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Application Notes: PDT10A & PDT20A

PDT10A and PDT20A compact neutron monitoring modules designed for high radiation environments connect directly He-3 detectors. Each instrument contains a precision wide band charge sensitive pre-amplifier followed by an amplifier, wave shaping and discriminator circuits. When connected to He-3 detectors, these instruments detect the presence of charged-sensitive particles. Our focus is to create a compact (1 inch diameter), simple to use, high performance instrument with a variety of features available.

Each instrument has an analog monitoring test point allowing direct monitoring of amplified charged events using an oscilloscope.  A twelve turn trim pot is available to finely tune the amplifier sensitivity to the desired level so you can obtain a good HV plateau for your application.
In addition, when a charged event is detected, a TTL OUT pulse is produced.
The TTL OUT pulse width is adjustable from 40nSeconds to 500nSeconds.
Each instrument contains a TTL IN input allowing you to sum outputs from other modules in a system. This allows quick and easy interconnection from 1 to over 100 detectors for digital pulse counting measurements.
Each instrument contains a threshold control trim pot allowing you to control the discriminator level for your application.
The PDT20 is equivalent in performance to the PDT10A but additionally contains a high performance high voltage generator that can be adjusted over a range of 0-2200 volts.
The PDT20 contains a HV MON test point that allows you to measure and adjust the high voltage setting.
The PDT10 is compact, housed in 1 inch diameter aluminum with a length of 1.5 inches.
The PDT20 is enclosed in 1 inch diameter aluminum with a length of 3 inches.

PDT10A and PDT20A modules have been designed for extended temperature and vibration environments. The products have been tested at temperatures ranging from 0-170 degrees Centigrade including vibration to 70g from 2Hz to 10kHz.This new generation of neutron monitoring modules allow new systems to be quickly configured and existing detector instruments to be easily upgraded. The charge sensitive amplifiers are designed for Helium-3, boron loaded, fission chamber, alpha-and proton-recoil neutron detector operation. Multiple modules can be easily interconnected providing a single digital counting channel that will process pulse rates exceeding 5 MHz.  Engineered in a compact ruggedized aluminum housing that interfaces directly to gas filled detectors, these modules provide an efficient clean solution in difficult environments.

Series 10A and 20A modules provide customers with considerable versatility. This application note provides users with a basic understanding in the proper using of the 10A and 20A instrument.

PDT10A and PDT20A Configurations
You can purchase PDT10A and PDT20A modules with a variety of connector interfaces for He-3 detectors.  Modules have the capability to connect directly to Helium-3 gas filled detectors that us HN, MHV, or SHV type of connectors. If necessary, you can even cable directly from the PDT10A modules to the detector using a high voltage cable with the 10A-C.The 10A-C uses a miniature high voltage connector rated for 5000 volts allowing cable connection to the detector. PDT can provide the cable with the desired connectors for your application. Please contact us and we will discuss the cable length, type of connection on the detector to help simplify your setup. When using the 10A-C it is important that the cable length is kept as small as possible to minimize both capacitance and noise. Most PDT modules connect directly to He-3 detectors to minimize capacitance with the added advantage of providing a more ruggedized interface.

10A Detector Interface 20A Detector Interface Description
10A-HN 20A-HN HN interface to He-3
10A-SHV 20A-SHV SHV interface to He-3
10A-MHV 20A-MHV MHV interface to He-3
10A-C Cable interface to He-3

20A High Voltage Options
The high voltage generator of the PDT 20A are available in three configurations, 20A (standard),20F (fixed) or 20R (Remote).Each high voltage configuration still allows you to select the one of three detector interface connections, either HN, MHV or SHV. If you desire to order the standard PDT20A,the proper selection would be PDT20A-HN, if however your application requires you to interface to a detector that uses an SHV connector and you want a fixed HV supply at 1400 Volts, then the proper selection would be PDT20F-SHV @1400 Volts.

HV BIAS Configuration Description
20A Standard: HV variable 0-2200 Volts
20F Fixed: HV set 100-2100 with +/-100V
20R Remote: HV controlled via external D.C. voltage

Basic Neutron Pulse Counting
The 10A and 20A series of products provide accurate neutron pulse counting in a compact user friendly package.  A 50nsec digital pulse from the connector labeled "TTL OUT " is produced each time a bipolar pulse ((neutron event) exceeds the discriminator threshold. The 50nsec adjustable digital pulse (adjustable available only on the 10A and modules) is achieved via a variable resistor located beneath a hole labeled "PW " on top of each module.  A light-emitting-diode (LED) emits a flash, providing visual indications of detected pulses exceeding the discriminator setting. Control of sensitivity or threshold occurs via a 12 turn trim pot located beneath the hole identified as "THRES ADJ " on top of the module.

Pulse Observation
10A and 20A modules contain analog pre-amplification and bipolar amplifier electronics for pulse generation of detector charge events. The amplifier produces a 0 to 1 volt bipolar pulse which is fed to the "PULSE MON " test point. The output impedance of the "PULSE MON " test point is 10K ohms providing effective monitoring using an oscilloscope and impedance loading of the test point will not affect the performance of the amplifier. Amplifier gain is set with the "THRES ADJ " trim pot as identified above.  Using an oscilloscope, observe the bipolar amplifier pulse at the "PULSE MON " test point. To simplify the monitoring of the analog pulse, apply the "TTL OUT " digital pulse to the external trigger of the oscilloscope.  Remember that each time a pulse is detected by the discriminator, the LED flashes. The LED provides a good troubleshooting aid when monitoring neutron events. With the "TTL OUT " connected to the external trigger of the oscilloscope, cable the "TTL OUT " to the input of a counter and begin counting pulses. Using the 20A module, you can easily increase and decrease the HV supply observing the change in the gain of the amplifier pulses. Use the HV bias as the amplifier course gain adjust and use the "THRES ADJ " trim pot as the amplifier fine gain adjust.

Determining the Proper HV, Gain and Threshold Setting - Obtaining a Good HV Plateau Curve
Use a HV plateau to determine the proper HV setting for a Helium-3 neutron counter. To generate a plateau curve, collect a set of scalar counts ("TTL OUT " pulses) at fixed time intervals at a series of voltage steps. Plot the counts versus HV steps producing a plateau curve. The HV region above the knee of the plateau curve is generally where the detector is operated. Refer also to the high voltage bias setting recommended by the detector manufacturer.

Both PDT10A and PDT20A products provide three mechanisms to control the HV Plateau: HV bias setting, gain setting and threshold seeing. The HV bias control is not available on the PDT10A series but is controlled indirectly via the PDT20A if the HV bias input is applied to the PDT20A.Both the 10A and 20A provide gain and threshold setting capability. The gain setting control is available on the top of the module where the cables originate. The gain control uses a 12 turn trim pot located beneath the hole labeled "Gain ADJ ".The threshold setting is on the side located beneath the decal. The decal does not identify the hole location because users are not generally encouraged to change the setting. The mention of this trim pot is available for the more sophisticated user. The threshold trim pot is a single turn adjustment. The threshold setting is set at the factory to provide a minimum of .015pC sensitivity. You have the option of either increasing or decreasing the sensitivity default setting. The gain setting allows the sensitivity to be varied by about a factor of 20:1.If the minimum threshold sensitivity is set to .015pC via the threshold adjustment, you can then finely tune the gain adjustment to a final sensitivity range between .015pC to .3pC.Combining both the gain and threshold adjustments, the sensitivity can be controlled over a range of .005pC to 2pC,a very large control range.NOTE:PDT100/110/120 instruments provide only one sensitivity control adjustment that is referred to at the "THRES ADJ " control trim pot.

Digital Pulse Routing: Digital Summing
10A and 20A modules can be daisy-chained by connecting the "TTL OUT " pulses from one module to the "TTL IN " inputs of another module. This connection sums the digital pulses in an OR ’ed fashion allowing several modules to be daisy-chained, providing a single pulse counting channel.

The "TTL OUT " Pulse
The "TTL OUT " uses a 50 ohm source and has been tested with good results using cable lengths in excess of 100 meters. The pulse width is preset at the factory to 50nS.On both the 10A and 20A modules, the pulse width can be varied from 40nSec to 500nsec by changing the setting of the trim pot located on the side of the product just beneath the hole labeled "PW ADJ ".

12A Support Modules for 10A/20A Allow both Analog and Digital Summing
A support module can be purchased to provide analog summing of the signal from detectors. Charged events are collected by the 12A support module and routed to the 10A or 20A module for amplification via the cable labeled "DETECTOR PULSE ".Each 10A or 20A module can support inputs from two to four support 12A modules. This interconnection scheme reduces cost, simplifies cabling, allowing a series of detectors to be operated as a single unit. Support (12A)modules have no adjustments, do not require a +Vcc (DC power supply) input, and provide the lowest cost solution, requiring only "HV IN " and "DETECTOR PULSE " connections. Note that each module contains two "DETECTOR PULSE " cabled connectors allowing modules to be daisy--chained in a analog fashion. Such a scheme allows both analog and digital pulse summing. Support modules channel the analog pulse from the detector to the 10A/20A amplifier via a cable that connects to the "DETECTOR PULSE " connector. Digital summing occurs by connecting the "TTL OUT " to the "TTL IN " between modules as described above in the Digital Pulse Routing section. Typically, two to three 12A support modules would be used for each 10A or 20A module.

20A Series HV Bias Supply
The 20A series modules provide an equivalent feature set to the 10A modules but additionally contains an adjustable high voltage bias supply changeable from 0 Volts to 2200 Volts. All modules are configured at the factory for power supply (+Vcc) setting of either +5 volt operation (default) or +12 volt operation.

High Voltage Bias Supply:0 Volt to 2200 Volt Adjustment Span

The 20A produces high voltages and safety precautions should always be administered when using these instruments. Before applying power, make sure all cables are properly attached and modules are properly fastened to detectors. Do not apply power until modules are properly configured in your application ensuring that the power supply and all cables and detectors are properly connected to earth ground.

The internal bias voltage (high voltage generator) of each 20A series module channels its HV output to two connectors, the "HV OUT " SHV cabled connector located on the top of the module and to the HV PIN ((with suitable filtering applied) located at the bottom of the module which attaches to the detector. The HV PIN interfaces to the gas filled detector and is not available to the operator once connected to a detector. Having the bias supply available at the SHV connector allows customers to use the 20A as the biasing of the 10A series modules, or other applications that might additionally require a HV bias supply. For low count rates, a 20A module can support several 10A modules (4-8).For high count rates (>50K counts/sec), a 20A module supports three 10A modules.

20A series modules are available in three basic configurations,20A (standard),20F (Fixed Voltage), and 20R (Remote control). Each configuration can be purchased with an HN, SHV or MHV interface to the He-3 detector. The 20A-HN is the standard configuration containing an internal twelve turn variable resistor (trim pot) allowing the user control of the high voltage bias supply over a range of 0 to 2200 volts by simply changing the setting on the trim pot. The 20F is nearly equivalent to the 20A,however the trim pot adjustment span is +/-100 volts about a preset fixed voltage set at the factory. The fixed voltage is determined by the user and units can be shipped with any preset voltage setting between 100-2100 volts. The advantage of the 20F is improved noise, temperature and mechanical vibration performance. Since the adjustment range is +/-100 volts, setting the voltage to .1 volt resolution (i.e..1700.1 Volt!) is easily achievable.

20A and 20F Modules
Both 20A and 20F modules provide access to a internal trim pot via a hole labeled "HV ADJ "located on side of the module. A test point labeled "HV MON " allows you to monitor the high voltage setting as the trim pot is adjusted. When adjusting a 20A module, as you turn the trim pot from its lowest level to its highest level (turning the trim pot clockwise using an adjustment tool) a voltage at the "HV MON "test point is observed that spans from 0 to 2.2 volts resulting in a voltage at the "HV OUT " cabled SHV connector spanning 0 to 2200 volts. The same approach is used for the 20F,although the voltage adjustment span is greatly limited (+/-100 volts). All 20A units are shipped from the factory with an "HV OUT " setting of 1700 volts which corresponds to a voltage of 1.7 volts at the "HV MON " test point. When monitoring the "HV MON " test point with a DMM meter, it is best to connect the LO (or ground) of the meter to the 100 module ground itself (via one of the connector cases) rather than at the power supply source ground. This avoids additional errors resulting from ground current returns. Connecting the meter LO to one of the connector grounds is best achieved using a alligator clip, since a ground test point is not provided on the module itself.

20R Modules
The 20R provides remote operation, allowing the user to control the high voltage by applying a precision DC voltage from 0 to 2.2 volts to produce an high voltage output of 0 to 2200 volts. The 20R is shipped with an permanently attached cable (for the remote operation) terminated to a BNC. To control the bias supply voltage, apply a precision DC voltage to the BNC. Be aware that the HV generator of the 100 series is engineered for low noise and excellent long term stability operation. When using the 20R,best performance can be achieved only with use of a low noise and highly stable precision DC voltage. It is best to use a DC source that provides 50 part per million (50 PPM) stability performance and the noise performance should be in the 10 PPM region. Generally, power supply sources will not provide suitable results.

For safety, if the remote cable is not attached to a source, the 20R is designed to produce a low voltage (near zero volts) at "HV OUT ".The input resistance of the remote cable is 10K ohms, high enough that loading of your DC source should not affect performance. The input resistance of the cable alternatively provides suitable protection if you happen to connect the remote cable to a voltage greater than the power supply voltage (+Vcc). The 20R will not be damaged for voltages up to 20 volts applied to the remote cable.

In your application, after the cables and detector(s) have been properly attached to the 20A series modules, apply power to the module. Monitor the "HV MON " test point and adjust either the internal trim pot or the remote DC voltage to obtain the desired "HV OUT " voltage. Use a 5-1/2 digit voltmeter to obtain good monitoring information and allow you to simultaneously monitor noise and stability performance. (After the bias voltage is set to the desired value, neutron pulse monitoring begins.)

Factory Options
Due to the wide variety of applications,10A and 20A modules are customized to support many of these needs. Some of the customized applications provided to our customers are stated below. These modules provide enormous versatility, and can be customized to your application needs.

Customers have requested cable options used to replace the test points. Having a cable replace the "PULSE MON " and or the "HV MON " allows users to more simply monitor the analog signal of the neutron event or the HV voltage by connecting directly to their monitoring equipment.
10A and 20A,modules can be ordered from the factory to interface to gas filled detectors using HN connectors, SHV or MHV connectors or direct cables.
Low power customized modules are available for applications requiring extremely low power. For +5 Volts operation, supply current less than 2mA are available. For +15 Volt operation, supply current less the 1mA are available.

Read the Data Sheet for the PDT10 and PDT20 Series Modules

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