresner models, and 1.1–1.7 with the CEM03 model. Both simulations were thus conservative with respect to the neutron fluxes transmitted through the concrete walls. This conservative behaviour is probably caused by a combination of several uncertainties, including for instance uncertainties on the proton and neutron interaction cross-sections and uncertainties on the concrete composition and density.

Neutron spectrometry and dosimetry in 100 and 300 MeV quasi-mono-energetic neutron field at RCNP, Osaka University, Japan

This paper describes the results of neutron spectrometry and dosimetry measurements using an extended range Bonner Sphere Spectrometer (ERBSS) with 3 He proportional counter performed in quasi-mono-energetic neutron fields at the ring cyclotron facility of the Research Center for Nuclear Physics (RCNP), Osaka University, Japan. Using 100 MeV and 296 MeV proton beams, neutron fields with nominal peak energies of 96 MeV and 293 MeV were generated via 7 Li(p,n)7 Be reactions. Neutrons produced at 0° and 25° emission angles were extracted into the 100 m long time-of-flight (TOF) tunnel, and the energy spectra were measured at a distance of 35 m from the target. To deduce the corresponding neutron spectra from thermal to the nominal maximum energy, the ERBSS data were unfolded using the MSANDB unfolding code. At high energies, the neutron spectra were also measured by means of the TOF method using NE213 organic liquid scintillators. The results are discussed in terms of ambient dose equivalent, H * (10), and compared with the readings of other instruments operated during the experiment.

First test of SP2: A novel active neutron spectrometer condensing the functionality of Bonner spheres in a single moderator

The NESCOFI@BTF (2011–2013) international collaboration was established to develop realtime neutron spectrometers to simultaneously cover all energy components of neutron fields, from thermal up to hundreds MeV. This communication concerns a new spherical spectrometer, called SP^2, which condenses the functionality of an Extended Range Bonner Sphere Spectrometer (ERBSS) into a single moderator embedding multiple active thermal neutron detectors. The possibility of achieving the complete spectrometric information in a single exposure constitutes a great advantage compared to the ERBSS. The first experimental test of the instrument, performed with a reference 241Am–Be source in different irradiation geometries, is described. The agreement between observed and simulated response is satisfactory for all tested geometries.

Spectrometry of 50 and 100 MeV quasi monochromatic neutron fields with extended range Bonner spheres

Quasi monochromatic neutron fields (QMN) from 50 to 180MeV are produced at TSL, The Svedberg Laboratory (Uppsala University, Sweden) by exploiting the 7Li(p,n) reaction in the forward direction. The energy distribution of these fields includes a main high-energy peak, with a few MeV width, superposed to a low-energy tail having its maximum in the tens of MeV region. Because this low-energy tail can be as important as 50% or more in terms of neutron fluence, the knowledge of its energy distribution is essential to completely characterize the QMN neutron fields. This is especially needed when QMN fields are used to irradiate devices with non-zero response to low-energies.This work presents the complete neutron spectrometry, from thermal energies up to the nominal peak energy, of the 50 and 100MeV QMN fields produced at TSL. This was done by means of two extended range Bonner Sphere Spectrometer (ERBSS) relying on different central detectors and independently calibrated. In spite of its known limitations such as the poor energy resolution, this spectrometer is still the only one capable to simultaneously respond to all energies of interest. Significant outcomes of this work are the neutron spectra and the determination of spectrum-integrated quantities such as the total fluence and the peak fluence.

WE‐E‐141‐08: Measurements of Secondary Neutron Spectrum and Dose Equivalent From a 250 MeV Passively Scattered Proton Beam Using An Extended Range Bonner Sphere Spectrometer

Purpose: Secondary neutrons from proton therapy have a wide energy range, from thermal to the maximum proton beam energy, which can be as high as ∼250 MeV. Most measurements reported in the literature to date have either not included spectral information or were obtained using detectors with limited response to neutrons above 10 MeV. Therefore, the primary objective of the present study was to measure a secondary neutron spectrum from proton therapy using a spectrometer that was sensitive to neutron energies over the entire energy spectrum. Methods: We performed measurements using an extended‐range Bonner sphere spectrometer (BSS) with high‐atomic‐number shells placed within polyethylene spheres to increase high‐energy response compared to a standard BSS. Measurements were performed for a 250‐MeV passively scattered proton beam with a medium snout and closed aperture. All measurements were performed in air 100 cm from isocenter. Data were unfolded using the MXD_FC33 unfolding algorithm. The a priori spectrum for neutron unfolding was determined using the Bayesian statistical estimator methodology. We used Monte Carlo N‐Particle Code to calculate a response matrix specifically for this measurement system. Results: Total measured fluence was 8.36 ncm2Gy‐1 and included a low energy tail from thermal energies to approximately 1 eV (16% fluence), an evaporation peak from 1 eV to 10 MeV (69% fluence), and a direct neutron peak from 10 MeV to 250 MeV (15% fluence). The total ambient dose equivalent was 2.03 mSvGy‐1 with neutrons in the evaporation peak contributing the highest dose equivalent and neutrons below 1 eV contributing essentially no dose. Conclusion: We report both neutron fluence and dose equivalent spectra for a 250 MeV passively scattered proton beam. These data were measured with a spectrometer that is sensitive over the entire measurable neutron energy spectrum (thermal to 250 MeV) and thus could be a valuable tool for benchmarking Monte Carlo simulations. This research was supported in part, by the National Cancer Institute award 1R01CA131463‐01A1 (W.D. Newhauser, P.I.) and a subcontract of that award (R.M. Howell, P.I).