Cosmic Neutron Research Articles

Low-level gamma-ray spectrometry for environmental samples

Low-level gamma-ray spectrometry with large volume HPGe detectors has been widely used in analysis of environmental radionuclides. The reasons are excellent energy resolution and high efficiency that permits selective and non-destructive analyses of several radionuclides in composite samples. Although the most effective way of increasing the sensitivity of a gamma-ray spectrometer is to increase counting efficiency and the amount of the sample, very often the only possible way is to decrease the detector’s background. The typical background components of a low-level HPGe detector, not situated deep underground, are cosmic radiation (cosmic muons, neutrons and activation products), radioactivity of construction materials, radon and its progenies. A review of Monte Carlo simulations of background components of HPGe detectors, and their characteristics in coincidence and anti-Compton mode of operation are presented and discussed.

Dating of syngenetic ice wedges in permafrost with 36Cl

A new method of permafrost dating with the cosmogenic radionuclide 36Cl is presented. In the first application, syngenetic ice wedges are dated using the ratio of 36Cl and Cl concentrations in ice as the signal. 36Cl is produced in the atmosphere by nuclear reactions of cosmic rays on argon. Stable chlorine enters the atmosphere from the oceans. Their ratio does not depend on chloride concentration in precipitations and on sublimation of snow. In situ production of 36Cl in permafrost ice via cosmic ray-induced reactions and neutron capture are calculated and the dating age limit is estimated as 3 million years. 36Cl/Cl ratios in permafrost samples from cape Svyatoy Nos (Laptev Sea coast), North-Eastern Siberia, are measured by accelerator mass spectrometry. Analysis of the first results and the calculated dates support the feasibility of the 36Cl permafrost dating method

Background study for the pn-CCD detector of CERN Axion Solar Telescope

The CERN Axion Solar Telescope (CAST) experiment searches for axions from the Sun converted into photons with energies up to around 10 keV via the inverse Primakoff effect in the high magnetic field of a superconducting Large Hadron Collider (LHC) prototype magnet. A backside illuminated pn-CCD detector in conjunction with an X-ray mirror optics is one of the three detectors used in CAST to register the expected photon signal. Since this signal is very rare and different background components (environmental gamma radiation, cosmic rays, intrinsic radioactive impurities in the setup, etc.) entangle it, a detailed study of the detector background has been undertaken with the aim to understand and further reduce the background level of the detector. The analysis is based on measured data taken during the Phase I of CAST and on Monte Carlo simulations of different background components. This study will show that the observed background level (at a rate of (8.00 ± 0.07) × 10 −5 counts cm −2 s −1 keV −1 between 1 and 7 keV) seems to be dominated by the external gamma background due to usual activities at the experimental site, while radioactive impurities in the detector itself and cosmic neutrons could make just smaller contribution.

Neutron detection on the Foton-M2 satellite by a track etch detector stack

In the frame of a European Space Agency (ESA) project called 'Biology and Physics in Space', a returning satellite, Foton-M2, was orbiting a container, the BIOPAN-5, loaded with biological experiments and facilities for radiation dosimetry (RADO) in the open space. One of the RADO experiments was dedicated to the detection of the primary cosmic rays and secondary neutrons by a track etch detector stack. The system was calibrated at high-energy particle accelerators and neutron generators. The developed detectors were investigated by an image analyser, and from the track parameters the linear energy transfer spectra and the absorbed dose were determined (26 microGy/d). Also, the neutron flux was estimated below 5 MeV and found to be 2.4 cm(-2) s(-1) directly from the space. The construction of the stack allowed to investigate the neutrons also from the direction of the carrying satellite, where the flux was found somewhat higher.

From the mass of the neutrino to the dating of wine

The Neutrino Ettore Majorana Observatory (NEMO) experiment, which is currently running in the Modane underground laboratory is looking for a very rare process of the natural radioactivity, called the neutrinoless double beta decay, in which a nucleus emits two electrons simultaneously. The calculated half-life being so large (∼10 25 years), few counts per year are expected which implies that all the background components have to be reduced by few orders of magnitude. Beside cosmic rays, external γ-rays, neutrons and radon, the detector must be built out of materials containing no measurable radioactive isotopes such as U, Th, Ra and their progenies. Most of the natural radioactive isotopes are γ-emitters. So, in order to be able to select and control the radiopurity of all materials, the Bordeaux group has been involved over several years in the development of low background γ-ray spectrometers, based on the use of HPGe crystals. When applied to the radioactivity measurements of wine, the first measurements have shown that beside the well-known isotope 40 K, the wine contains also weak traces of 137Cs with an activity depending on the vintage. This technique has therefore led to the possibility to date the wine bottles having vintage between 1950 and 1980 or at least to control the year written on the label or on the cork. Furthermore, it has been shown that it is not necessary to open the bottle.

Muon and neutron observations in connection with the corotating interaction regions

Ground cosmic ray observations are used for studying several kinds of interplanetary structures. The cosmic ray data has different responses to each kind of interplanetary structure. This article has as objective to study cosmic ray muon and neutron signatures due to the passage of corotating interaction region (CIR) in the interplanetary medium, and identify the signatures in the cosmic ray data due to these events. The cosmic ray muon data used in this work are recorded by the multidirectional muon detector installed at INPE’s Observatório Espacial do Sul – OES/CRSPE/INPE-MCT, in São Martinho da Serra, RS (Brazil) and the neutron data was recorded by the neutron monitor installed in Newark (USA). The CIR events were selected in the period from 2001 to 2004. CIRs clearly affect cosmic ray density in the interplanetary medium in the Earth’s vicinity, where the magnetic field plays an important role.

Erratum: Measuring the cosmic ray muon-induced fast neutron spectrum by (n,p) isotope production reactions in underground detectors [Phys. Rev. C72, 025807 (2005)

While cosmic ray muons themselves are relatively easy to veto in underground detectors, their interactions with nuclei create more insidious backgrounds via: (i) the decays of long-lived isotopes produced by muon-induced spallation reactions inside the detector, (ii) spallation reactions initiated by fast muon-induced neutrons entering from outside the detector, and (iii) nuclear recoils initiated by fast muon-induced neutrons entering from outside the detector. These backgrounds, which are difficult to veto or shield against, are very important for solar, reactor, dark matter, and other underground experiments, especially as increased sensitivity is pursued. We used fluka to calculate the production rates and spectra of all prominent secondaries produced by cosmic ray muons, in particular focusing on secondary neutrons, due to their importance. Since the neutron spectrum is steeply falling, the total neutron production rate is sensitive just to the relatively soft neutrons, and not to the fast-neutron component. We show that the neutron spectrum in the range between 10 and 100 MeV can instead be probed by the (n, p)-induced isotope production rates 12C(n, p)12B and 16O(n, p)16N in oil- and water-based detectors. The result for 12B is in good agreement with the recent KamLAND measurement. Besides testing the calculation of muon secondaries, these results are also of practical importance, since 12B (T1/2 = 20.2 ms, Q = 13.4 MeV) and 16N (T1/2 = 7.13 s, Q = 10.4 MeV) are among the dominant spallation backgrounds in these detectors.

A note on 207Bi in environmental samples

Traces of the radionuclide 207Bi were identified in soil and cryoconite (glacier sediment) samples from Alpine regions of Austria. This nuclide has been produced in thermonuclear explosions mainly in the early 1960s and subsequently dispersed in the atmosphere. Activity concentrations up to 22 Bq/kg d.m. have been found. The ratio 207Bi: 137Cs(global fallout) equals (1.70 ± 0.12)10 −3, which is in accordance with literature data. When low levels of 207Bi are assessed by gamma spectrometry, corrections must be made for a gamma line produced in the lead shield by neutron activation due to cosmic neutrons.

The Hiroshima thermal-neutron discrepancy for 36Cl at large distances. Part I: New 36Cl measurements in granite samples exposed to A-bomb neutrons

The long-lived radioisotope (36)Cl (half-life: 301,000 years) was measured in granite samples exposed to A-bomb neutrons at distances from 94 to 1,591 m from the hypocenter in Hiroshima, by means of accelerator mass spectrometry (AMS). Measured (36)Cl/Cl ratios decrease from 1.6 x 10(-10) close to the hypocenter to about 1-2 x 10(-13), at a distance of 1,300 m from the hypocenter. At this distance and beyond the measured (36)Cl/Cl ratios do not change significantly and scatter around values of 1-2 x 10(-13). These findings suggest that the (36)Cl had been predominantly produced by thermalized neutrons from the A-bomb via neutron capture on stable (35)Cl, at distances from the hypocenter smaller than about 1,200 m. At larger distances, however, confounding processes induced by cosmic rays or neutrons from the decay of uranium and thorium become important. This hypothesis is theoretically and experimentally supported in a consecutive paper. The results are compared to calculations that are based on the most recent dosimetry system DS02. Close to the hypocenter, measured (36)Cl/Cl ratios are lower than those calculated, while they are significantly higher at large distances from the hypocenter. If the contribution of the cosmic rays and of the neutrons from the decay of uranium and thorium in the sample was subtracted, however, no significant deviation from the DS02 calculations was observed, at those distances. Thus, the Hiroshima neutron discrepancy reported in the literature for (36)Cl for samples from large distances from the hypocenter, i.e., higher measured (36)Cl/Cl ratios than predicted by the previous dosimetry system DS86, was not confirmed.

Monte-Carlo Calculation of the Response Functions for Two Prototype Cosmic Neutron Metrology Instruments

The response functions for two cosmic neutron detection systems have been calculated using Monte-Cario computational methods. The detection systems that form the focus of this research are modified Leake detector designs in which a central thermal neutron detector is surrounded by a sphere of high-density polyethylene. In this arrangement, the surrounding polyethylene moderates the incident fast neutrons that are then detected by the central detector; in this case a 3He-filled proportional counter. In order to extend the response of these detector systems to cater for cosmic neutron environments, a shell of high-Z material has been included in each to promote (n, xn) reactions in the polyethylene moderator. We have used shells of lead and copper for this purpose to bring the high-energy component of the cosmic field, extending up to several GeV, within the capability of the detector systems. In particular, copper has been used in comparison with lead since the former is easier and safer to machine and handle. The overall diameter of the instruments studied in this work is 208 mm.Calculations of the neutron response have been performed with MCNP4C, for the thermal - 20 MeV energy range, and with MCNPX 2.1.5 / LA150N neutron libraries for the higher-energy cosmic region of the spectrum beyond 20 MeV. The results of these calculations are compared with experimental data that have been recorded with the instruments at the CERN Cosmic Reference Field Facility (CERF), Geneva, Switzerland. This comparison is discussed in respect of the likely applications of these detector systems to high-energy neutron field measurement on-board aircraft and in the vicinity of high-energy particle accelerators. The former application is gaining considerable research attention following the revised estimates of relative biological effectiveness of cosmic neutron fields and the related recommendation that aircrew be regarded occupationally-exposed radiation workers, on behalf of the International Commission on Radiological Protection (ICRP) and the European Union. The latter application is of interest to accelerator operators and users who can be exposed to neutron fields with 100 MeV components as a result of scattering reactions from the materials comprising the accelerator and associated shielding. This is of growing concern to the medical field where the use of cyclotrons is often a key aspect of radiation treatment and therapy, in which the control and estimation of administered doses is a key objective.

Analysis of the neutron component at high altitude mountains using active and passive measurement devices

The European Council directive 96/29/Euratom requires dosimetric precautions if the effective dose exceeds 1 mSv/a. On an average, this value is exceeded by aircrew members. Roughly half of the radiation exposure at flight altitudes is caused by cosmic ray-induced neutrons. Active ( 6LiI(Eu)-scintillator) and passive (TLDs) Bonner sphere spectrometers were used to determine the neutron energy spectra atop Mt. Sonnblick (3105 m) and Mt. Kitzsteinhorn (3029 m). Further measurements in a mixed radiation field at CERN as well as in a proton beam of 62 MeV at Paul Scherrer Institute, Switzerland, confirmed that not only neutrons but also charged particles contribute to the readings of active detectors, whereas TLD-600 and TLD-700 in pair allow the determination of the thermal neutron flux. Unfolding of the detector data obtained atop both mountains shows two relative maxima around 1 MeV and 85 MeV, which have to be considered for the assessment of the biologically relevant dose equivalent. By convoluting the spectra with appropriate conversion functions the neutron dose equivalent rate was determined to be 150±15 nSv/h. The total dose equivalent rate determined by the HTR-method was 210±15 nSv/h. The results are in good agreement with LET-spectrometer and Sievert counter measurements carried out simultaneously.

Neutron-induced 10B fission as a major source of soft errors in high density SRAMs

The impact of cosmic neutron induced 10B fission in production submicron SRAM devices is reported for the first time. Using a cold neutron beam to accelerate soft error rate events, we unambiguously demonstrate that neutron induced 10B fission can be a significant source of soft errors in deep-submicron SRAMs fabricated with borophosphosilicate glass.

A Monte Carlo model of sea-level neutron background: Directionality, spectra, and intensity

We are developing a model of the cosmic ray-induced neutron backgroundat sea-level using the high-energy/Monte Carlo transport code MCNPX. The modelaccounts for and records anisotropy in neutron production from cosmic rayinteractions, as well as the known “ground effect” neutron albedo.Initial results with an isothermal, constant density atmosphere in slab geometryshow good agreement with experimental data on neutron intensity and low-energydirectionality predicted previously. Our model also indicates significantnet downward directionality above the energy range 0.1–1.0 MeV dependingon the sea-level material composition.

Improved Analysis of COMPTEL Solar Neutron Data, with Application to the 15 June 1991 Flare

Direct solar flare neutrons are a valuable diagnostic of high-energy ion acceleration in these events, and COMPTEL improves over all previous cosmic neutron detectors in its capacity for neutron energy measurement. Previous studies of COMPTEL neutron data have worked with an incomplete model of the instrumental response, applying energy-by-energy detection efficiencies. Here we employ statistical regularisation techniques with the full (Monte Carlo simulation derived) response matrix to produce improved estimates of neutron numbers and energy distribution. These techniques are applied to data from the well-observed 15 June 1991 flare. Our improved treatment of the instrumental response results in a reduction of 73% in total neutron numbers, compared with previously deduced values. Implications for the picture of primary ion acceleration in this flare are briefly discussed.

Cosmogenic nuclide production rate systematics in terrestrial materials: Present knowledge, needs and future actions for improvement

The growing applications of cosmogenic nuclides produced in the Earth’s atmosphere, and in situ in a variety of terrestrial materials, as tracers in a wide ranging Earth science problems, has put a greater demand on accurate determination of the nuclide production rates in a variety of targets exposed in different settings on the Earth in the troposphere. The present state of our knowledge of cosmogenic production rates is reviewed briefly, in conjunction with the phenomenological and theoretical framework for: (i) the cosmic ray flux incident in the near Earth environment, (ii) the nucleonic cascade set off in the Earth’s atmosphere by the primary cosmic radiation, and (iii) the rate for production of nuclides in terrestrial materials, in widely different settings. These considerations set the stage for the diverse questions, which must be taken into account for determining the source functions of isotopic changes in terrestrial materials. We discuss the different approaches which have been adopted earlier to obtain the source strengths of nuclear interacting particles of the cosmic radiation, and direct measurements of nuclide production rates made by exposing targets to cosmic radiation at sea level and at mountain altitudes. We show that rapid progress in determining nuclide source functions with sufficient information on temporal variability is indeed expected in the near future as a result of: (i) dramatic improvements in the past 2–3 decades in our understanding of the character of propagation of cosmic radiation within the heliosphere, (ii) experiments now being conducted by a few groups to determine the source strengths of cosmic ray slow neutrons, and nuclide production rates in cosmic ray exposed targets, and finally (iii) the emergence of better nuclear codes which deal with the development of nucleonic cascades in the Earth’s atmosphere.

Rapid Radiation Dose Determined by Bubble Neutron Dosemeters

The bubble neutron dosemeter is a new device that can yield quantitative neutron dose by the number of bubbles therein. The dosemeter is easy to carry and is re-usable. Bubble neutron dosemeters made by Bubble Technology Industries have been tested to determine their performance regarding saturation, durability and accumulated dose range. The bubble neutron dosemeters are used to map the neutron dose rate distribution in a nuclear reactor containment and to record in-flight cosmic neutron dose in commercial flights.

Cosmic neutron intensity at sea level near the zero degree geomagnetic latitude

The cosmic neutron intensity at sea level was monitored in the South China Sea where the vertical cut-off of geomagnetic rigidity is highest. The lowest neutron intensity, measured near the geomagnetic latitude at zero degrees, is 1.13 ± 0.12 n Sv h −1. Land survey on the nearby coral reef yields a lower reading. Shipboard data in the open sea are also compared to those measured in harbors and further inland. The progressive decrease of cosmic neutron dose rates from ship to ground indicates that the air-sea-land interfaces play an essential role in attenuating the cosmic neutron intensity.

Some radiation environment estimation data from 10–12 km altitude aircraft

The results of probing the radiation environment on board different civil aviation planes with singletype detectors (nuclear emulsions), with particular emphasis to the cosmic radiation flux measured inside aircraft, are presented. The measurement results make it possible to find the absorbed and equivalent doses induced by the cosmic radiation neutrons and charged particles.

Atmospheric Neutrons, Dose Equivalent and Average Build-up-attenuation Coefficient in Buildings (Estimated by CR-39 Detector).

Long term (three years from Aug. 1994 up to Sep. 1997) irradiated prechemically etched CR-39 solid state nuclear track detectors were used for the estimation of recoil proton track densities and related dose equivalent of atmospheric neutrons whose energies (Enabove-0.5 MeV which we call the effective fast neutrons) cause observable recoil proton tracks. The CR-39 detectors were exposed to the ordinary atmosphere and to up and down sides of building ceilings to estimate the total effect of buildings, ceilings and air voids, on the effective fast neutrons fluences. Also the attenuation of such neutrons within concrete was investigated. It was found that atmospheric cosmic neutrons of fluences above -0.5 Me V decrease with the increasing of concrete thicknesses (0-12 cm) . The average build up factor B times the attenuation terme-Σt.d (build up-attenuation coefficient) B⋅e-Σt.dwas calculated for each ceiling and for concrete blocks. The results give agreements with experiments done hitherto.

Cosmic‐ray‐induced neutron intensity in Taiwan and its variations during a typhoon

Cosmic neutrons as a secondary cosmic radiation, strike the ground with intensity which varies at different latitude, altitude, and solar activity. In a 3‐year extensive survey period, the cosmic neutron intensity was measured over Taiwan islandwide; the average neutron dose equivalent rate is 0.65 nSv h−1 near sea level, up to 14.5 nSv h−1 at the highest mountaintop (altitude of 3952 m). In a rare event, the cosmic neutron intensity at a monitoring station was recorded during the entire period that Typhoon Herb swept across the island in 1996. The anomalous reading of cosmic neutron intensity in the typhoon eye, 2.17±0.40 nSv h−1 (or 4 times higher than the normal reading) is reported here for the first time. The onset of cosmic neutron intensity in the typhoon is closely associated with the steady, rapid air pressure drop as well as the 6×106 km3 spiral cloud; the dips in the onset are correlated with sustained gale winds and heavy precipitation. It is believed that the air pressure drop provides thinner air within the typhoon, and the spiral cloud containing dense moisture further slows down high energy neutrons from the ground, where we observed the neutrons showering down from the stratosphere where the cosmic neutrons have the maximum density. These combined effects result in more medium‐energy neutrons penetrating down to the ground through the typhoon; while the gale wind and heavy rain deflect, attenuate, and, to a lesser extent, absorb the cosmic neutrons from the atmosphere.