11institutetext: Center for Space Research, North-West University,
Potchefstroom, South Africa 11email<EMAIL_ADDRESS>22institutetext:
Space Physics and Astronomy Research Unit, University of Oulu, Finland
33institutetext: Sodankylä Geophysical Observatory, University of Oulu,
Finland 44institutetext: Institut für Experimentelle und Angewandte Physik,
Universität Kiel, Kiel, Germany 55institutetext: South African National Space
Agency, Hermanus, South Africa 66institutetext: Universidad de Alcalá (UAH),
Dpto. Física y Matemáticas and Dpto. Automática, Campus Científico-
Tecnolóógico (Externo), Alcalá de Henares, Madrid, Spain 77institutetext:
Instituto de Geofísica, Universidad Nacional Autónoma de México, México D.F.,
México 88institutetext: Deutsches Elektronen-Synchrotron DESY in Zeuthen,
Germany 99institutetext: Department of Space Science and Applications
Research, Ethiopian Space Science and Technology Institute, Addis Ababa,
Ethiopia 1010institutetext: Department of Chemistry “Ugo Schiff”, University
of Florence, Italy

# The mini-neutron monitor: A new approach in neutron monitor design

Du Toit Strauss Corresponding author: 11<EMAIL_ADDRESS>Stepan
Poluianov 2233 Cobus van der Merwe 11 Hendrik Krüger 11 Corrie Diedericks 11
Helena Krüger 11 Ilya Usoskin 2233 Bernd Heber 44 Rendani Nndanganeni 55
Juanjo Blanco-Ávalos 66 Ignacio García-Tejedor 66 Konstantin Herbst 44 Rogelio
Caballero-Lopez 77 Katlego Moloto 11 Alejandro Lara 77 Michael Walter 88
Nigussie Mezgebe Giday 99 Rita Traversi 1010

###### Abstract

The near-Earth cosmic ray flux has been monitored for more than 70 years by a
network of ground-based neutron monitors (NMs). With the ever-increasing
importance of quantifying the radiation risk and effects of cosmic rays for,
e.g., air and space-travel, it is essential to continue operating the existing
NM stations, while expanding this crucial network. In this paper, we discuss a
smaller and cost-effective version of the traditional NM, the mini-NM. These
monitors can be deployed with ease, even to extremely remote locations, where
they operate in a semi-autonomous fashion. We believe that the mini-NM,
therefore, offers the opportunity to increase the sensitivity and expand the
coverage of the existing NM network, making this network more suitable to
near-real-time monitoring for space weather applications. In this paper, we
present the technical details of the mini-NM’s design and operation, and
present a summary of the initial tests and science results.

###### keywords:

neutron monitors – space weather instrumentation – cosmic rays – neutron
monitor multiplicity

##  1 Introduction

Cosmic rays, including sporadic solar energetic particle (SEP) events, are the
dominant source of hazardous atmospheric radiation at, and above, aviation
altitudes, making the long-term monitoring of cosmic ray levels essential. The
latter is especially important since the International Civil Aviation
Organization’s (ICAO’s)111ICAO State Letter, Proposals for the amendment of
Annex 3 and consequential Amendments to Annex 15, PANS‐ABC and PANS‐ATM, AN
10/1‐17/41, 7 April 2017. recent amendments to the meteorological procedures
to include space weather warnings and/or predictions as part of flight
planning. A reliable near real-time estimate of the cosmic ray flux is
therefore essential. Moreover, ICAO requires ground-based cosmic ray
observations that can serve as a proxy for the levels of radiation at aviation
altitudes, making the neutron monitor (NM) a reference space weather
instrument for aviation dosimetry studies, but also more generally for
monitoring the levels of cosmic radiation that may cause unwanted effects,
including single-event upsets (see e.g. Taber and Normand, 1993; Dyer et al.,
2007, 2009), in avionics and other sensitive electronics devices (see also the
IEC 62396-1:2016 standard222Available at
https://webstore.iec.ch/publication/24053.).

For more than 70 years, NMs have been the most cost-effective instrument to
monitor the levels of cosmic radiation near Earth (see the recent review by
Bütikofer, 2018). These instruments, however, do not measure the cosmic
radiation directly, but rather the flux of secondary and tertiary neutrons
produced in the atmosphere, and in the detector itself. The count rate of an
NM can thus be written, in the most basic formulation, as

$N(P_{c},t)=\sum_{i}\int_{P_{c}}^{\infty}j_{i}(P,t)Y_{i}(P,t,\ldots)dP,$ (1)

where $P_{c}$ is the cutoff rigidity (minimum rigidity particle that can reach
the detector), $i$ represents the particle distribution under consideration
(e.g. protons and heavier nuclei), and $Y_{i}(P,t,\ldots)$ is the so-called
yield function that represents the response of the instrument on the unit flux
of primary cosmic rays with rigidity $P$, including atmospheric and
instrumental effects. The latter therefore depends on various parameters that
are unique to each monitor (see e.g. Caballero-Lopez, 2016; Clem and Dorman,
2000). In order to reconstruct the primary cosmic ray flux, $j_{i}(P,t)$, the
yield function must be known for a number of stations characterized by
different $P_{c}$ values (e.g. Mishev et al., 2020). Once the atmospheric
yield functions are known, continuous spectral information about cosmic ray
modulation (primary spectra above the atmosphere and magnetosphere) can be
obtained from neutron monitor differential response functions. These response
functions can be derived from the count rates of the world-wide neutron
monitor network if individual NMs are inter-calibrated (e.g. Moraal et al.,
2001).

To inter-calibrate the world-wide network of NMs, Moraal et al. (2001)
initiated the development of a smaller and lighter version of the standard
neutron monitor, a calibration NM. The aim was to design a portable NM that
can easily be shipped and installed, even at very remote locations. Although
this calibration process was mostly successful (e.g. Krüger et al., 2003), it
was quickly realized that, under the right circumstances, a calibration NM’s
data can be of sufficient quality to supplement the measurements of
traditional NMs. It was subsequently proposed by Krüger et al. (2015) to refer
to these smaller monitors as mini-NMs, with mini-NMs installed at the
Concordia (Poluianov et al., 2015; Usoskin et al., 2015) and Neumayer III
(Heber et al., 2015) research stations in Antarctica. Since then, the mini-
NM’s design has been constantly optimized and the counting electronics
updated. In this paper, we describe the most recent version of the mini-NM,
including mechanical design, electronics systems, and software implemented,
along with initial tests of the data acquired.

Figure 1: A sketch of the standard mini-NM set-up. The reflector (yellow),
moderator (black), front- and end-cap (blue and green), and lead rings (dark
and light gray) are indicated. Figure 2: Mini-NMs undergoing testing. The
front covers were removed on two monitors to show the lead rings and the
internal moderators.

##  2 Basic neutron monitor design

The design of the basic NM has changed very little over the years since its
first development (see the review by Simpson, 2000), until the most recent
implementation (e.g. Signoretti and Storini, 2011; Medina et al., 2013; Jung
et al., 2016), consisting of an outer reflector (shielding the instrument from
external thermal neutrons, while containing neutrons produced within the NM),
a lead producer (producing additional low energy neutrons via nuclear
interactions to increase the monitor’s detection efficiency), and an inner
moderator (thermalizing the newly produced neutrons). The thermal neutrons are
then counted via pulses produced in a proportional counter tube, traditionally
filled with either BF3 or 3He gas. For both of these filling gases, an
electrical pulse is produced via a neutron capture process (Knoll, 2010), i.e.

${}^{10}\mathrm{B}+\mathrm{n}$ $\displaystyle\rightarrow$
$\displaystyle\left\\{\begin{array}[]{ccc}^{7}\mathrm{Li}+\alpha+2.78\,\mathrm{MeV}\\\
^{7}\mathrm{Li}^{*}+\alpha+2.30\,\mathrm{MeV}\end{array}\right.$ (4)

or

${}^{3}\mathrm{He}+\mathrm{n}\rightarrow\,^{3}\mathrm{H}+\mathrm{p}+0.76\,\mathrm{MeV}$
(5)

that ionizes a small portion of the filling gas. Note that only a small
fraction ($\sim 6\%$) of the reactions leaves 7Li in the ground state,
releasing 2.78 MeV in kinetic energy, while the majority of the reactions
leave 7Li in the first excited state, releasing only 2.3 MeV in kinetic
energy. Some NMs, sometimes referred to as neutron moderated detectors (NMDs),
do not have the lead producer and are sensitive to neutrons in a slightly
lower energy range. These NMDs are referred to as “bare”, lead-free detectors,
or even “wax” NMs in certain texts where the reflector and moderator in older
set-ups were manufactured from paraffin wax. The physics of neutron detection
in NMs and NMDs are identical, but the count rates of NMDs are usually lower,
and the instrument responds to lower energy incident particles, due to the
absence of a producer.

###  2.1 Mechanical design

A sketch of the standard mini-NM design is shown in Figure 1. The outer
reflector consists of the main cylinder (yellow) and front and end covers
(blue and green), manufactured from 75mm thick rolled polyethylene sheets.
Lead rings of a maximum diameter of 206mm are shown in light and dark gray,
while the inner moderator (black) is also manufactured from rolled
polyethylene sheets of thickness 15mm. The inner diameter of the lead rings,
and the diameter of the moderator, is determined by the type of tube used in
the monitor. For the purpose of this study, we have used three different
tubes, with details summarized in Table 1. The completed mini-NM set-up, while
undergoing testing, is shown in Figure 2. The dimensions of a completed mini-
NM, with a trolley stands, are approximately 900mm (height) $\times$ 700mm
(width) $\times$ 800mm (length) and the weight is approximately 250/100 kg
with/without the lead rings.

The volume of a mini-NM detector is notably smaller than that of a full-size
standard NM64 (the 64 referring to the standardized NM set-up implemented in
1964 during the International Quiet Sun Year; IQSY). However, with a change in
regulations of the ICAO, related to the transport of dangerous
goods333https://www.icao.int/safety/DangerousGoods/Pages/DGP23-Report.aspx, it
became possible to significantly increase the allowed maximal pressure of the
BF3 tubes. The pressure in a mini-NM detector LND2043 is 930 mbar, while it is
only 300 mbar in a standard NM64 detector. This partly compensates for the
reduced efficiency of the mini-NM as compared to the much larger full-size
NM64.

Table 1: Comparison of the different tubes used for testinga.

Tube naming |  | Gas | Outer diameter (mm) | Length (mm) | Recommended voltage (V)
---|---|---|---|---|---
BF3 | LND2043 | BF3 | 89 | 697 | 2120 – 2370
BF3(thin) | LND20366 | BF3 | 51 | 652 | 900 – 1150
He3 | LND25373 |  3He | 51 | 652 | 1200 – 1450

  * a

Tubes are manufactured by LND Inc., USA and available through
https://www.lndinc.com/.

Figure 3: The left panel shows a schematic representation of the mini-NM
electronics head while the right panel shows the actual components, with the
front cover removed.

###  2.2 Electronics

Although the mechanical design of a mini-NM, similar to a standard NM, is
simple, the associated electronics are usually the most complicated part. In
designing the new version of the mini-NM’s electronics, it was decided to use,
as much as possible, affordable, off-the-shelf components. In addition, it was
decided to design the electronics in a modular fashion, where defective parts
can be quickly (and cheaply) replaced. A sketch of the electronics head is
shown in the left panel of Figure 3, while the right panel shows a finished
unit. The different components are briefly discussed below.

####  2.2.1 Raspberry Pi micro-computer

A Raspberry Pi 3 B444https://www.raspberrypi.org/, a fully-functional single-
board computer with GNU/Linux operating system is used to interface with the
system components. Not only does the Raspberry Pi offer a significant amount
of computing power but it also simplifies a number of data processing tasks:
The on-board computing power enables data processing and data validation in
real-time. This allows functionality like digital filters and enables units to
alert or automatically reset if an error occurs. The built in network
interface allows for remote monitoring, administration, and data
synchronisation. The USB (universal serial bus) interface allows for a local
data backup when network connectivity is interrupted or unavailable. The
weakness of the Raspberry Pi is the limited write cycles on the SD (secure
digital) card. This can be overcome by using Linux shared memory for
intermediate data recording and higher quality (e.g. industrial rated) SD
cards.

####  2.2.2 High voltage supply

The high voltage supply consists of a microprocessor and a set of voltage
doubler circuits also known as a Cockroft-Walton multiplier. A PID
(proportional integral derivative) controller adjusts a pulse width modulated
input signal to regulate the output voltage. The desired voltage is sent to
the microcontroller over a serial connection, and the voltage value is
returned to the Raspberry Pi. The output range of the high voltage supply can
be set between 0.8 – 4 kV, where the maximum voltage is also dependent on the
inductance of the transformer, which can also be adjusted. The 5 V signal from
the microcontroller switches a transistor which supplies a flyback transformer
before the multiplication stage. This high-voltage power supply design is
feasible as the neutron monitor tube requires very little current. The only
real concern is the long term voltage stability and minimal high-frequency
noise, which is less than 1 mV on 4 kV.

####  2.2.3 Pre-amplifier

The pre-amplifier uses a low noise MOSFET (metal-oxide-semiconductor field-
effect transistor) for its first amplification stage. A DC (direct current)
blocking capacitor is used to filter the pulses from the high voltage on the
neutron monitor cathode. The MOSFET’s low gate current allows the filtered
pulses to trigger the amplifier. Careful design considerations were made with
regard to the input impedance and sensitivity of the pre-amplifier. The input
impedance determines the shape of the amplified pulse, and over biasing will
make the MOSFET too sensitive with the result that amplified noise will then
dominate the later stages of the system. The pre-amplifier is a critical
component, and variations between heads are most likely related to
manufacturing tolerances in the MOSFET and its biasing components. Band-pass
filters are used in the second stage of amplification to suppress any noise
from the pre-amplifier and to shape the amplified pulses.

####  2.2.4 Analogue-to-digital converter

A dedicated PIC32 microcontroller is used to sample the incoming pulses, after
amplification, with a 10-bit analog-to-digital converter (ADC). The pulses are
sampled at 2 MHz and stored in the microcontroller’s buffer. The sampling
discriminator can be set in software, only triggering samples above the set
value. This software filter allows for further elimination of noisy data. The
Raspberry Pi software interfaces with the PIC32 via SPI (serial peripheral
interface) and clears the sample buffer. The raw pulse data is stored and
processed by the Raspberry Pi software.

####  2.2.5 Peripherals

The environmental conditions are measured with two
Adafruit555https://www.adafruit.com/product/2652 BME280 sensor boards (the BME
sensor itself is produced by Bosch.666https://www.bosch-
sensortec.com/products/environmental-sensors/gas-sensors-bme680/) The BME280
sensor has a maximum absolute offset of 1 mbar, and a relative error of 0.12
mbar. We have, however, performed long-term tests of this sensor and have
found any possible offsets to be constant over time. The relative (noise)
error is further decreased in the mini-NM by sampling the pressure every
second and using one-minute averages for all calculations. We believe that the
BME sensor provides a very affordable and accurate alternative to the more
expensive barometers currently used. Data is sent to the Raspberry Pi via an
I2C (inter-integrated circuit) connection. Internal (meant to be placed
against the tube inside of the moderator) and external (placed inside the
electronics enclosure) pressure/temperature sensors are available. A redundant
pressure measurement can also help with the identification of any possible
absolute calibration issues when installing a new pressure sensor or to test
for possible drifts in a defective sensor.

The default time synchronization is realized with the Network Time Protocol
(NTP), when the mini-NM receives time signals from an NTP server via a network
connection. However, there is an optional GPS (global positioning system) unit
that can be connected via a USB connector on the Raspberry Pi that sends
location data and a signal for time synchronisation (when an NTP server is not
available) over the USB-serial connection.

###  2.3 Software

Similar to the electronics, the software used on the mini-NM was designed to
be modular with scripts written using the Python programming language
777https://www.python.org/ or simple shell scripts. Specific scripts,
therefore, have very specific purposes, e.g., a stand-alone Python script is
used to read the raw GPS data over a serial connection and write the raw data
to a file located on the Raspberry Pi. Similar scripts read data from the ADC,
the BME sensors, and communicate with the Arduino nano
888https://www.arduino.cc/en/Guide/ArduinoNano which controls the HV. Once the
raw data are collected in their native time resolution, the data is averaged
into one-minute time resolution.

These scripts are continually being refined and updated, with the latest
version available through a Creative Commons non-commercial
license999https://fskbhe1.puk.ac.za/neutronmonitor/Mini_NM/Software_images/.

Figure 4: Left panel: A selection of pulses registered from the thin BF3 tube.
Red pulses are assumed to be observations, while blue pulses indicate noise.
Right panel: A 2D pulse width-amplitude histogram as registered from the same
tube. The distribution is also projected onto each plane. The dashed lines
show the cut applied to the pulses in order to discriminate between
measurements and noise. On the projected histograms, all the pulses are
indicated by a black histogram, while the red histogram indicated pulses
assumed to be measurements free from noise.

Figure 5: The waiting time distribution (blue histogram) for cosmic ray
neutrons as observed with the 3He tube (left panel) and for the test source
(right panel). The red curves show fitted Poisson distributions, while the
dashed gray and green lines indicate bins containing either 1 or 10 counts.

Figure 6: The dependence of the observed count rate on variations of the HV,
as measured for different tubes. The average count rate for each tube is
indicated by the dashed line, while the red line shows the results of a linear
regression. For the middle panel, green data points were excluded when
performing the regression.

##  3 Testing: Neutron source

Initial testing of the mini-NM was performed with an 241Am-9Be neutron source
to improve measurement statistics, but also to eliminate any possible
environmental dependences.

###  3.1 Pulse height-width diagrams

The left panel of Figure 4 shows a selection of pulses read-out of the mini-NM
with the thin BF3 tube inserted. Based on the maximum pulse height and length
(i.e., width) of the pulses, they are divided into counts (indicated by red)
and noise (blue curves). The ADC threshold is indicated on this figure as the
dashed line at 0.2 V. Once the ADC voltage crosses this threshold, the start
of a pulse is registered, and the pulse is digitized until the voltage drops
below this level. For each pulse, the amplitude (i.e., maximum pulse height)
and length are recorded, and, as discussed in the next section, this is used
to discriminate between data and noise.

The right panel of Figure 4 shows a binned 2D histogram of the pulse width and
pulse histogram, again for the thin BF3 tube. The distributions are also
projected onto each plane. From the 2D histogram, we can clearly identify low
amplitude noise below $\sim 0.5$ V, consisting of short and longer pulses.

Two peaks are observed in the pulse width-amplitude plot, near 1.6 V and 2 V,
which is presumably due to the decay into 7Li∗ and 7Li, respectively. Also
note the continuous decreasing pulse amplitude towards lower pulse amplitudes,
until the noise threshold is reached. This is, most likely, due to the wall
effect, resulting in some fraction of energy being deposited into the wall of
the detector (see the discussion by e.g. Knoll, 2010).

By calculating and monitoring these distributions on, e.g., an hourly basis,
allows us to determine and track the accuracy of the detector: Any external
noise (e.g., mechanical vibrations or high-voltage ripple currents) will
change the shape of the distributions, allowing us to identify and correct for
such events.

Visualizing the pulses in such a 2D plane also allows us to perform a detailed
separation of the pulses into noise and/or data (counts) by applying an
appropriate cut through the pulse amplitude/width plane. As an example, here
we perform a simple square cut (indicated by the dashed lines) and count all
the pulses in these regions as data. The projections show all the pulses as
black histograms, and selected data as the red histograms. In the future, more
complex separation algorithms can be applied to the pulses.

###  3.2 Multiplicities

The new hardware design allows us to also examine the waiting time, $\Delta
t$, between pulses down to the low $\mu$s range. For the purpose of this
study, we define the waiting time simply as the time between pulses, i.e. the
time that elapses from the end of one pulse (ADC drops below the threshold)
and the onset of the next (ADC crosses the threshold). The electronics sample
at a constant rate of 2MHz, and, in principle, the detector dead-time is only
determined by the length of the pulses observed. In a standard NM with the
lead producer in place, these so-called multiplicity plots show a double-
Poisson distribution. Using cosmic ray measurements from the 3He tube, Figure
5 shows a distribution of the waiting time between pulses for a full day of
observation of cosmic ray neutrons as the blue histogram in the left panel.
The grey and green dashed lines indicate bins with either 1 or 10 counts (data
below these lines are considered statistically insignificant). The shaded
parts of the distribution is used to fit two separate Poisson distributions in
the form

$\mathcal{D}(\Delta t)=\mathcal{D}_{0}\exp\left(\frac{\Delta t-\Delta
t_{0}}{\tau}\right),$ (6)

with the resulting values of $\tau$ indicated in the legend. The calculated
distributions seem consistent with previous observations (e.g. Bieber et al.,
2004), where the low $\Delta t$ distribution is formed by evaporated neutrons
produced in the lead producer. These neutrons have a high level of
multiplicity, i.e., every incident high-energy proton leads to the formation
of a number of neutrons. The high $\Delta t$ distribution is formed by a
combination of low energy neutrons created in the atmosphere, and low-
multiplicity neutrons created in the lead producer by low energy protons.

The results for the neutron test source (right panel) shows only a single
Poisson distribution, as these low energy neutrons probably interact directly
with the 3He gas, and do not undergo nuclear reactions with the producer.

###  3.3 High-voltage plateau

We run the mini-NM tube in the proportionality mode and set the voltage near
the observed (and recommended) plateau-region (see, e.g., Kamal, 2014).
Although the different tubes’ recommended operating voltage regime is given in
Table 1, we have also tested the response of each tube’s count rate to voltage
changes to characterize any possible changes in the count rate due to changing
voltages, and to make sure the operating voltages are, in fact, within the HV
plateau for each tube. As a default setting, we operate the tubes at the
following voltages: BF3 @ 2300 V, BF3 (thin) @ 1100 V, and 3He @ 1350 V.
However, with the HV system discussed above, it is easy to adjust this value
with simply changing a variable in the corresponding software script.

The results of the changing HV values are shown in Figure 6. For each tube the
count rate was taken for five minutes at each voltage, and where the tests
were still performed in the presence of the neutron source. The top panel
shows the results for the thin BF3 tube, and these indicate that, for this
tube and in this voltage range, there is virtually no response in the count
rate. A linear regression is performed (red line), resulting in a gradient of
$<0.001$ %/V. Similar analyses for the 3He (middle panel) and thick BF3
(bottom panel) tubes give gradients of $0.01$ %/V and $0.03$ %/V,
respectively. Keeping in mind that the HV typically varies less than 1 – 3
V/min, any errors from a slowly changing HV are indeed negligible within the
voltage range shown here.

Figure 7: The top panel shows the calculation of the barometric coefficient
(the values indicated in the legend) for the different tubes (the thin BF3
indicated by green symbols and lines, the BF3 tube by blue, and the results
for the 3He tube by red). The bottom panel is similar, but now shows a test
for any temperature dependence.  Figure 8: Probability distribution of the
count rate for each tube (histograms), fitted with a Gaussian curve (dashed
lines). The legend shows the average and standard deviation of each fit.

##  4 Cosmic ray measurements: Laboratory testing

After testing with the neutron source, all three tubes were operated over
December 2019 in the physics laboratory to test the pressure and temperature
dependence of each tube. We also compare the count rates of the different
tubes, this time using cosmic ray neutron measurements over a time interval
where the cosmic ray levels were approximately constant. The tests were done
in Potchefstroom, South Africa, at a cutoff rigidity of $P_{c}\sim 7.2$ GV
($26^{\circ}$ 43’ S, $27^{\circ}$ 06’ E, 1335 m.a.s.l.).

###  4.1 Pressure correction

The primary correction that needs to be performed on the count rate is a
pressure correction due to the changing atmospheric depth above the monitor.
The measured count rate, $N$, changes according to the following relation,

$N=\langle N\rangle\exp\left[-\beta\left(P-\langle P\rangle\right)\right],$
(7)

where $P$ indicates pressure, $\beta$ is the barometric coefficient of each
tube, and $\langle P\rangle$ indicates the reference pressure (average over a
long-time interval). Note that $-\Delta P=-(P-\langle P\rangle)$. More details
can be found in e.g. Carmichael et al. (1968), Paschalis et al. (2013), Lara
et al. (2016), and Bütikofer (2018).

To calculate the barometric coefficient of each tube, we plot $\ln(N/\langle
N\rangle)$ as a function of $-\Delta P$, and perform a linear regression, with
the barometric coefficient being the gradient thereof. This is shown in the
top panel of Figure 7 for the different tubes. The calculated barometric
coefficients are indicated in the legend and are all within the range $0.65$ –
$0.75$ %/mbar. Once these coefficients are known, we apply the pressure
correction by

$N^{\prime}=N\exp\left(\beta\Delta P\right),$ (8)

where $N^{\prime}$ is the pressure corrected count rate of each tube.

###  4.2 Temperature dependence

Previous studies (e.g. Evenson et al., 2005; Krüger et al., 2008) have shown
that NM count rates are affected by temperature variations. It should be noted
that, in contrast to the real barometric effect, related to the physics of
atmospheric cascade, the temperature effect is spurious and related to the
stability of the detector rather than to the real changes of the neutron flux.
In traditional NMs, the temperature is usually carefully maintained, and these
effects are not important. However, the more mobile mini-NMs are often placed
in environments with large temperature changes, and hence, we need to check
for (and correct if necessary) any possible systematic temperature effects.
Any temperature effects are hard to predict, and different components of the
mini-NM probably react differently to temperature variations (see the
discussion by Evenson et al., 2005).

The bottom panel of Figure 7 shows $N^{\prime}/\langle N^{\prime}\rangle$
(note this is already the pressure-corrected count rate) as a function of the
temperature change $\Delta T=T-\langle T\rangle$ for the different tubes. Note
that we use the internal temperature sensor that is placed in-between the tube
and the moderator. All three tubes show a negative temperature dependence,
with the count rate decreasing with increasing temperature. For the thin BF3
tube, the temperature dependence is, however, negligible, while it may become
significant for the other tubes, especially if these are placed in an
environment with large temperature changes. The correction for the temperature
changes can be implemented as

$N_{T}^{\prime}=N^{\prime}-\beta_{T}\Delta T,$ (9)

where $N^{\prime}_{T}$ is now the temperature and pressure corrected count
rate, and $\beta_{T}$ the so-called temperature coefficient (i.e., the
gradient of the linear regression curve). Both linear regressions discussed
above go, by definition, through the origin. However, depending on the mini-NM
set-up and its dependence on the local environmental conditions, a linear
regression may not be sufficient, and higher-order polynomials might be
required. Some earlier works have also implemented exponential functions
(e.g., Heber et al., 2015), which can be reduced to a linear approximation if
$\beta_{T}\Delta T$ is sufficiently small.

Similar to the barometric coefficient, we should note that the temperature
coefficient might be highly dependent on the local environmental conditions
where the monitor is placed and that the pressure and/or temperature
coefficients given here cannot be universally applied (see, e.g., Krüger and
Moraal, 2010; Aiemsa-ad et al., 2015). Generally we do not test for any
humidity or atmospheric water vapour effects (see Banglieng et al., 2020).

###  4.3 Comparison between the different tubes

The pressure corrected count rate from each tube is binned as a histogram, and
shown in Figure 8. Gaussian distributions are fitted to each histogram (dashed
lines), showing that the remaining variations are roughly consistent with
statistical cosmic ray variations. The average count rate (as derived from the
fitted distribution) from each tube is indicated in the legend, along with the
standard deviations. The efficiency of the different tubes are clearly
illustrated in this figure: The thin BF3 tube is, as expected, the least
efficient with an average count rate of 23 counts/min, followed by the BF3
tube with 66 counts/min, and the 3He tube being the most efficient with a
count rate of 84 counts/min. However, the increasing efficiency comes with a
price, with the 3He being, by far, the most expensive, and the thin BF3 being,
by far, the most inexpensive of the tubes tested. As such, the BF3 tube is
recommended for a standard mini-NM set-up, balancing statistical accuracy with
operating costs. It should be noted that the tube used depends on the
application, i.e., if one is interested in low resolution data (i.e., data or
weekly averages for soil moisture measurements), then a thin BF3 tube could
still be sufficient for the task at hand.

In addition, a neutron monitor is often considered as an instrument for long-
time measurements. Some cosmic ray stations have neutron monitors in operation
for 50 years or even longer. In such applications, BF3 tubes show very good
stability while 3He-filled ones can experience a long-term drift of parameters
because of leakage of helium gas. On the other hand, the efficiency of BF3
counters may also gradually decline because of deposition on the anode wire
that can become notable for high-latitude, high-elevation NMs with very high
count rates, such as South Pole (Bieber et al., 2007).

Table 2: Current and planned mini-neutron monitor stations. Station name | Lat. (∘) | Long. (∘) | Alt. (m.a.s.l.) | Tube | Comments
---|---|---|---|---|---
Current stations |  |  |  |  |
Dome C (Antarctica) | 75∘06’S | 123∘23’E | 3233 | BF3 | Lead and lead-free unit
Sierra Negra Peak (Mexico) | 18∘59’N | 97∘19’W | 4100 | BF3 | Uses older electronics
Riyadh (Saudi Arabia) | 24∘38’N | 46∘43’E | 612 | BF3 |
Zugspitze (Germany) | 47∘25’N | 10∘58’E | 2650 | BF3 |
Future/planned stations |  |  |  |  |
Addis Ababa (Ethiopia) | 9∘06’N | 38∘46’E | 3293 | BF3 | Deployment in 2021
Chacaltaya (Bolivia) | 16∘21’S | 68∘07’W | 5240 | BF3 | Deployment in 2021
Mobile stations |  |  |  |  |
SA Agulhas II (South Africa) |  $\sim$33∘S – 72∘S |  $\sim$ 37∘W – 37∘E | $\sim 0$ | He3 |
Polarstern (Germany) |  $\sim$85∘N – 70∘S |  $\sim$ 70∘W – 20∘E | $\sim 0$ | He3 | Uses older electronics
BIO Hespérides (Spain) |  $\sim$37∘N – 62∘S |  $\sim$ 60∘W – 0∘E | $\sim 0$ | BF3 |

##  5 Discussion

Below we briefly describe measurements from three mini-NM stations producing
long-term data, while Table 2 summarizes the current and planned mini-NM
stations.

###  5.1 Concordia research station

A pair of mini-NMs was installed at the French-Italian research station
Concordia (Dome C, Antarctic plateau) at 75∘06’ S, 123∘23’ E, 3233 m a.s.l. in
2015 (Poluianov et al., 2015; Usoskin et al., 2015). One of those instruments
(called DOMC) is a standard-design unit with the lead producer and the the
other is “bare” (lead-free, DOMB). Both use the thick BF3 tube discussed here.
The unique location of the station (high elevation and close proximity to the
geomagnetic pole) provides low atmospheric and negligible geomagnetic cutoffs.
Moreover, DOMC and DOMB are ones of few NMs whose asymptotic cone point south
of the equatorial plane, making it possible to study the 3D distribution of
cosmic-ray variability. The average count rates of DOMC and DOMB are about
1150 and 315 counts/min, respectively, during 2019. The high altitude polar
location makes these mini-NMs exceptionally sensitive to low-energy cosmic
rays, which is particularly important for registration of solar energetic
particles (SEPs). In confirmation of that, several SEP events have been
registered since the start of the operation of mini-NMs at Concordia,
including very weak ones (e.g., Mishev, Alexander et al., 2017). For example,
GLE #72 on 10 September 2017 caused count rate increases of +10% (maximum 1210
cts/min with pre-increase 1102 cts/min) and +13% (maximum 330 cts/min with
pre-increase 293 cts/min) at DOMC and DOMB, respectively (see the
International GLE database http://gle.oulu.fi). Introduction of those
instruments to the neutron monitor global network significantly increased its
overall sensitivity to SEPs and led to a revision of the ground level
enhancement (GLE) definition with the introduction of the sub-GLE class of
events (Poluianov et al., 2017). In 2019, neutron monitors DOMC and DOMB were
upgraded with the electronics described in this paper (in November and August,
respectively). Before that, their electronics were similar to one in Krüger
and Moraal (2013).

###  5.2 Neumayer III

A mini-NM (using a thick BF3 tube) was installed at the German Antarctic
Neumayer III research station early in 2011 (Heber et al., 2015). Neumayer III
is located on the Ekström Ice-shelf in the Atka-bay in the Weddell-sea at 70∘
40 S, 8∘16 W. The corresponding cutoff rigidity $P_{c}=0.1$ GV has been
calculated utilizing PLANETOCOSMICS (Desorgher et al., 2009) with the
International Geomagnetic Reference Field (Finlay et al., 2010) and the
Tsyganenko (1989) as model for the inner and outer Earth magnetosphere,
respectively (Herbst et al., 2013). The monitor provided data from 2013 –
2017, and was primarily used to study the long-term stability of the
instrument as there were few transient cosmic ray events during this period.
The monitor was transport back to Kiel, Germany, in 2018, and retro-fitted
with the new electronics discussed in this paper in early 2020. It is
currently undergoing testing, and will be redeployed in the near future.

###  5.3 Polarstern

In 2011, a mini-NM with a 3He tube, was installed on the German research
vessel, the Polarstern (Heber et al., 2015). The instrument has provided data
since 2012, and is used to study both the long-term stability and sensitivity
of the instrument, as well as the latitude effect as the ship sails each year
from Germany to Antarctica.

###  5.4 Sierra Negra

This mini-NM was deployed on a plateau at the slope of the Sierra Negra
volcano in the central part of Mexico (19.0∘N, 97.3∘W), where the High
Altitude Water Cherenkov (HAWC) gamma ray observatory is located. The cutoff
rigidity at this site is $\sim 7.8$ GV and the altitude is 4100 m.a.s.l.,
allowing relatively high count rates of $\sim 840$ counts/min (on January
2018). The mini-NM has been taking data since 2015, with a data gap of three
months during 2016. Taking advantage of the portability of the mini-NM, an
altitude survey was performed during 2014 going from the sea level up to 4580
m.a.s.l. (Lara et al., 2016). This study showed that the mini-NM is stable and
performs well in the field.

##  6 Summary and outlook

The original purpose of the mini-NM was to develop a small and portable NM
that can be used to inter-calibrate the world-wide network of NMs. These
stationary monitors, placed at different altitudes and magnetic latitudes,
also have different environmental conditions, and react to environmental
changes differently. We believe that such an inter-calibration study could
still be relevant, and should, at some point, be continued.

On the other hand, mini-NMs stations have produced valuable data and
scientific insights, especially at polar regions and high altitudes where the
mini-NM count rates are comparable to full-scale sea-level NMs. We firmly
believe that this trend will continue, and, in the future, a world-wide
network of mini-NMs will co-exist with the current network of NMs, especially
covering current observational gaps (e.g., the sparse coverage over the
Southern Hemisphere including African and South America). The mini-NMs are
much more cost-effective to construct and operate than traditional NMs,
providing an intriguing possibility to also retrofit older stations that were
decommission in the past due to financial constraints with low maintenance
mini-NMs. Most of these sites already have the necessary infrastructure
(including power and, in some cases, internet access). Potential sites include
‘Sulphur Mountain, Canada, at 2383 m, the site of the original NM64 monitor.

###### Acknowledgements.

Operation of DOMC/DOMB mini-NMs is supported by projects CRIPA and CRIPA-X
(No. 304435) of the Academy of Finland, Finnish Antarctic Research Program
(FINNARP). DOMC/B NM data can be obtained from http://cosmicrays.oulu.fi,
courtesy of the Sodankylä Geophysical Observatory. Operation of DOMB/DOMC NMs
are possible thanks to the hospitality of the Italian polar programme PNRA
(via the LTCPAA PNRA 2015/AC3 project) and the French polar Institute IPEV.
The neutron monitor database NMDB (http://nmdb.eu) founded under the European
Union’s FP7 programme (contract no. 213007) is acknowledged for hosting and
providing data. We acknowledge support from the MINECO - FPI 2017 program, co-
financed by the European Social Fund. This work has been supported by the
project CTM2016-77325-C2-1-P, funded by Ministerio de Economía y
Competitividad, and by the European Regional Development Fund, FEDER. This
work is based on the research supported in part by the National Research
Foundation of South Africa (NRF grant number: 119424). Opinions expressed and
conclusions arrived at are those of the authors and are not necessarily to be
attributed to the NRF. Figures prepared with Matplotlib (Hunter, 2007).

## References

  * Aiemsa-ad et al. (2015) Aiemsa-ad, N., D. Ruffolo, A. Sáiz, P. S. Mangeard, T. Nutaro, et al., 2015.  Measurement and simulation of neutron monitor count rate dependence on surrounding structure.  _Journal of Geophysical Research (Space Physics)_ , 120(7), 5253–5265.  10.1002/2015JA021249.
  * Banglieng et al. (2020) Banglieng, C., H. Janthaloet, D. Ruffolo, A. Sáiz, W. Mitthumsiri, et al., 2020.  Tracking Cosmic-Ray Spectral Variation during 2007 - 2018 Using Neutron Monitor Time-delay Measurements.  _ApJ_ , 890(1), 21.  10.3847/1538-4357/ab6661.
  * Bieber et al. (2007) Bieber, J. W., J. Clem, D. Desilets, P. Evenson, D. Lal, C. Lopate, and R. Pyle, 2007.  Long-term decline of South Pole neutron rates.  _Journal of Geophysical Research (Space Physics)_ , 112(A12), A12102.  10.1029/2006JA011894.
  * Bieber et al. (2004) Bieber, J. W., J. M. Clem, M. L. Duldig, P. A. Evenson, J. E. Humble, and R. Pyle, 2004.  Latitude survey observations of neutron monitor multiplicity.  _Journal of Geophysical Research (Space Physics)_ , 109(A12), A12106.  10.1029/2004JA010493.
  * Bütikofer (2018) Bütikofer, R.  Ground-Based Measurements of Energetic Particles by Neutron Monitors, 95–111.  Springer International Publishing, 2018.  ISBN 978-3-319-60051-2.
  * Caballero-Lopez (2016) Caballero-Lopez, R. A., 2016.  An estimation of the yield and response functions for the mini neutron monitor.  _Journal of Geophysical Research (Space Physics)_ , 121(8), 7461–7469.  10.1002/2016JA022690.
  * Carmichael et al. (1968) Carmichael, H., M. Bercovitch, M. A. Shea, M. Magidin, and R. W. Peterson, 1968.  Attenuation of neutron monitor radiation in the atmosphere.  _Canadian Journal of Physics Supplement_ , 46, 1006.  10.1139/p68-405.
  * Clem and Dorman (2000) Clem, J. M., and L. I. Dorman, 2000.  Neutron Monitor Response Functions.  _Space Sci. Rev._ , 93, 335–359.  10.1023/A:1026508915269.
  * Desorgher et al. (2009) Desorgher, L., K. Kudela, E. Flückiger, R. Bütikofer, M. Storini, and V. Kalegaev, 2009.  Comparison of Earth’s magnetospheric magnetic field models in the context of cosmic ray physics.  _Acta Geophysica_ , 57(1), 75–87.  10.2478/s11600-008-0065-3.
  * Dyer et al. (2009) Dyer, C., A. Hands, F. Lei, P. Truscott, K. A. Ryden, P. Morris, I. Getley, L. Bennett, B. Bennett, and B. Lewis, 2009.  Advances in Measuring and Modeling the Atmospheric Radiation Environment.  _IEEE Transactions on Nuclear Science_ , 56(6), 3415–3422.
  * Dyer et al. (2007) Dyer, C., F. Lei, A. Hands, and P. Truscott, 2007.  Solar Particle Events in the QinetiQ Atmospheric Radiation Model.  _IEEE Transactions on Nuclear Science_ , 54(4), 1071–1075.
  * Evenson et al. (2005) Evenson, P., J. W. Bieber, J. Clem, and R. Pyle, 2005.  Neutron Monitor Temperature Coefficients: Measurements for BF3 and 3He Counter Tubes.  In Proceedings to the 29th International Cosmic Ray Conference, vol. 2 of _International Cosmic Ray Conference_ , 485.
  * Finlay et al. (2010) Finlay, C. C., S. Maus, C. D. Beggan, T. N. Bondar, A. Chambodut, et al., 2010.  International Geomagnetic Reference Field: the eleventh generation.  _Geophysical Journal International_ , 183(3), 1216–1230.  10.1111/j.1365-246X.2010.04804.x.
  * Heber et al. (2015) Heber, B., D. Galsdorf, J. Gieseler, K. Herbst, M. Walter, A. Stoessl, H. Moraal, H. Krüger, and G. Benade, 2015.  Mini neutron monitor measurements at the Neumayer III station and on the German research vessel Polarstern.  In 34th International Cosmic Ray Conference (ICRC2015), vol. 34 of _International Cosmic Ray Conference_ , 122.
  * Herbst et al. (2013) Herbst, K., A. Kopp, and B. Heber, 2013.  Influence of the terrestrial magnetic field geometry on the cutoff rigidity of cosmic ray particles.  _Annales Geophysicae_ , 31(10), 1637–1643.  10.5194/angeo-31-1637-2013.
  * Hunter (2007) Hunter, J. D., 2007.  Matplotlib: A 2D graphics environment.  _Computing in Science & Engineering_, 9(3), 90–95.  10.1109/MCSE.2007.55.
  * Jung et al. (2016) Jung, J., S. Oh, Y. Yi, P. Evenson, R. Pyle, G. Jee, J.-H. Kim, C. Lee, and J. Sohn, 2016.  Installation of Neutron Monitor at the Jang Bogo Station in Antarctica.  _Journal of Astronomy and Space Sciences_ , 33(4), 345–348.  10.5140/JASS.2016.33.4.345.
  * Kamal (2014) Kamal, A., ed., 2014.  Particle physics.  Graduate Texts in Physics, Springer.
  * Knoll (2010) Knoll, G. F., ed., 2010.  Radiation Detection and Measurement, 4th ed.  Wiley.
  * Krüger and Moraal (2010) Krüger, H., and H. Moraal, 2010.  A calibration neutron monitor: Statistical accuracy and environmental sensitivity.  _Advances in Space Research_ , 46(11), 1394–1399.  10.1016/j.asr.2010.07.008.
  * Krüger and Moraal (2013) Krüger, H., and H. Moraal, 2013.  Neutron monitor calibrations: progress report.  _J. Phys.: Conf. Ser._ , 409(1), 012,171.  ”10.1088/1742-6596/409/1/012171”.
  * Krüger et al. (2003) Krüger, H., H. Moraal, J. W. Bieber, J. M. Clem, P. A. Evenson, K. R. Pyle, M. L. Duldig, and J. E. Humble, 2003.  First Results of a Mobile Neutron Monitor to Intercalibrate the Worldwide Network.  In Proceedings of the International Cosmic Ray Conference, vol. 6 of _International Cosmic Ray Conference_ , 3441.
  * Krüger et al. (2008) Krüger, H., H. Moraal, J. W. Bieber, J. M. Clem, P. A. Evenson, K. R. Pyle, M. L. Duldig, and J. E. Humble, 2008.  A calibration neutron monitor: Energy response and instrumental temperature sensitivity.  _Journal of Geophysical Research (Space Physics)_ , 113(A8), A08101.  10.1029/2008JA013229.
  * Krüger et al. (2015) Krüger, H., H. Moraal, R. Nel, H. G. Krüger, and M. O’Kennedy, 2015.  The mini neutron monitor programme.  In Proceedings to the 34th International Cosmic Ray Conference, vol. 34 of _International Cosmic Ray Conference_ , 223.
  * Lara et al. (2016) Lara, A., A. Borgazzi, and R. Caballero-Lopez, 2016.  Altitude survey of the galactic cosmic ray flux with a Mini Neutron Monitor.  _Advances in Space Research_ , 58(7), 1441–1451.  10.1016/j.asr.2016.06.021.
  * Medina et al. (2013) Medina, J., J. J. Blanco, O. García, R. Gómez-Herrero, E. J. Catalán, et al., 2013.  Castilla-La Mancha neutron monitor.  _Nuclear Instruments and Methods in Physics Research A_ , 727, 97–103.  10.1016/j.nima.2013.06.028.
  * Mishev et al. (2020) Mishev, A. L., S. A. Koldobskiy, G. A. Kovaltsov, A. Gil, and I. G. Usoskin, 2020.  Updated Neutron-Monitor Yield Function: Bridging Between In Situ and Ground-Based Cosmic Ray Measurements.  _Journal of Geophysical Research (Space Physics)_ , 125(2), e27433.  10.1029/2019JA027433.
  * Mishev, Alexander et al. (2017) Mishev, Alexander, Poluianov, Stepan, and Usoskin, Ilya, 2017.  Assessment of spectral and angular characteristics of sub-GLE events using the global neutron monitor network.  _SWSC_ , 7, A28.  10.1051/swsc/2017026, URL https://doi.org/10.1051/swsc/2017026.
  * Moraal et al. (2001) Moraal, H., A. Benadie, D. de Villiers, J. W. Bieber, J. M. Clem, P. E. Evenson, K. R. Pyle, L. Shulman, M. L. Duldig, and J. E. Humble, 2001.  A mobile neutron monitor to intercalibrate the worldwide network.  In International Cosmic Ray Conference, vol. 10 of _International Cosmic Ray Conference_ , 4083.
  * Paschalis et al. (2013) Paschalis, P., H. Mavromichalaki, V. Yanke, A. Belov, E. Eroshenko, M. Gerontidou, and I. Koutroumpi, 2013.  Online application for the barometric coefficient calculation of the NMDB stations.  _New A_ , 19, 10–18.  10.1016/j.newast.2012.08.003.
  * Poluianov et al. (2015) Poluianov, S., I. Usoskin, A. Mishev, H. Moraal, H. Kruger, G. Casasanta, R. Traversi, and R. Udisti, 2015.  Mini Neutron Monitors at Concordia Research Station, Central Antarctica.  _Journal of Astronomy and Space Sciences_ , 32(4), 281–287.  10.5140/JASS.2015.32.4.281.
  * Poluianov et al. (2017) Poluianov, S. V., I. G. Usoskin, A. L. Mishev, M. A. Shea, and D. F. Smart, 2017.  GLE and Sub-GLE Redefinition in the Light of High-Altitude Polar Neutron Monitors.  _Sol. Phys._ , 292(11), 176.  10.1007/s11207-017-1202-4.
  * Signoretti and Storini (2011) Signoretti, F., and M. Storini, 2011.  A new modular cosmic-ray detector.  _Astrophysics and Space Sciences Transactions_ , 7(1), 11–14.  10.5194/astra-7-11-2011.
  * Simpson (2000) Simpson, J. A., 2000.  The Cosmic Ray Nucleonic Component: The Invention and Scientific Uses of the Neutron Monitor - (Keynote Lecture).  _Space Sci. Rev._ , 93, 11–32.  10.1023/A:1026567706183.
  * Taber and Normand (1993) Taber, A., and E. Normand, 1993.  Single event upset in avionics.  _IEEE Transactions on Nuclear Science_ , 40(2), 120–126.
  * Tsyganenko (1989) Tsyganenko, N. A., 1989.  A magnetospheric magnetic field model with a warped tail current sheet.  _Planet. Space Sci._ , 37(1), 5–20.  10.1016/0032-0633(89)90066-4.
  * Usoskin et al. (2015) Usoskin, I., S. Poluianov, H. Moraal, H. Krüger, G. Casasanta, R. Traversi, and R. Udisty, 2015.  A mini neutron monitor in Central Antarctica (Dome Concordia).  In 34th International Cosmic Ray Conference (ICRC2015), vol. 34 of _International Cosmic Ray Conference_ , 217.