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Year : 2017  |  Volume : 40  |  Issue : 3  |  Page : 121-125  

Thermal neutrons of the terrestrial origin in the Brazilian region

1 Department of Space Geophysics, Space Research Institute of the n Academy of Sciences (IKI) Moscow, Russia
2 Department of Physics, Technological Institute of Aeronautics (ITA), São José dos Campos, Brazil

Date of Submission03-Apr-2017
Date of Decision31-May-2017
Date of Acceptance11-Jul-2017
Date of Web Publication16-Feb-2018

Correspondence Address:
Anatoly Alexandrovich Gusev
Department of Space Geophysics, Space Research Institute of the Russian Academy of Sciences, IKI, 84/32, Profsoyuznaya Str, Moscow 117997
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/rpe.RPE_16_17

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Dynamics of near ground concentration of ambient thermal neutrons (TNs) was studied using data of neutron detectors of the COsmic-ray Soil Moisture Observing System (COSMOS) network and those of the Laboratory of environmental radiation of the Instituto Tecnológico de Aeronáutica (Brazil). A significant excess of TN count rate over that of rapid ones was found in two COSMOS probes located in Brazil and other sites in North America and Europe. The effect was explained in terms of neutron production in nuclear (α, n) reactions initiated by decay of radon gas comprised in the ground rather than in the atmosphere. A revealed strongly marked diurnal variation of the TN concentration proved to be in antiphase with air temperature that is characteristic for particles in thermal equilibrium with air. A previously unknown effect of saturation of nighttime TN concentration was observed. It is explained in terms of a dynamic equilibrium state, which sets in the absence of vertical convection between the neutrons escaping from the ground and their losses in nuclear reaction in the air. The results described can be used for the determination of ground concentration of ambient radon and its dynamics in boundary layer of atmosphere as well as in other geophysical studies.

Keywords: Atmosphere, radon, soil, thermal neutrons

How to cite this article:
Gusev AA, Martin IM. Thermal neutrons of the terrestrial origin in the Brazilian region. Radiat Prot Environ 2017;40:121-5

How to cite this URL:
Gusev AA, Martin IM. Thermal neutrons of the terrestrial origin in the Brazilian region. Radiat Prot Environ [serial online] 2017 [cited 2018 Mar 19];40:121-5. Available from: http://www.rpe.org.in/text.asp?2017/40/3/121/225579

  Introduction Top

Neutron radiation is a potentially dangerous health hazard due to its high penetration power and ability to produce radionuclides in most substances, including the body tissues. The main part of the neutrons of the natural origin is produced in air showers initiated by cosmic rays. For several decades after their discovery, the interest in atmospheric neutrons was limited by studies in nuclear physics and astrophysics. Some new application of ambient neutron observation emerged at the end of the last century, propelled by the concern on the exposure of aircrews to atmospheric cosmic radiation.[1],[2] At altitudes of commercial airlines atmospheric neutrons account for about half of the equivalent dose. Atmospheric neutron flux depends on above mean sea level (AMSL) elevation and initial energies of cosmic ray particles entering the atmosphere. A cosmic ray energy spectrum is controlled by a local geomagnetic cutoff rigidity.

Currently, the atmospheric neutron background of cosmic ray origin is quite well described, that allow its quite accurate simulation for ≥1 MeV energies at distances exceeding ≈1 km from the ground [3],[4] (do not confuse with the AMSL). At smaller than 1 km distances from the ground, the neutron flux is added with one more component-albedo neutrons. These are neutrons of the same atmospheric origin but scattered and moderated in the ground or produced by cascade energetic particles in nuclear reaction with the ground substance. The albedo flux constitutes a distinct peak of thermal (≈2.5 × 10−8 MeV) and epithermal energies (≈2.5 × 10−8–4 × 10−7 MeV) in the lethargy (i.e., neutron flux × energy vs. energy) spectrum [Figure 1][5] increasing as a distance from the ground decreases.
Figure 1: Lethargy (i.e., flux × neutron energy vs. energy) neutron spectrum measured in Sao Jose dos Campos.[5] A distinct peak at 2 × 10−7 MeV is due to thermal and epithermal albedo neutrons

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The albedo thermal neutron (TN) production rate is believed to be quite stable and be controlled by presence in the soil of elements such as B, Cl, K, and Gd, effectively absorbing the neutrons. Being in thermal equilibrium with the air, the TNs participate in atmospheric motion that sets their dynamics quite apart from the faster neutron one.

Opposite to the TNs, the egress of albedo of higher energies ones, essentially depends on soil moisture and ground cover (vegetation, water, snow, and ice) due to the ability of hydrogen-containing compounds effectively slow down neutrons.[6],[7] This leads to an inverse dependence of the fast neutron flux on soil moisture that is the basis of a noninvasive method of measuring of moisture content. [8]

Both the direct and albedo neutron fluxes are proportional to a total energy of the parent cosmic ray particles.[9] Due to that, a corresponding near ground ratio of thermal and fast neutron fluxes does not vary with AMSL, under the same soil composition, moisture, and cover. Depending on the latter characteristics, numerical simulations result in the ratio within 0.3–1.[7] A higher ratio indicates a potential presence of a TN source different of the atmospheric one.

A natural source of such neutrons could be nuclear reactions of α-particles produced in the decays of radon gas diffusing from the Earth's crust with ground substance ([α, n] reactions). This component of natural radiation was studied only in a few experiments [10],[11] aimed to reveal a potential connection between a seismic activity and transient increases of near-ground neutron intensity not related with cosmic ray variations: it is supposed that the crust deformations provoked by seismic activity results in additional radon release or/and piezonuclear fission reactions.[12] It should be noted that all that studies were performed indoor, and the weather-related neutron dynamics was out of their scope.

Biosphere processes not being in themselves a source of neutrons at the same time significantly affect the dynamics of the near surface neutron fluxes.[13] For example, several events of significant increase (up to two orders) of TN concentration with the duration of minutes to hours were observed in forest biocenoses.[14] A similar phenomenon was registered over phytoplankton fields in Atlantic.[15] It is supposed that this effect may be used to assess a biomass volume and composition.[16]

Results of the studies [17],[18] indicate that even weak TN fluxes affect metabolic and physiological status of the organisms causing their potential sensitivity to both space and terrestrial weather. The near-ground neutron flux becomes the object of a growing interest in recent years, due to a threat which thermal and epithermal neutrons pose for microelectronic devices: soft-fail studies show that ambient neutrons became the main cause (more important than α-particles) of these errors in SRAM and DRAM units with boron glass.[19] Due to a permanent increase of number of microelectronic devices in use, it becomes a serious problem.

The above demonstrates a variety of phenomena determining TN concentration in the near-ground atmosphere. The aim of the present work is to reveal weather-related effect on near ground outdoor TN concentration.

  Data and Method Top

The work is based on the data of a continental scale COsmic-ray Soil Moisture Observing System (COSMOS) network of soil moisture monitoring by measuring a flux of ambient fast neutrons.[20] The network includes several tens of probes located in the USA, Europe, Africa, Australia, and Brazil. The neutrons are observed in two energy ranges: TN s (parameter UNMO) are registered with a bare 3 He or BF3 proportional counter (sensitive area of 0.3 m 2) and the fast ones (parameter MOD)-with the same type one but shielded with 2.5 cm of a polyethylene moderator. Data of the shielded detector are used for soil moisture simulation. The bare counter is designed for calibration. The probes are also equipped with barometric pressure, humidity, and temperature sensors (parameters PR, RH, and TEM). Data sampling interval is 1 h.

The work also uses results of indoor and outdoor neutron measurements with a set of 3 He counters (total sensitive area of 0.0250 m 2) performed in the Laboratory of environmental radiation of the Instituto Tecnológico de Aeronáutica (ITA), in Sao Jose dos Campos, Brazil.

  Results Top

[Table 1] lists probes with an increased (>1) thermal to fast neutron ratio (UNMO/MOD). The cutoff rigidity parameter determines a primary cosmic ray flux reaching the atmosphere boundary from space above the probe (the lower cutoff value, the higher the flux); the relative cosmic ray flux is proportional to a secondary particles flux incident on the ground in the same place (refer introduction part). One can see no apparent correlation of these parameters with the registered neutron count rates (that is number of neutrons registered by a detector per time units). This speaks in favor that the observed TN fluxes are governed rather by local soil parameters than the incident fluxes of secondary atmospheric particles.
Table 1: The station geophysics parameters and measured neutron fluxes

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Typical examples of the corresponding count rate records for the TNs are depicted in [Figure 2]. One can see that a very pronounced diurnal variation of the TNs is a clear antiphase with the atmospheric temperature.
Figure 2: Examples of the diurnal variations of the thermal neutron count rates and the temperature observed in CPTEC and UFSM sites. The temperature scale is inverted for more clear demonstration of anticorrelation between the parameters

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[Figure 3] and [Figure 4] show diurnal TN variation observed outdoor and indoor in the ITA site.
Figure 3: An example of the outdoor diurnal variation of the thermal neutron count rate in Instituto Tecnológico de Aeronáutica site

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Figure 4: Daily profile of the indoor thermal neutron flux in Instituto Tecnológico de Aeronáutica site obtained with superposition epoche method is shown with gray and thick black curves. Corresponding result of the Barsan group is shown with a thin black curve. The numbers in bold denout amplitudes of daily variations

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  Discussion Top

The increased UNMO/MOD ratio suggests the presence of a neutron source different of the atmospheric one. The only known source of natural TN, which could be specified as an alternative to the atmospheric one is nuclear reactions initiated by decay of 220 Rn and 222 Rn, already mentioned in the introduction.

A profile of the diurnal variation presented in [Figure 2] is characteristic for particles (molecules, clusters, and aerosols) in thermal equilibrium state with the atmospheric atoms [21] and thus participating in their collective motion. Such a type of variation results from vertical convection in the boundary layer of the atmosphere. The convection is controlled in turn by a vertical temperature gradient determined by a temperature difference between the air and ground surface. In a rough approximation the higher the ground temperature, the stronger the convection and the smaller the particle concentrations.

In substance, the observed TN concentration variation is identical to that of outdoor radon.[22],[23] However, the atmospheric radon, its progenies and other radionuclides suspended in the air in free state or attached to aerosol do not produce neutrons because energies of decay α-particles are less than energy threshold for (α, n) reactions for nitrogen and oxygen. Opposite to that corresponding cross-section values for such soil constituencies as Al, Fe, and Na exceed hundreds millibarns.[24]

Characteristic thickness of a soil slab, from which the generated neutrons can reach the ground-atmosphere interface, is determined by neutron slow-down and diffusion lengths, which are of tens centimeters. The radon concentration at this depths increases by 10%–100%[25],[26] in comparison with that at the ground surface, and this difference is greater, the smaller the radon intensity at the ground surface (Pearson correlation coefficient is −0.7. The effect trivially implies diffusion propagation in soil). In this way, the atmospheric TN concentration should be related with the radon concentration in corresponding soil layer. This is confirmed by direct observation of a strong anticorrelation between temperature gradient and radon concentration at depth of 5 cm, where radon concentration was twenty times higher than at the ground surface.[26] Thus, it is to be expected that an indoor TN concentration is also related with chemical and physical properties of surrounding radon-containing construction materials rather than with local radon concentration.

Simultaneous indoor and outdoor observations of TN concentration performed in ITA are consistent in general with the above consideration: An outdoor counter [Figure 3] registered a distinct diurnal variation of the same pattern as in [Figure 2]. In the same time, the analogs counter located at the second floor of the concrete building [Figure 4] registered quite a weak variation with an afternoon maximum similar that observed indoor in Baksan group [27] A different pattern of the indoor variation is clearly explained by the absence of indoor convection.

In contrast to radon, whose lifetime is constant, the TN lifetime is controlled by losses in radioactive capture in matter and due to that proved to be dependent on density of the latter. It is ~1 ms in soil and ~100 ms in air. This results in a faster decrease of TN concentration with altitude. According to [28] the average, TN concentration at 27.6 m (canopy height) of altitude is three times less than at the ground surface. The maximal decrease of the radon concentration at this altitude happened at 18 h local time is only 1.5.[29]

Another effect related with a shorter TN lifetime is a well-pronounced saturation of the morning maxima corresponding to the temperature minima. In fact, just because of that the TN daily minima in [Figure 2] anticorrelates (Pearson coefficient is −0.9) with the day temperature amplitude (i.e., a residual of a daily temperature maximum and minimum), rather than with the daily temperature itself. The effect is explained by very small or negative nighttime vertical convection because a cooled ground surface that shut down the neutron outflow. Due to that a dynamic equilibrium between emerging and lost neutrons establishes just in ≈0.3 s. For 222 Rn it needs ≈16 days that is the reason of the absence of saturation in the radon daily variation.

The neutron fluxes, i.e., (count rate per se nsitive area) registered by the CPTEC, UFSM, and ITA detectors are about 15, 20, and 1.5 n/m 2 s, respectively. In the last case, the counters were wrapped with paraffin moderator decreasing the registered flux by at least an order of magnitude. With this in mind, the agreement between the measurements looks quite satisfactory.

Indore measurements and Monte-Carlo simulation performed by Baksan group [27] resulted in a TN flux value of 1–4 n/m 2 s. Taking into account difference in experimental techniques used (solid-state scintillation detector instead of a gas- discharge counter), building construction, and difference in chemical and physical characteristics of the soil, the result could be considered to be consistent with the above and not contradictory to the hypothesis of the ground radon origin of TNs. The simulation is also reproduces an abrupt increase of the TN concentration while crossing soil-air interface, related with corresponding increase in neutron lifetime.

  Conclusions Top

Data of TN detectors located in Brazil were analyzed. A strong diurnal variation of the outdoor TN concentration was revealed. This variation pattern is typical for constituents in thermal equilibrium and is controlled by vertical convection in the boundary layer of the atmosphere. No analogs indoor variation was observed that is explained by atmospheric convection absence. A hypothesis of neutron production in nuclear reaction initiated radon decay in the upper slab of the ground is considered and was found to be consistent both with this observation and published data. A short TN lifetime in the air results in a previously unknown effect of saturation of the night TN concentration.

The effects described can be useful in studying of radon propagation in soil and physical and chemical characteristics of the latter as well as in other geophysical studies related with radon dynamics.


The authors are grateful to the COSMOS staff for (http://cosmos.hwr.arizona.edu) providing access to the probes data and to the ITA-IEEF and CNPq (Proc. 303511/03-6, Proc. 505756/03-0) supported this work.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4]

  [Table 1]


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