Scintillation Counters and Multichannel Analyzers (MCAs)

The Precision Radiation Instrument (PRI) Model 111B “Deluxe Scintillator” was one of the earliest commercial NaI survey meters and, like most of the company’s instruments, it employed an eye-catching chromed finish.  The PRI 111B’s main specifications are:

Detector:      1.5” x 1”NaI crystal.

Range:          0 – 0.025, 0 – 0.05, 0 – 0.25, 0 – 0.5, 0 – 2.5 and 0 – 5 mR/h

Batteries:      four 1.5 volt flashlight batteries, two 22.5 volt batteries and two 67.5 volt

Price:            $495.00 new in 1954, $300 to $500 on eBay in 2005



Restoring a Precision Radiation Instrument (PRI) Model 111B “Deluxe Scintillator”

David Prutchi’s Mod of the DT-590A/PDR-56 “X-Ray” Probe into a Scintillation Counter and MCA Probe

◄  The Radiac Set, AN/PDR-56 is a portable scintillation type instrument used for detection of alpha contamination. The system includes a large and small interchangeable probe with a probe extension. The AN/PDR-65/65A system measures gamma intensities to 10,000  cGy/hr; and records cumulative doses to 9,999 cGy/hr.  I do not have the Am-141 checksource.

  The AN/PDR-56 has been replaced by the AN/PDR-77, which resulted in many new scintillation probes to be placed in the surplus market.

In addition to emitting 5.1 MeV alpha particles, the element Plutonium-239 also emits gamma rays in the energy range of 14 to 21 keV. Because these gamma rays are more penetrating than the alpha particles, they travel further in matter and air and can be detected at further distances from the ground. The “x-ray” probe for the AN/PDR-56 uses a CaF2(Eu) scintillator/photomultiplier combination to detect the 14-21 keV gammas from Pu-239. The photomultiplier tube fits into a tube socket with a voltage divider network to supply the dynode voltages. A mu-metal shield surrounds the phototube to protect it from gain changes due to stray magnetic fields or from changes in orientation with respect to the earth’s magnetic field. The x-ray probe is an assembly which includes the amplifier-discriminator circuits integral to the phototube scintillator housing. The discriminator is a single channel analyzer adjusted to detect Pu-239 gamma rays.


CLICK HERE for the complete maintenance manual (including schematics) for the PDR-56

  Coming soon:  Modifying the DT-590A/PDR-56 “X-Ray” Probe into a Scintillation Counter and MCA Probe

David Prutchi’s Scintillation Crystal / PMT Evaluation Setup

David Prutchi’s Home-Brewed Multichannel Analyzer (MCA)

  Current state of my PIC-based portable MCA.  The pulse processing and peak-detection circuit is shown at the bottom.  A PIC applications development board is my breadboard platform.  Spectra are displayed on a Scott Edwards Electronics graphics LCD.  I’ll post schematics when the system is debugged.  Keep an eye on this page!

David Prutchi’s Mod of the CD V-700 Survey Meter for Scintillator Operation

  My “CDV-700 Pro” can be connected  either to a 900V Geiger Mueller tube or to a 900V or 1200V photomultiplier-based scintillation probe. The CDV-700 Pro has an internal preamplifier which makes it possible to detect low-amplitude pulses related to low-energy gamma interactions with the scintillation crystal.


CLICK HERE to go to my CD V-700 mods page

  In my home-brew scintillation probe, a surplus 8” long x 2” dia NaI(Tl) crystal is coupled to a Photonis XP2102 photomultiplier tube. The complete assembly is wrapped in optical-black cloth and placed inside a thick cardboard tube closed using PVC caps. CLICK HERE for Schematic [PDF], CLICK HERE [3MB PDF] for detailed instructions.

CLICK HERE for pictures of my restored PRI 111-B

CLICK HERE for PRI 111B repair by Allyn Goshy's

CLICK HERE for a source of NaI(Tl) crystals - Contact Dr. Ival Toepke at Radiation Sensors.  They do sell to hobbyists.

CLICK HERE for instructions on replacing the batteries using a DC/DC converter

CLICK HERE for the schematic diagram for the PRI 111B

CLICK HERE for data on the type 6199 end-viewing, 1 1/2 inch diameter, 10-stage photomultiplier tube used in the PRI-111B

CLICK HERE for a source of PMT replacements:  Sphere Research Corporation

CLICK HERE for data on the CK526 pentode tube

CLICK HERE for data on the 6007 radiation-cooled pentode tube

CLICK HERE for data on the 5841 gas regulator tube

© 2005 David Prutchi.  All rights reserved.

  My 1” disc check sources (from top right, clockwise): 10uCi Cs-137, PMI checksource, 0.1uCi Sr-90, 1uCi Ba-133, 1uCi Eu-152, 1uCi Co-60.  Center: a GeoElectronics thorium mantle.  These are all legal to have without a license!

Radioactive Sources

My Miniature Detectors

  This is a complete PMT/NaI(Tl) scintillation probe manufactured by Scionix in Holland.  It has a 9.5mm diameter x 20mm long NaI(Tl) crystal.  It incorporates a 0.4” PMT with integral resistor divider and combined power/signal connector via single coax connector.

  This is a CdZnTe (CZT) room-temperature, Single Point Extended Area Radiation (SPEAR) Detector by eV.  It is capable of measuring ionizing radiation

(X- and gamma-rays, as well as β and α-particles). The SPEAR detector combines a robust design, CdZnTe detector and signal preamplifier in a small package.

A damaged pixilated CZT detector (left) still has quite a few active pixels left to be the basis for an amateur miniature gamma camera.  The CZT detector on the right needs a preamplifier and a bias power supply to yield signals suitable for pulse-height analysis.

My friends at our sister company Spectrum Dynamics kindly gave me these treasures from their junkbox.  Take a look at their website!!  Spectrum Dynamics developed very cool technology to yield high-resolution 3D SPECT medical images using miniature gamma cameras and high-speed reconstruction algorithms.

CCFL-Inverter-Based High Voltage Power Supplies

  This circuit implements Charlie Thompson’s 0 to 1500V+, regulated, low-ripple HV power supply for PMT's and Geiger tubes  Unfortunately, the CCFL inverter used by Charlie cannot drive PMT dividers with relatively low total resistance.

  In this low-ripple HV power supply, I followed the design of the power supply section of the Spectrum Techniques model 450 SCA.   However, instead of their HV transformer and oscillator, I used a BXA-12579 CCFL inverter.  This circuit has enough power to drive all of my commercial PMTs (up to 1,600VDC) with low-total-resistance integrated voltage dividers.   Click here for the schematic diagram.

  This is my setup to evaluate scintillation crystals and pieces of scintillation plastic.  It consists of a 12-stage, high-gain bialkaly PMT enclosed in a light-tight chassis.  Optical black film is used to wrap the assembly to ensure that stray photons that managed to get through the sealing don’t make it to the PMT.  The chassis top is hinged, and a light-tight seal is ensured by tightening some wingnut bolts.

David Prutchi’s Mod of the DT-590A/PDR-56 “X-Ray” Probe into a General-Purpose Gamma Probe

I recently purchased a PDR-56F radiac set on eBay. The AN/PDR-56 is a portable scintillation type instrument used for detection of alpha contamination (e.g. from Plutonium).  Gladly, I don’t have much plutonium contamination at home to worry about

I decided to purchase an extra DT-590A/PDR-56F x-ray probe from and modify it by replacing the CaF2(Eu) crystal with a NaI(Tl) sealed assembly. I also changed the settings of the SCA to count 100keV to 1.3MeV gammas (can also be used to hunt for a specific line by narrowing the SCA window).

Europium doped calcium fluoride CaF2(Eu) is a low density scintillation crystal with a high light output. Thanks to its low Z value it is well suited for the detection of low-energy gamma photons ("x-rays"). However, the higher-density NaI(Tl) scintillation crystals are used in most standard applications for detection of gamma-radiation because of their unequalled high light output and the excellent match of the emission spectrum to the sensitivity of photomultiplier tubes.

The probe works great! Using a 1" dia x 2" long NaI(Tl) crystal and setting the discrimination floor at 100keV, I have a background count of ~800 CPM. I get 10,000 CPM from a 10uCi Cs-137 checksource from 2' away.


CLICK HERE for complete instructions on modifying the DT-590A/PDR-56 x-ray probe into a general-purpose gamma probe.

I don’t want to compromise the security of our country.  If you are a government agent (FBI, CIA, DEA, FDA, FAA, etc.) and find material in these pages that you find objectionable, please contact me so that I can rectify the situation.