1. A mini neutron monitor: The effect of instrumental temperature and each structural component
Helena Krüger
PP Krüger, D Malan, C Diedericks
Centre for Space Research
School of Physical en Chemical Sciences
North-West University
Potchefstroom, South Africa

2. The North-West University, Potchefstroom campus, operates four neutron monitor stations:
Hermanus, South Africa (since 1957),
SANAE, Antarctica (since 1964),
Potchefstroom, South Africa (since 1971),
Tsumeb, Namibia (since 1976).

3. Neutron Monitors: NWU Network
Tsumeb Station
NWU, Potchefstroom
Hermanus (SANSA)
SANAE Base, Antarctica

4. Calibration Monitor – 2002-2010
Two mobile calibration NMs were built and operated between 2002 and 2010, to calibrate the stationary NMs worldwide.
Fully transportable as a unit. Length: 753 mm and mass: 223 kg (including the trolley).
Various tests were performed:
Stability
Instrumental temperature
Latitude sensitivity
Environment dependence

5. Change in vision – mini NMs
Rapid developments in electronics, in particular in interfacing technologies and data transmission, required a redesign of the electronics.
3He became expensive, but when the more efficient 10BF3 counter tubes became available, the vision broadened to the concept of a mini neutron monitor, i.e. a permanent detector in its own right.
(c) An affordable, permanent network of mini NMs, which can extended the current aging network of predominantly NM64 super neutron monitors, by placing the mini NMs in existing buildings with existing infrastructure (only needs power and network connectivity) and, if possible, at mountain altitudes of > m.

6. Outline Development of the electronics Pressure sensors
Instrumental temperature tests done on the newest version
Influence of the structure (moderator, lead producer and reflector) on the count rate.

7. 1. The electronics ( )
Records individual pulse arrival times with an accuracy of 1 ms, from which the count rates per time interval are calculated.
Small, lightweight, compact design: mass of 1.8 kg, and dimensions of 10 cm x 18 cm x 24 cm.
Easy to transport and exchanged.
More sensitive pre-amplifier with shielding to eliminate effect of environmental electromagnetic noise.
A custom-made cross-platform software package, qGraph, was developed for data capture, transmission and analysis.

8. Development of modular electronics (2016/7)
Easy maintainability components (Raspberry pi)
Data accessible in realtime through internet
Easily maintainable hardware and software
Recording individual pulse arrival times with an accuracy of 1 µs
Records the pulse profile of each event; giving the pulse height and width
Modular design: Easily adapted to different NMs (stationary and mobile)

9. Development of modular electronics (2016)
Technical Aspects:
Off-the-shelf embedded processing for easy maintainability
Modern, affordable pressure sensors and new humidity sensor
New efficient high voltage source with exact software calibration
Accurate Real Time Clock. GPS optional by either using NTP or NMEA over TCP/IP.
Pulse shape recording module for:
Pulse profiling
Auto check integrity of electronics
Long-term stability

10. First phase pre-amplifier
Main board
HV filter
Pressure sensor
From HV
Raspberry pi
Second phase pre-amplifier
& pulse registration
Ethernet
USB ports

12. The threshold is approximately 0.25 V

13. The electronics head is also designed in such a way that it can be used on existing aging stationary neutron monitors.
Three of the six counters of the Sanae NM64, and all four counters of the lead-free monitor at Sanae were fitted with these heads in January 2014.
At the end of 2017 we want to replace them with the new modular electronics.

14. Sanae 4-counter NMD

15. Onboard Polarstern – Science Laboratory

16. Comparison of counter tubes
Mini Neutron Monitor
NM64
IGY
Counter Type
LND25382
LND2043
LND20366
LND25373
BP28
NW G-15-34A
Fill Gas
3He
10BF3
Fill Pressure (mm Hg)
3040
700
196
200
450
Operating Voltage (V)
2800
1950
Length (mm)
1992
1910
870
Diameter (mm)
50.8
88.9
148 38
Counts/minute 84 63 25
660 61
Counts/minute: Approximate, at sea level and cutoff rigidity < 1 GV.

17. 2. Pressure sensors
A major cost component of the registration system is a sufficiently accurate pressure sensor.
From almost 30 years’ experience with a Paroscientific Digiquartz sensor we could verify that it is accurate and long-term stable to within ± 0.1 mb (The required pressure accuracy is ± 0.2 mm Hg or ± 0.14 mb). However, it costs of the order of $US 5000.
Vaisala, PTB1101A1AB, costs of the order of $US 2000.
Affordable Bosch BME280 sensors: measure pressure, temperature and relative humidity, cost of the order of $US 60.

18. Preliminary results

19. These preliminary results show that the pressure accuracy of this Bosch sensor is ~ 0.1%.

20. 3. Instrumental temperature sensitivity
Unexpected large instrumental temperature sensitivity was discovered with the calibration NM at SANAE and confirmed with temperature tests at Potchefstroom.
The Bartol-group discovered the same effect at one of their neutron monitor stations.

21. Temperature sensitivity
Independent studies indicate that the calibrator, as well as the stationary neutron monitors, has significant instrumental temperature sensitivity.
3He Calibrator %/oC
3He in 3NM64 configuration (Nain) 0.09%/oC
3He 3NM64 simulation %/oC
10BF3 IGY (Potchefstroom) %/oC
10BF3 in 3NM64 configuration (Thule) 0.04%/oC
10BF3 3NM64 simulation %/oC
This means that NMs have to be kept at a fixed temperature and have to be normalized to a standard temperature.

22. At Dome C a quite large temperature effect on the count rates was experienced with the two mini NMs, at the end of 2016.
The mini NM (DOMC) with lead had a negative temperature effect of 1.99%/ºC, while the unleaded one (DOMB) had a positive temperature coefficient of 0.68%/ºC.
Tests were performed to obtain the temperature coefficient for two mini NMs, one with a BF3 counter tube and another one with a 3He tube.

23. Preliminary results

25. Temperature coefficients (%/oC)
(preliminary)
With lead
Without lead
3He tube
0.10 ± 0.02
0.16 ± 0.02
BF3 tube
-1.22 ± 0.04
-1.15 ± 0.03

26. Pulse height histogram for 3He mini NM
A threshold level is determined (dotted line), to discriminate between noise and CR events.

27. Pulse height histogram for 3He mini NM
When temperature increases, gain of preamplifier decreases, diagram moves to the left, fixed threshold and counts are lost.
And vice versa. Record the amplitude of each pulse and then apply threshold cut afterwards.

28. E.g., if the temperature changes by 5oC, the gain of the preamplifier changes by 5%. If the theshold is kept fixed (black line), the change in gain will result in a 0.1% change in the count rate.
When the threshold is adapted (red line), the count rate becomes gain independent.

29. 4. Influence of the structure on the count rate
The effect of each structural component (reflector, lead producer and moderator) on the count rates is determined experimentally for two mini neutron monitors, one with a BF3 counter tube and another one with a 3He tube.

30. A mini neutron monitor Counter Inner moderator of polyethylene
Lead producer
Outer reflector of polyethylene

31. Normalised count rate – preliminary results
BF3
3He
Mini NM
100%
Lead + moderator
81.7 %
97.4%
Moderator
55.9%
61.0%
Tube only
49.6%
62.3%
Lead
45.9%
54.1%
Moderator + reflector
41.9%
35.3%
Without the reflector the count rate decreases with 18% for the BF3 and 3% for 3He.
Inner moderator increases the count rate with 6% for the BF3 and decreases with 1% for 3He.
Question: how does the count rate change with temperature in each case?

32. The effect of the moderator+lead is the largest.
There is multiplicity only when there are a producer (of neutrons) and a moderator to thermalize them.

34. Mini Neutron Monitor – Summary
The current NM network can be extended by a second generation network of affordable mini NMs, at almost any chosen site with existing infrastructures.
The electronics boxes can also be used as replacements for the older electronics of existing NMs.
Using modern electronics, the pulse profile information can be used to reduce the temperature dependence of the count rate, as well as to detect problems in NMs, such as drift effects.
Possible to perform multiplicity analysis.

35. Thank you

37. P ≈ P0 exp[– h (m)]
β ≈ 1 %/mm Hg
N = N0 exp[0.01(P0 – P)], with P in mm Hg
A mini NM at 3 000 m altitude will have a count rate 10 times larger than at sea level. That gives the same count rate as a single-counter NM64 at sea level.