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Next, a radiation officer at a local university brought a neutron source near the Helium 3 tube, resulting in the graph seen below. Since the neutron source was emitting high energy neutrons, a neutron moderator (such as polyethylene) would increase count rate by perhaps 500 times.
A low noise, JFET, diode protected, high pass filter circuit amplifies input signals from the detector tube. This is passed to a unique CMOS, temperature tracking, threshold detector. Threshold level is adjusted by way of a 25 turn cermet trim pot. Next the signal is amplified and shaped into a square wave by way of a direct connect, 100,000 gain CMOS section. The signal is then passed to a silicon output transistor. The silicon output transistor generates a square wave from rail to rail.
The high voltage section employs a high efficiency voltage converter with quad filtering on the output. Output voltage is continually sampled by way of a high precision resistor - op amp comparator. The voltage is compared to a precision voltage reference with op-amp buffer. The reference voltage input to the comparator is adjusted by way of a 25 turn, 5K cermet trim pot. HV ripple after high-pass input filter is undetectable.
Power for the high voltage section and low voltage section is drawn from the DC power coax jack. If no DC power plug is in place, the unit will try to draw necessary power from the computer serial port. This allows operation with just one cable leading to the PMI-30.
The voltage for the JFET - CMOS section and the op-amp voltage reference section is supplied by way of a high efficiency CMOS voltage regulator. A low voltage LED lights when DC power drops to approx. 7 volts.
All shielding, including the high voltage coax shield, is connected to the signal ground
lead from the serial port.
(Circuit updated Jan. 12th 2006 including greater resistance to damage from electrostatic discharge upon the body of the external detector tubes and to signal ground. Reduced noise from the already low noise high voltage section. An improvement was also made to the threshold detector).
Threshold: Adjustable with 25 turn CERMET trim pot from ~0.5 mV through 60 mV.
Impedance ~40K ohm. (DC impedance: 1M ohms)
H.V.: Adjustable with 25 turn CERMET trim pot from 400 to 1400 VDC.
(2000+ VDC option is available. PMI-30 2000+ VDC option: $49. Higher voltages also available)
H.V. Regulation: Less than +- 5 volts with input voltage change from 16 volts to 6 volts. HV ripple after high-pass input filter is undetectable.
Detectors: Scintillation, proportional tubes, neutron counting tubes, Geiger tubes, etc.
Construction: ABS case material has flame retardant and threaded brass inserts. Internal EMI shielding.
Detector Connector: series "MHV". (Other connectors available).
Power: 6 VDC to 12 VDC. Power draw: 10ma depending on H.V. setting and detector draw.
Paired pulse resolving time: Less than 5 micro seconds.
Temperature Range: -40C to +65C (-40F to 149F).
Size: 4.4" x 2.44" x 1.06"
Weight: 7 oz. (200 gms.)
The PMI-30 allows the use of standard 9 volt batteries, 12 volt batteries, small wall plug mounted DC power supplies, etc. to supply power for the PMI-30.
Also included in the PMI-30 is a MOD RJ-11 telephone jack for connection to the computer (or LCD-90) by way of 4 conductor telephone wire. A seven foot telephone cable is included but this can be extended to perhaps several hundred feet with telephone extension cable. We also supply a nine volt battery snap and plug with the PMI-30 so one can run the system from a standard nine volt battery.
We normally supply the PMI-30 with a MHV COAX connector. A three feet COAX cable with MHV connectors costs $79. (Used to connect the PMI-30 to the detector tube).
The PMI-30 includes the standard set of DOS software. To this you might
want to add AW-GRAPH software ($74). Otherwise one should purchase
our new Windows program Aw-Radw ($129). The LCD-90 device
works with the PMI-30, as well as HP palm top PCs (200LX or 1000CX).
Click Here For Info About LCD-90: Aware's New LCD-90 MicroController - Data-Logger
Helium 3 Neutron Tubes: Approx. $Call.
Phone - write for more details.
Note the six inch coax. This length includes the MHV male connectors on both ends. A short coax is necessary in that a longer coax might absorb too much of the tiny signal from the HE-3 tube.
H3 tube (He3/25291):
Gas Pressure---------------3800 Torr
Effective Volume-----------32.6 cubic cm
Cathode Material-----------Stainless Steel
Operating Voltage Plateau--1200 to 1450 volts.
Count life approx.-----------10 to the 12th
The sensitivity figure cps/nv represents CPS when tube is exposed to 1 thermal neutron per sq. centimeter per second. If the above tube, with a sensitivity of 12 cps/nv, is placed within a suitable moderator, for example a 22.9 cm (9.0 in.) diameter cadmium-loaded polyethylene sphere, then its sensitivity can be expressed as typically 20 cpm/mS/h (2000 cpm/mrem/hr) (241AmBe fast neutrons).
The detector is approx. 5000 times more sensitive to thermal neutrons as compared to MeV level neutrons. The cross section of He3 to neutrons at 2 MeV energy is ~0.9 (barns). The cross section at 1 MeV is also ~0.9 (barns). The cross section at 10 eV is ~280 (barns) and the cross section at 0.025 eV is ~5000 (barns). (See Radiation Detection and Measurement by Glenn F. Knoll, second edition, page 484 (cross section versus neutron energy)).
You may want to surround the tube with a moderator to slow down higher energy neutrons. Polyethylene or paraffin are most often used. Surround tube with a few centimeters of either. An 8" or 12" diameter polyethylene sphere is sometimes utilized. (See Radiation Detection and Measurement by Glenn F. Knoll, second edition, page 516 and 517). Moderator should surround tube on all sides because some neutrons, after having past tube, will bounce back.
Note as a general neutron monitor one could place a few centimeters of polyethylene or paraffin around one end of tube but allow some of the tube body to stick out. This would allow system to pick-up both high energy neutrons and thermal neutrons. Thermal neutrons would hit tube directly and be detected whereas high energy neutrons would be moderated to lower energy by the hydrogen in the polyethylene or paraffin.
Click Here for example use of PMI-30 starting on page 156: Wide-Area Sensor Network
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