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Cosmic Ray Neutron Monitor

Using a neutron detector to measure cosmic rays may sound odd, but this has been a common way to measure the level of cosmic ray levels since 1948. This is because if the primary cosmic ray that starts a cascade has an energy well over 500 MeV, and so many of its secondary by-products will be neutrons that will reach ground where they can be detected.  These systems are commonly called Neutron Monitors

Russian Neutron Pulse Corona boron Counter SI-19N

Recently some Russian Boron Coated Cathode Corona Pulse Neutron Tubes (SI-19N) became available on ebay at low cost and so I thought I'd see if I could also make one these guys.

List of materials required to build a Neutron Monitor:

  1. Reflector - 75mm of UHMWPE (Ultra-high-molecular-weight polyethylene)  or 280mm of Paraffin Wax.  Low energy neutrons cannot penetrate this material, but are not absorbed by it. consequently, non-cosmic ray induced neutrons are kept out of the monitor and low energy neutrons generated in the lead are kept in which is largely transparent to the cosmic ray induced cascade of neutrons.
  2. Producer - 40mm of Lead . Fast neutrons that get through the reflector interact with the lead to produce, on average about 10 much lower energy neutrons. This both amplifies the cosmic signal and produces neutrons that cannot easily escape the reflector.
  3. Moderator20mm of UHMWPE (Ultra-high-molecular-weight polyethylene) or 32mm of Paraffin Wax.  This slows down the neutrons now confined within the reflector, and makes them more likely to be detected.
  4. Nuetron Detector Tube - Boron Coated Cathode Corona Pulse Neutron Tube SI-19N. After very slow neutrons are generated by the reflector, producer, moderator, and so forth, they encounter a nucleus within the proportional counter which causes it to disintegrate. This nuclear reaction produces energetic charged particles that ionizes the gas in the proportional counter, producing an electrical signal. 
  5. Electronics - Very similar to a typical Geiger Counter circuit but with an adjustable High Voltage Regulated DC Supply 2-3kv

Neutron Monitor Layout

Draft Neutron Detector Circuit

Boron Coated Cathode Corona Pulse Neutron Tube SI-19N

Russian Neutron Pulse Corona boron Counter SI-19N

Operating  Voltage - 2000-2800 V
Working Voltage - 2400 V
Slop of plateau, % per 100 V - ≤ 3
Neutron sencitivity, pulses per neutron/ cm2 - 100
Plateau Length - 800 V
Own background, pulses/min - 5
Temperature range , 0 C  -  -50... +150
Diameter - 33.3 mm
Length  - 218 mm