CERN-EU High-energy Reference Field Research Articles

Calibration and simulation of a silicon dosemeter for ambient dose equivalent in low-earth orbit space.

A particle dosemeter (PD) is a payload of NEXTSat-2 in the low-earth orbit (LEO). The absorbed dose in LEO needs to be converted into the ambient dose equivalent (H*(10)). Due to a mixed field in LEO, the calibration factors (klow and khigh) should be determined for the low-and high-linear energy transfers (LET) (below and above 1.5keV/μm), respectively. The PD was irradiated with a 137Cs source at the Korea Radiation Solution facility to obtain H*(10) and absorbed doses. However due to the lack of sources for the high-LET calibration, H*(10) and an absorbed dose were calculated by simulating PD for the high-energy neutron field at CERN-EU high-energy Reference Field. The measured klow of PD had a difference of 5.1% and 9.5% from the calculated value of PD and the measured value of Liulin detectors, respectively. However, a difference in khigh between PD and Liulin was explained by the contribution of non-neutron components to Liulin in the measurements.

Evaluation and modelling of the lithium fluoride based thermoluminescent detector response at the CERN-EU high-energy reference field (CERF)

Lithium fluoride (LiF) thermoluminescent detectors (TLDs) are commonly used for measurements in mixed high energy radiation fields such as space, hadron radiotherapy and other high energy accelerator facilities. However, the results of these measurements are often challenging to interpret since the radiation field compositions are not known a priori. The goal of this work was to model and understand the LiF TLD response in the CERN-EU high-energy Reference Field (CERF).Six different LiF TLD types, namely MTS (LiF:Mg,Ti) and MCP (LiF:Mg,Cu,P) TLDs with different Li abundances, were irradiated at CERF. For the modelling of their response, CERF reference fluence energy spectra from FLUKA simulations and TLD relative luminescence efficiencies according to the Microdosimetric d(z) Model were collected from previous works and supplemented with new relative luminescence efficiency calculations for neutrons with energies above 0.01 MeV. Dedicated MCNP6.2 simulations were performed to obtain the required fluence to absorbed dose conversion factors.Results demonstrated that the response of LiF detectors can be well predicted even for complicated, mixed radiation fields. The modelling allowed to assess the dose contributions from different radiation types in mixed fields. Based on the results of this work, a methodology was proposed to perform LiF TLD measurements in well-characterized mixed fields in terms of absorbed dose in water or tissue. In unknown mixed radiation fields MTS-7 TLDs are recommended to assess the non-neutron component of the absorbed dose in water or tissue. Assessing the dose from neutrons requires dedicated neutron detectors.

Characterization of Thermoluminescent Dosimeters for Neutron Dosimetry at High Altitudes.

Neutrons constitute a significant component of the secondary cosmic rays and are one of the most important contributors to natural cosmic ray radiation background dose. The study of the cosmic ray neutrons’ contribution to the dose equivalent received by humans is an interesting and challenging task for the scientific community. In addition, international regulations demand assessing the biological risk due to radiation exposure for both workers and the general population. Because the dose rate due to cosmic radiation increases significantly with altitude, the objective of this work was to characterize the thermoluminescent dosimeter (TLDs) from the perspective of exposing them at high altitudes for longtime neutron dose monitoring. The pair of TLD-700 and TLD-600 is amply used to obtain the information on gamma and neutron dose in mixed neutron-gamma fields due to the present difference in 6Li isotope concentration. A thermoluminescence dosimeter system based on pair of TLD-600/700 was characterized to enable it for neutron dosimetry in the thermal energy range. The system was calibrated in terms of neutron ambient dose equivalent in an experimental setup using a 241Am-B radionuclide neutron source coated by a moderator material, polyethylene, creating a thermalized neutron field. Afterward, the pair of TLD-600/700 was exposed at the CERN-EU High-Energy Reference Field (CERF) facility in Geneva, which delivers a neutron field with a spectrum similar to that of secondary cosmic rays. The dosimetric system provided a dose value comparable with the calculated one demonstrating a good performance for neutron dosimetry.

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.

The CERN-EU high-energy Reference Field (CERF) facility: New FLUKA reference values of spectral fluences, present and newly proposed operational quantities

The CERN-EU high-energy Reference Field (CERF) facility provides an almost unique high-energy workplace radiation field for calibrating and testing radiation protection instrumentation employed at high-energy accelerators and for aircraft and space dosimetry. For the available exposure locations, this work provides the particle fluences of the various components of the radiation field, and a new set of reference values for the currently used operational quantities, i.e. ambient dose equivalent, H*(10), and personal dose equivalent Hp(10), as well as for the newly proposed ICRU/ICRP quantities ambient dose, H*, and personal dose, Hp, that may be adopted in the future for radiation protection. Particle spectra and radiological quantities were computed with the latest versions of the FLUKA code and benchmarked against experimental data taken with one of the reference instruments used at the facility.

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.

Activation experiment at the CERN-EU high-energy Reference Field (CERF) facility to benchmark ActiWiz and FLUKA predictions: A comparison after 30 days of cooling time

Abstract An experiment was carried out in June 2018 at CERN in order to benchmark FLUKA and ActiWiz activation calculations with experimental data. A first set of samples made of Copper, Aluminium, Stainless Steel and Kovar with a well-known elemental composition was irradiated by the direct impact of a mixed hadron beam of the CERF facility with a momentum 120 GeV/ c. A second set of samples was located downstream of a 50 cm long copper target and irradiated by the secondary particles generated from this impact. This paper discusses first the experimental results obtained by γ -spectrometry measurements after 30 days of cooling time for the two set of samples. Then, it discusses the FLUKA simulations performed in order to get the radionuclide inventory as well as the mixed particle fluence spectra in the samples for the two positions. In a third part, it discusses the predictions given by the ActiWiz software compared with the FLUKA results. Finally, the experimental and theoretical results are compared and discussed.

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.

Zenith: A Radiosonde Detector for Rapid-Response Ionizing Atmospheric Radiation Measurements During Solar Particle Events

Solar energetic particle events create radiation risks for aircraft, notably single event effects (SEEs) in microelectronics along with increased dose to crew and passengers. In response to this, some airlines modify their flight routes after automatic alerts are issued. At present these alerts are based on proton flux measurements from instruments on-board satellites, so it is important that contemporary atmospheric radiation measurements are made and compared. This paper presents the development of a rapid-response system built around the use of radiosondes equipped with a radiation detector, Zenith, which can be launched from a Met Office weather station after significant solar proton level alerts are issued. Zenith is a compact, battery-powered solid-state radiation monitor designed to be connected to a Vaisala RS-92 radiosonde which transmits all data to a ground station as it ascends to an altitude of ~33 km. Zenith can also be operated as a stand-alone detector when connected to a laptop, providing real-time count rates. It can also be adapted for use on unmanned aerial vehicles. Zenith has been flown on the Met Office Civil Contingency Aircraft (MOCCA), taken to the CERN-EU high energy Reference Field (CERF) facility for calibration and launched on a meteorological balloon at the Met Office's weather station in Camborne, Cornwall, UK. During this sounding, Zenith measured the Pfotzer-Regener maximum to be at an altitude of 18 - 20 km where the count rate was measured to be 1.15 counts s-1 cm-2 compared to 0.02 counts s-1 cm-2 at ground level.

DETERMINATION OF THE RESPONSE TO THE ATMOSPHERIC COSMIC RADIATION OF A NEUTRON DOSIMETER ASSISTED BY MONTE CARLO SIMULATION.

A TLD-based dosimeter of polyethylene-lead-polyethylene, was developed and characterized with Monte Carlo simulations, using the MCNPX code. This passive system for the determination of the ambient dose equivalent (H*(10)) for neutrons over a wide energy range can be used for the dosimetry of neutrons from atmospheric cosmic radiation, on the ground, and onboard aircraft. A method assisted by Monte Carlo simulations that improves the calibration of fast neutron dosimeters based on moderation and thermalization of the incident fast flux and the measurement of the thermal flux by a sensor, which respond mainly to thermal neutrons, is presented in this work. The H*(10) energy response of this dosimeter was obtained from simulations for monoenergetic neutrons from 10-10 to 104 MeV. The validation of the modeling was done with irradiations for ISO standard neutron fields of 241Am-Be, 252Cf and 252Cf(D2O) at Instituto de Radioproteção e Dosimetria (IRD, Brazil) and at CERN-EU high-energy reference field (CERF).

The CERN-EU high-energy Reference Field (CERF) facility: applications and latest developments

The CERF facility at CERN provides an almost unique high-energy workplace reference radiation field for the calibration and test of radiation protection instrumentation employed at high-energy accelerator facilities and for aircraft and space dosimetry. This paper describes the main features of the facility and supplies a non-exhaustive list of recent (as of 2005) applications for which CERF is used. Upgrade work started in 2015 to provide the scientific and industrial communities with a state-of-the-art reference facility is also discussed.

FIELD CALIBRATION OF A TLD ALBEDO DOSEMETER IN THE HIGH-ENERGY NEUTRON FIELD OF CERF.

The new albedo dosemeter-type AWST-TL-GD 04 has been calibrated in the CERF neutron field (Cern-EU high-energy Reference Field). This type of albedo dosemeter is based on thermoluminescent detectors (TLDs) and used by the individual monitoring service of the Helmholtz Zentrum München (AWST) since 2015 for monitoring persons, who are exposed occupationally against photon and neutron radiation. The motivation for this experiment was to gain a field specific neutron correction factor Nn for workplaces at high-energy particle accelerators. Nn is a dimensionless factor relative to a basic detector calibration with 137Cs and is used to calculate the personal neutron dose in terms of Hp(10) from the neutron albedo signal. The results show that the sensitivity of the albedo dosemeter for this specific neutron field is not significantly lower as for fast neutrons of a radionuclide source like 252Cf. The neutron correction factor varies between 0.73 and 1.16 with a midrange value of 0.94. The albedo dosemeter is therefore appropriate to monitor persons, which are exposed at high-energy particle accelerators.

INVESTIGATION OF THE EXTENDED RANGE REM-COUNTER SMARTREM-LINUS IN REFERENCE AND WORKPLACE FIELDS EXPECTED AROUND HIGH-ENERGY ACCELERATORS.

Radiation survey instrumentation is adequate for the use around high-energy accelerators if capable to measure the dose arising from neutrons with energies ranging from thermal up to a few gigaelectronvolts. The SmartREM-LINUS is a commercial extended range rem-counter, consisting of a central (3)He-proportional counter surrounded by a spherical moderator made of borated polyethylene with an internal shield made of lead. The dose rate indicated by the SmartREM-LINUS was investigated for two different irradiation conditions. The linearity and the angular dependence of the indicated dose rate were investigated using reference neutron fields produced by (241)AmBe and (252)Cf. Additional measurements were performed in two different workplace fields with a component of neutrons with energies >20 MeV, namely the CERN-EU high-energy reference field and near the beam dump of the SwissFEL injector test facility. The measured dose rates were compared to a commercial rem-counter (WENDI2) and the results of Monte Carlo simulations.

Design and characterization of an SRAM-based neutron detector for particle therapy

A neutron detector based on registration of radiation effects in Static Random Access Memories (SRAMs) has been developed at the University of Bergen for applications in particle therapy. Nine different SRAMs were tested and a 16 Mibit SRAM from Cypress was chosen for the final detector. The SRAMs were irradiated in beam lines at PTB Braunschweig, the Oslo Cyclotron Laboratory, The Svedberg Laboratory, The Institute for Energy Technology (IFE, Kjeller) and the CERN-EU high-energy reference field. The results from the measurements demonstrate the feasibility of using the selected SRAMs for neutron detection. The results indicate low or no sensitivity to thermal neutrons while the cross section for fast neutrons increases with neutron energy before reaching a more stable level at energies of several tenths of MeV.

SEE Measurements and Simulations Using Mono-Energetic GeV-Energy Hadron Beams

Single Event Upset (SEU) measurements were performed on the ESA SEU Monitor using mono-energetic GeV-energy hadron beams available in the North Experimental Area at CERN. A 400 GeV proton beam in the H4IRRAD test area and a 120 GeV mixed pion and proton beam at the CERN-EU high Energy Reference Field facility (CERF) were used for this purpose. The resulting cross section values are presented and discussed as well as compared to the several hundred MeV case (typical for standard test facilities) from a physical interaction perspective with the intention of providing a more general understanding of the behavior. Moreover, the implications of the cross section dependence with energy above the several hundred MeV range are analyzed for different environments. In addition, analogous measurements are proposed for Single Event Latchup (SEL), motivated by discussed simulation results. Finally, a brief introduction of the future CHARM (CERN High-energy AcceleratoR Mixed facility) test installation is included.

Characterization of the Neutron Environment and SEE Investigations at the CERN-EU High Energy Reference Field and at the Pic du Midi

A new platform dedicated to the long-term characterization of the Atmospheric Natural Radiative Environment at mountain altitude (Pic du Midi, +2885 m) is presented. The performance of a high energy neutron spectrometer is established with measurements performed in a realistic neutron field and by comparison with the neutron monitors. A SEE-rate prediction approach is also used on a 90 nm SRAM memory array which was irradiated at CERF and set up at the Pic du Midi during almost one year.

Comparison of SOI Microdosimeter and Tissue Equivalent Proportional Counter Measurements at the CERF Facility

The CERN-EU High Energy Reference Field (CERF) facility is used in the calibration of neutron dosimeters for radiation protection applications in aviation and high energy physics. A comparison of the facility's microdosimetric spectra obtained with a new Silicon on Insulator (SOI) Microdosimeter and a HAWK Tissue Equivalent Proportional Counter (TEPC) is presented. Experimental data obtained with both devices indicates the presence of a small charged particle flux within the neutron dominated field which makes a non negligible contribution to the dose equivalent.

Characterization of extended range Bonner Sphere Spectrometers in the CERF high-energy broad neutron field at CERN

The accurate determination of the ambient dose equivalent in the mixed neutron–photon fields encountered around high-energy particle accelerators still represents a challenging task. The main complexity arises from the extreme variability of the neutron energy, which spans over 10 orders of magnitude or more. Operational survey instruments, which response function attempts to mimic the fluence-to-ambient dose equivalent conversion coefficient up to GeV neutrons, are available on the market, but their response is not fully reliable over the entire energy range. Extended range rem counters (ERRC) do not require the exact knowledge of the energy distribution of the neutron field and the calibration can be done with a source spectrum. If the actual neutron field has an energy distribution different from the calibration spectrum, the measurement is affected by an added uncertainty related to the partial overlap of the fluence-to-ambient dose equivalent conversion curve and the response function. For this reason their operational use should always be preceded by an “in-field” calibration, i.e. a calibration made against a reference instrument exposed in the same field where the survey-meter will be employed. In practice the extended-range Bonner Sphere Spectrometer (ERBSS) is the only device which can serve as reference instrument in these fields, because of its wide energy range and the possibility to assess the neutron fluence and the ambient dose equivalent (H⁎(10)) values with the appropriate accuracy. Nevertheless, the experience gained by a number of experimental groups suggests that mandatory conditions for obtaining accurate results in workplaces are: (1) the use of a well-established response matrix, thus implying validation campaigns in reference monochromatic neutrons fields, (2) the expert and critical use of suitable unfolding codes, and (3) the performance test of the whole system (experimental set-up, elaboration and unfolding procedures) in a well controlled workplace field. The CERF (CERN-EU high-energy reference field) facility is a unique example of such a field, where a number of experimental campaigns and Monte Carlo simulations have been performed over the past years.With the aim of performing this kind of workplace performance test, four different ERBSS with different degrees of validation, operated by three groups (CERN, INFN-LNF and Politecnico of Milano), were exposed in two fixed positions at CERF. Using different unfolding codes (MAXED, GRAVEL, FRUIT and FRUIT SGM), the experimental data were analyzed to provide the neutron spectra and the related dosimetric quantities. The results allow assessing the overall performance of each ERBSS and of the unfolding codes, as well as comparing the performance of three ERRCs when used in a neutron field with energy distribution different from the calibration spectrum.

Mapping of cosmic radiation dose in Croatia

The Earth is continually bombarded by high-energy particles coming from the outer space and the sun. These particles, termed cosmic radiation, interact with nuclei of atmospheric constituents and decrease in intensity with depth in the atmosphere. Measurements of photon and gamma radiation, performed with a Radiameter at 1 m above the ground, indicated dose rates of 50–100 nSv/h. The neutron dose rate was measured with the CR-39 track etch detector calibrated by the CERN-EU high-energy Reference Field (CERF) facility. Correlation between neutron dose rates and altitudes at 36 sites was examined in order to obtain a significant positive correlation coefficient; the resulting linear regression enabled estimation of a neutron dose at particular altitude. The measured neutron dose rate in Osijek (altitude of 89 m, latitude of 45.31° N) was 110 nSv/h.

Measurements of neutron radiation in aircraft

Radiation environment is a complex mixture of charged particles of the solar and galactic origin, as well as of secondary particles created in an interaction of galactic cosmic particles with the nuclei of the Earth's atmosphere. A radiation field at aircraft altitude consists of different types of particles, mainly photons, electrons, positrons and neutrons, with a large energy range. In order to measure a neutron component of the cosmic radiation, we investigated a few combinations of a track etch detector (CR-39, LR-115) with a plastic converter or boron foil. Detector calibration was performed on neutrons coming from the nuclear reactor, as well as in the CERN-EU high-energy Reference Field (CERF) facility. From November 2007 to September 2008, the neutron dose equivalent was measured by the track detectors during five aircraft flights, in the north geographical latitude from 21° to 58°; the respective average dose rate, determined by using the D-4 detector (CR-39/B), was Ḣ(n)=5.9 μSv/h. The photon dose rate, measured by the electronic dosimeter RAD-60 SE, had the average value of Ḣ(f)=1.4 μSv/h.