Neutron Dose Equivalent Rates Research Articles

Evaluation of fluence-to-dose response function of neutron survey meter using singular value decomposition method

This paper presents the evaluation of the fluence-to-ambient dose equivalent response function (referred to as F2D response function) of the Aloka TPS-451C neutron dose equivalent rate meter (hereafter referred to as a neutron meter) based on a singular value decomposition (SVD) approach. Measurements of neutron ambient dose equivalent rates, Ḣ∗(10), using the neutron meter were conducted in various neutron standard fields of 241Am-Be source with different neutron fluence rate spectra. A series of Ḣ∗(10) values and corresponding neutron fluence rate spectra were used as the input data for unfolding the F2D response function of the neutron meter. To verify the SVD-based method, the fluence response functions of a well-known Bonner sphere spectrometer system were unfolded using the SVD method, and the results were compared with those provided in the IAEA technical report No. 403. The SVD method was then applied to determine the F2D response function of the Aloka TPS-451C neutron meter with the use of the ICRP-74 F2D response function and that predicted in a previous work as initial guesses. The consistency between initial guesses and the SVD unfolded F2D response function of the Aloka TPS-451C neutron meter implies the reliability of the SVD method for determining the response functions of neutron meters.

Design and testing of an irradiation room with low room-scattering for neutron calibration

IntroductionThis laboratory plans to establish neutron reference radiation fields with three neutron sources to calibrate neutron-measuring devices. To perform calibration at multiple dose rates, neutron ambient dose equivalent rate H˙*(10) needs to range 1 μSv/h to 10 mSv/h. The lower limit requires that the maximum available calibration distance should be at least 4.5 m. MethodsTo reduce room-scattered neutrons and extend the available calibration distance, MC simulations were conducted to determine the material and size of the irradiation room. A 3″ Bonner sphere and a LB6411 environmental neutron dosimeter were used to characterize the irradiation room. ResultsA 14.32 × 14.32 × 12.00 m3 irradiation room was built based on simulation results. Floor, roof, and walls are made of 75 cm concrete covered by a coating layer of 2 cm BPE and 3 cm PE. Experimental maximum available calibration distance reaches 4.65 m. The range of H˙*(10) for calibration covers 1 μSv/h to 10 mSv/h. Neutron and photon H˙*(10) outside the room are within 0.19 μSv/h and 0.22 μSv/h, respectively.

Developing and using international standards for neutron ambient dose equivalent rate meters

Measurements of neutron ambient dose equivalent rates are challenging because there are no perfect instruments that cover the entire 9 orders of magnitude neutron energy range (thermal to 20 MeV). Hence quantifying performance requirements with the international standard IEC 61005 for neutron ambient dose equivalent rate meters presents unique challenges. This paper discusses the requirements of this standard, which have been refined and improved over the last 20 years. Examples of type testing results for the constancy of ambient dose rate response, variation of the response due to neutron energy, response time, and various environmental disturbances are examined. IEC 61005 provides manufacturers with internationally acceptable requirements and provides consistent test methods for compliance with stated performance requirements.

Results from the Radiation Assessment Detector on the International Space Station, Part 2: The fast neutron detector

We report the results of the first six years of measurements of so-called fast neutrons on the International Space Station (ISS) with the Radiation Assessment Detector (ISS-RAD), spanning the period from February 2016 to February 2022. ISS-RAD combines two sensor heads, one nearly identical to the single sensor head in the Mars Science Laboratory RAD (MSL-RAD). The latter is described in a companion article to this one. The novel sensor is the FND, or fast neutron detector, designed to measure neutrons with energies in the range from 200 keV to about 8 MeV. ISS-RAD was deployed in February 2016 in the USLAB module, and then served as a survey instrument from March 2017 until May 2020. Data were acquired in Node3, the Japanese Pressurized Module, Columbus, and Node2. At the conclusion of the survey portion of RAD’s planned 10-year campaign on ISS, the instrument was stationed in the USLAB; current plans call for it to remain there indefinitely. The radiation environment on the ISS consists of a complex mix of charged and neutral particles that varies on short time scales owing to the Station’s orbit. Neutral particles, and neutrons in particular, are of concern from a radiation protection viewpoint, because they are both highly penetrating (since they do not lose energy via direct ionization) and, at some energies, have high biological effectiveness. Neutrons are copiously produced by GCRs and other incident energetic particles when they undergo nuclear interactions in shielding. As different ISS modules have varying amounts of shielding, they also have varying neutron environments. We report results for neutron fluences and dose equivalent rates in various positions in the ISS.

Results from the Radiation Assessment Detector on the International Space Station: Part 1, the Charged Particle Detector

We report the results of the first six years of measurements of the energetic particle radiation environment on the International Space Station (ISS) with the Radiation Assessment Detector (ISS-RAD), spanning the period from February 2016 to February 2022. The first RAD was designed and built for MSL, the Mars Science Laboratory rover, also known as Curiosity; it has been operating on Mars since 2012 and is referred to here as MSL-RAD. ISS-RAD combines two sensor heads, one nearly identical to the single MSL-RAD sensor head, the other with greatly enhanced sensitivity to fast neutrons. These two sensor heads are referred to as the Charged Particle Detector (CPD) and Fast Neutron Detector (FND), respectively. Despite its name, the CPD is also capable of measuring high-energy neutrons and γ-rays, as is MSL-RAD. ISS-RAD was flown to the ISS in December 2015 and was deployed in February 2016, initially in the USLab module. RAD was used as a survey instrument from January 2017 through May 2020, when the instrument was positioned in the USLab and set to a zenith-pointing orientation. The energetic particle environment on the ISS is complex and varies on short time scales owing to the orbit, which has a 51.6∘ inclination with respect to the equator and has had an altitude in the 400–440 km range in this time period. The ISS moves continuously through the geomagnetic field, the strength of which varies with latitude, longitude, and altitude. The orbit passes through the South Atlantic Anomaly (SAA) several times a day, where magnetically trapped protons and electrons produce large but transient increases in observed fluxes and absorbed dose rates. The environment inside the ISS is affected by the solar cycle, altitude, and the local shielding, which varies between different ISS modules. We report results for charged particle absorbed dose and dose equivalent rates in various positions in the ISS. In an accompanying paper, we report similar results for neutron dose equivalent rates obtained with the ISS-RAD Fast Neutron Detector.

Photoneutrons and Gamma Capture Dose Rates at the Maze Entrance of Varian TrueBeam and Elekta Versa HD Medical Linear Accelerators

Herein, we evaluated the neutron and gamma capture dose equivalent rates at the maze entrance of Varian TrueBeam and Elekta Versa HD™ medical linear accelerators (linacs) using experimental measurements as well as empirical calculations. Dose rates were measured using calibrated neutron and gamma area survey meters placed side-by-side at the measurement point of interest. Measurements were performed at a source-to-detector distance of 100 cm, with a 10 × 10 cm2 field size therapeutic X-ray beam, and a 30 × 30 × 15 cm3 solid water patient equivalent phantom, with a linac operating at 15, 10 MV, and 10 MV flattened filter-free (FFF). Dose rates were also measured at different points at the centerline along the maze towards the maze entrance. The measured dose equivalent rates at the maze entrance were comparable to those reported in the literature. The dose rates along the maze decreased exponentially towards the maze entrance and were significant for short maze lengths. The evaluated empirical methods for estimating neutron dose rates at the maze entrance of a linac proposed by Kersey, the modified Kersey method and Falcão method, agree by a factor of two from the experimental measurements. The results revealed vital radiation protection considerations owing to neutron contamination in external beam therapy.

The neutron dose equivalent rate measurements by R3DR/R2 sp ectrometers on the international space station

The data from two Bulgarian-German instruments with the basic name “Radiation Risk Radiometer-Dosimeter” (R3D) are discussed. The R3DR instrument worked inside the ESA EXPOSE-R facility (2009–2010), while R3DR2 worked inside the ESA EXPOSE-R2 facility (2014–2016). Both were outside the Russian Zvezda module on the International Space Station (ISS). The data from both instruments were used for calculation of the neutron dose equivalent rate. Similar data, obtained by the Russian “BTNNEUTRON” instrument on the ISS are used to benchmark the R3DR/R2 neutron dose equivalent rate. The analisys reveals that the “BTNNEUTRON” and R3DR/R2 values are comparable both in the equatorial and in the South Atlantic Anomaly (SAA) regions. The R3DR/R2 values are smaller than the “BTNNEUTRON” values in the high latitude regions. The comparison with the Monte Carlo simulations of the secondary galactic cosmic rays (GCR) neutron ambient dose equivalent rates (El-Jaby and Richardson, 2015, 2016) also shows a good coincidence with the R3DR/R2 spectrometer data obtained in the equatorial and high latitude regions.

Ensuring the unity of measurements of the neutron radiation rate absorbed dose in the field of clinical neutron radiation dosimetry

Development of neutron radiation therapy arises the question of ensuring the uniformity of measurements of the absorbed dose and the absorbed dose rate of neutron radiation. FGUP “VNIIFTRI” improved the primary method and means of reproducing the unit of neutron radiation absorbed dose rate within the framework of improving the State primary standard of units of absorbed dose rate and neutron dose equivalent rate GET 117-2010. A set of ionization chambers has been created and the upper value of the reproduction of the unit of absorbed dose rate of neutron radiation has been expanded. A hardware-methodical complex has also been developed for transferring a unit of absorbed dose rate of neutron radiation to nuclear physics facilities used in neutron radiation therapy. As a result the expanded relative uncertainty of reproduction of the unit of absorbed dose rate does not exceed 3%, the expanded relative uncertainty of the transmission method does not exceed 0.5%. The resulting level of accuracy is in line with the recommendations used in neutron beam therapy.

Design of a neutron shielding performance test system base on Am–Be neutron source

A system for neutron shielding performance test was designed based on an Am–Be neutron source and 3He proportional counter. The Am–Be neutron source was shielded by paraffin wax block with a thickness of dozens of centimeters and collimated by polyethylene rod. The neutron energy spectra in front of the collimator were simulated by MCNP. The detector system was designed based on 3He proportional counter and different response tuning assembly (RTA) to optimize the neutron energy region of interest. A method for testing neutron shielding performance for thermal, epithermal, fast neutrons and neutron ambient dose equivalent rate was proposed by employing different response tuning assembly (RTA). Common radiation shielding materials including polyethylene (PE), borated polyethylene (BPE) and graphite were conducted with the designed system. The results demonstrated that the proposed method is efficient for investigating the neutron shielding performance of materials.

Feasibility study of Isfahan MNSR as a calibration thermal neutron source

In the present work, the thermal beam-tube of Isfahan Miniature Neutron Source Reactor (MNSR) with known neutron dose-equivalent rate and flux is investigated as a calibration source of thermal neutrons. It is shown that 90% of the neutrons emerging from this tube are thermal. Measuring the response of TLD-600 and TLD-700 dosimeters exposed to these neutrons reveals that the both dosimeters respond linearly to neutrons in the range of 0.1 to 38 mSv. Moreover, after subtracting the contribution of mixed gamma rays using the calibration curves obtained in a standard 60Co gamma rays field, the response of TLD-600 dosimeter still behaves linearly with dose-equivalent. Finally, it is concluded that the dose-equivalent rate and flux of thermal neutrons in Isfahan MNSR are stable with time and this reactor can reliably be used as a calibration source for thermal neutrons.

Cosmic ray neutron spectrometry and dosimetry at High Altitude Research Laboratory, Gulmarg, Kashmir, India

Cosmic ray neutrons are produced by the interaction of primary cosmic rays with the air molecules present in the atmosphere. At the ground level, these secondary neutrons are one of the important contributors of natural cosmic ray radiation background dose. With two major peaks around 1 MeV and 100 MeV, the neutron energies can extend up to 10 GeV. At the ground level, considerable presence of thermal neutrons can also be observed due to collisions and moderation of fast neutrons with the soil and other surrounding materials. A study on the evaluation of cosmic ray neutron spectra, and the resulting ambient dose equivalents was carried out at High Altitude Research Laboratory, Gulmarg, India (altitude 2650 m, N34.0484°, E74.3805°) considering the enhanced contribution of cosmic ray background in the region. Extended energy Bonner Sphere Spectrometer (BSS) was used for the continuous measurement of cosmic ray neutrons during the period of October 2018 to June 2019. Neutron spectra and ambient dose equivalents were unfolded from the BSS counts with the Monte Carlo generated response matrix and the FRUIT unfolding code. Along with Bonner sphere spectrometer, Tissue Equivalent Proportional Counter (TEPC) was also used for a limited period for independent measurement of cosmic ray dose equivalents and Linear Energy Transfer (LET) distribution. For comparison, neutron spectra and dose equivalent rates were also calculated using Monte Carlo code FLUKA and EXPACS software program. The ambient dose equivalent rates were found to vary in the range of 24–34 nSv. h−1 during the entire observation period of 9 months. No considerable seasonal variations were observed during the course of the study. Dose equivalent rates, estimated by the TEPC were slightly higher than the neutron dose equivalent rates measured by BSS. Experimentally measured values match reasonably well with theoretically simulated results.

Observations of neutron radiation environment during Odyssey cruise to Mars

In April 2001, Mars Odyssey spacecraft with the High Energy Neutron Detector (HEND) onboard was launched to Mars. HEND/Odyssey was switched on measurement mode for most of transit to Mars to monitor variations of spacecraft background and solar activity. Although HEND/Odyssey was originally designed to measure Martian neutron albedo and to search for Martian subsurface water/water ice, its measurements during cruise phase to Mars are applicable to evaluate spacecraft ambient radiation background. The biological impact of the neutron component of this radiation background should be understood, as it must be taken into account in planning future human missions to Mars. We have modeled the spacecraft neutron spectral density and compared it with HEND measurements to estimate neutron dose equivalent rates during Odyssey cruise phase, which occurred during the maximum period of solar cycle 23. We find that the Odyssey ambient neutron environment during May – September 2001 yields 10.6 ± 2.0 μSv per day in the energy range from 0 to 15 MeV, and about 29 μSv per day when extrapolated to the 0–1000 MeV energy range during solar quiet time (intervals without Solar Particle Events, SPEs). We have also extrapolated HEND/Odyssey measurements to different periods of solar cycle and find that during solar minimum (maximum of GCR flux), the neutron dose equivalent rate during cruise to Mars could be as high as 52 μSv per day with the same shielding. These values are in good agreement with results reported for a similar measurement made with an instrument aboard the Mars Science Laboratory during its cruise to Mars in 2011–2012.

Establishment of Neutron Reference Fields in Vietnam: A Review

This paper summarizes the characterizations of neutron reference fields using radionuclide sources at the Institute for Nuclear Science and Technology (INST), Hanoi, Vietnam for the purposes of radiation protection and calibrations. The dosimetric quantities of spectral neutron fluence rates and neutron ambient dose equivalent rates have been calculated and presented using the Monte Carlo N-Particle transport code, version 5 (MCNP5) and Bonner sphere spectrometer (BSS) measurements in the combination with MAXED unfolding code. The characterizations of neutron reference fields have been done, complying with the ISO 8529 and 12789 series. The results confirm the neutron calibration capability at the INST in order to fulfill the domestic calibration demands of neutron ambient dose equivalent rate meters.

Characteristics of Simulated Workplace Neutron Standard Fields

This paper presents the development of simulated workplace neutron standard fields at the Institute for Nuclear Science and Technology with the 241Am-Be source moderated by polyethylene spheres with diameters of 15 cm and 30 cm. The characterization of the standard fields (in terms of neutron fluence rates and neutron ambient dose equivalent rates) was performed using Bonner sphere spectrometer system together with MAXED and FRUIT unfolding codes. The related quantities such as neutron dose equivalent-averaged energies and fluence-to-ambient dose equivalent conversion coefficients were also determined. The discrepancies of values are satisfied the standard uncertainty criteria as recommended by the International Standard Organization 12789 series. It implies that the simulated workplace neutron standard fields can be applied in the practical works for calibration purposes.

Shielding designing of 241Am-Be neutron source housing experiment and Monte Carlo simulation

Shielding of a neutron source housing for 1 Ci 241Am-Be source has been designed and fabricated based on the simulation carried out using Monte Carlo code FLUKA. Neutron and photon dose equivalent rates at the surface and at 1 m distance from the surface of the housing were calculated using FLUKA simulation and measured using gamma and neutron dose rate meters. The calculated and measured gamma and neutron dose equivalent rates agree well. Neutron spectra outside the source housing were generated using FLUKA simulation and measured with the ROSPEC + simple scintillation spectrometer neutron spectrometry system and also agree reasonably well. Gamma spectra outside the source housing and residual activity due to activation products in stainless steel and lead of the housing were also generated through simulation.

Evaluation of the calibration factors of neutron dose rate meters in a 241Am–Be neutron field

Calibrations were performed for three types of neutron ambient dose equivalent rate meters, i.e., Aloka TPS-451C (Hitachi), KSAR1U.06 (Baltic Scientific Instruments), and Model 12-4 (Ludlum), using a standard field of a 241Am–Be source. The measured total neutron ambient dose equivalent rates, $$H^*(10)_{\text{tot}}'$$ , were analyzed to obtain the direct neutron ambient dose equivalent rates, $$H^*(10)_{\text{dir}}'$$ , using the ISO 8529-2-recommended generalized-fit method, semiempirical fit method, and reduced-fitting method (RFM) fit methods. The calibration factor (CF), defined as the ratio between the conventional true value of the neutron ambient dose equivalent rates in a free field, $$H^*(10)_{\text{FF}}'$$ , and $$H^*(10)_{\text{dir}}'$$ , was evaluated as one of the important characteristics of the neutron meters in the present work. The fitting results show that the $$H^*(10)_{\text{dir}}'$$ values of the meters are in good agreement within the theoretical data within 4%. The averaged CFs of the three neutron meters were evaluated as $$0.99 \pm 0.01$$ , $$1.00 \pm 0.03,$$ and $$0.99 \pm 0.08$$ , respectively. The largest standard uncertainty of these values was determined to be approximately 18.47% ( $$k=1$$ ). The standard uncertainty of the CFs obtained using the RFM method was less than 4.23% ( $$k=1$$ ), which is the smallest uncertainty among the three methods.

Evaluation of neutron fluence in the treatment room and along the maze of an 18 MV linear accelerator

Neutron fluence and dose equivalent rate in the treatment room and along the maze of an 18 MV medical linear accelerator was evaluated using Monte Carlo code MCNP5. Scope of the study was to test the hypothesis that a simple accelerator head model can adequately estimate the neutron fluence in the treatment room and along the maze without the requirement of a detailed modeling of the complex accelerator head configuration. The accelerator head was modeled either as a spherical cell source of 10 cm in radius or as a point source in the centre of a spherical cell of 10 cm internal and 20 cm external radious, both made of tungsten. The calculations were compared against measurements performed using gold and indium activation foils. A good agreement between calculated and experimental results was observed in the treatment room. However, the calculations at the maze entrance overestimated the neutron fluence by a factor of 3. The results of the study contribute towards the development of a simple and computationally cost effective methodology for the calculation of neutron dose in the treatment room and the maze of high energy medical accelerators providing a representative simulation of the complex accelerator, bunker and maze configurations that cannot be described by presently used analytical expressions.

Development of neutron survey meter using prescila neutron probe

This paper presents the design and validation of a neutron survey meter. The meter consists of a PRESCILA neutron probe (with good sensitivity, directional response, gamma rejection, and enhanced high-energy response to 20 MeV) and an electrometer developed at Non-Destructive Evaluation center. The homogeneity response of the PRESCILA neutron probe was investigated as a function of distances from the 241Am - 9Be source in order to obtain the appropriate distance for accurate count-rate measurements using the neutron survey meter. A system consists of the PRESCILA neutron probe and the Ludlum Model 2326 electrometer was then used for measuring neutron ambient dose equivalent rates in the range from 50 cm to 200 cm with the step of 25 cm. The relationship between the count-rates and neutron dose equivalent rates (in the distance ranged from 50 to 200 cm) were deduced to validate the proper operation of the neutron survey meter.

STUDY OF NEUTRON RADIATION FIELD AT THE FIRST RADIOACTIVE ION BEAM LINE IN LANZHOU.

The first Radioactive Ion Beam Line in Lanzhou was a projectile fragment separator located in the HIRFL. The process of production and separation of radioactive ion beams can induce a strong and complex radiation field. The neutron dose equivalent rates were measured in four positions with a 70 MeV/u 40Ar18+ beam. The results were compared with that simulated by the FLUKA code. New shielding walls were installed to reduce the neutron background for spectroscopy measurement in the experimental terminal. In addition, the induced radioactivity of accelerator components and corresponding residual dose rates were analyzed for the radiation safety of accelerator workers. The airborne radioactivity as well as occupational exposure due to immersion in and inhalation of activated air were also estimated. This work aims to provide a valuable experience for the radiation study in the future fragment separator HFRS at HIAF.

NEUTRON CHARACTERIZATION OF ENSA-DPT TYPE SPENT FUEL CASK AT TRILLO NUCLEAR POWER PLANT.

The Neutron Standards Laboratory of CIEMAT has conducted the characterization of the independent spent fuel storage installation at the Trillo Nuclear Power Plant. At this facility, the spent fuel assemblies are stored in ENSA-DPT type dual purpose casks. Neutron characterization was performed by dosimetry measurements with a neutron survey meter (LB6411) inside the facility, around an individual cask and between stored casks, and outside the facility. Spectra measurements were also performed with a Bonner sphere system in order to determine the integral quantities and validate the use of the neutron monitor at the different positions. Inside the facility, measured neutron spectra and neutron ambient dose equivalent rate are consistent with the casks spatial distribution and neutron emission rates, and measurements with both instruments are consistent with each other. Outside the facility, measured neutron ambient dose equivalent rates are well below the 0.5 μSv/h limit established by the nuclear regulatory authority.