We will need a radiation counter to continue our exploration of the subatomic world, so let’s discuss gas-filled radiation detectors, especially the type commonly known as a Geiger counter.

As shown in Figure 56, a gas-filled radiation detector is simply a metal cylinder filled with an inert gas. A thin center wire is kept at high potential, usually between 300 and 3,000 V. The gas is ionized whenever a high-energy particle or a gamma ray enters the cylinder, causing a current pulse that can be detected after it passes through an amplifier. German physicists Hans Geiger and Walther Müller developed a tube in which radiation strips electrons off the neutral gas atoms, yielding positively charged ions and electrons. Both of these accelerate toward their oppositely charged electrodes. As they move, the ion pairs gain sufficient energy to ionize further gas molecules through collisions on the way, creating an avalanche of charged particles. This causes an intense pulse of current that cascades from the negative electrode to the positive electrode. This pulse is easily detected by very simple electronics.

Figure 56 When ionizing radiation enters a gas-filled detector, the gas inside the cylinder is ionized, causing a pulse of current between the anode and cathode. In a GM tube, the gas mixture and operating voltage are chosen so that a single ionizing event causes a cascade of ionization to produce a more easily detectable pulse.

The material used for the window determines the types of radiation that can enter the tube to be detected. Tubes with a glass window will not detect alpha radiation, since it is unable to penetrate the glass, so they are limited to detecting beta radiation and gamma rays (including X-rays). Tubes with a mica or Mylar window will detect alpha radiation, but the window area is very fragile. Geiger–Müller tubes don’t usually detect neutrons, since these do not ionize the gas inside the cylinder. We will discuss the different types of radiation.

The design of a Geiger counter is relatively simple and straightforward. In addition, complete units are widely available in the surplus market. These counters were distributed to fallout shelters by the Civil Defense in the 1950s and 1960s for use by civilian survivors in case of nuclear war. However, most have been taken out of service since the end of the Cold War, making new units plentiful.

Radiation-survey enthusiast George Dowell (owner of GeoElectronics) started the trend of modifying the Electro-Neutronics (ENi) Civil Defense V-700 model 6b into a clone of the same type of unit manufactured by Lionel.¹⁸ The version made by Lionel has a much better circuit, but is much more difficult to find. The ENi model has a great mechanical layout, so the modifications are easy to make.

We took the concept further and made a number of changes, improvements, and addition of features that turn a stock ENi CD V-700 into a great instrument for lab use (Figure 57). We affectionately call our version the “CD V-700 Pro.” Our version is based on George’s “LENi” modification, but adds the following features:

Figure 57 We modified a surplus Civil Defense V-700 radiation survey meter made by Electro Neutronics (model 6-b) into a very capable radiation counter capable of working with both GM and PMT scintillation probes. (a) We modified the front panel to accommodate the new switches, connectors, and panel light. (b) We placed a Veeder-Root count totalizer module on the side of the box. (c) The new electronic components, including a Zener diode stack and a PMT preamplifier are wired directly to the original printed circuit board.

• Preamplifier to make it compatible with photomultiplier scintillation probes (GM tubes can still be used).
• Selectable, regulated bias voltage (900 or 1,200 V) for connection to GM tubes and PMTs. A blinking indicator warns of the high-voltage selection.
• Noise-reduction circuitry eliminates hum.
• Internal piezo clicker.
• Power input jack saves batteries when powered from car or AC-operated power supply.
• 8-digit digital counter.

As a first step, study the schematic diagram of the original ENi CD V-700 (Figure 58) and identify the components on the PCB. Then study the schematic diagram of the instrument with the “LENi” modification (Figure 59) and final version (Figure 60). Start the modification by disconnecting the GM probe from the circuit. Then dig into the circuit and modify the power supply section:

Figure 58 This is the circuit schematic for the V-700 model 6-b Geiger-Müller Survey Meter made by Electro-Neutronics (ENi).

Figure 59 The ENi CD V-700 model 6-b can be modified to improve its performance, which turn it into a great lab instrument at a bargain price. The LENi modification by George Dowell is the most popular adaptation among radiation-survey enthusiasts.

Figure 60 If you are already modifying an ENi CD V-700 model 6-b, you may want to follow our circuit. Our version adds a piezo clicker, a digital totalizer, and compatibility with scintillation probes based on PMTs. We affectionately call this version the “CDV-700 Pro.”

• Remove Zener diode CR6 (sometimes 2 diodes in series) and discard, it will not be used.
• Remove R13 and reinstall it in series with the base lead going to the oscillator transistor.
• Add a 0.0022-μF, 50-V capacitor between the base and collector of transistor V4 (Q4).
• Substitute CR5 (D5) by a modern >5-kV silicon diode. We used a Fuji ESJA53-20-A, 20-kV, 5-mA diode.
• Substitute C8 with a modern 0.01-μF, 3-kV capacitor.
• Substitute C5 with a modern 0.0022-μF, 3-kV capacitor.
• Replace R13 by a 1.8-MΩ resistor and a 3.3-MΩ resistor in series.

At this point you will need to build a specialized × 10 high-voltage probe by stringing nine 10-MΩ resistors in series. An empty ballpoint pen makes a handy enclosure for the probe. Connect the probe to a digital multimeter with 10-MΩ input impedance when set to a range suitable for measuring 200 VDC (use a second multimeter to measure the resistance across the input terminals of your DMM), and measure the voltage across C8. With no GM tube connected, you should measure between 1,300 and 1,900 VDC (130 to 190 V on your multimeter).

Next, build the Zener diode stack on a small piece of perforated board. Build the Zener diode stack using 9 1N5383 150 V Zeners and 2 NTE5081A 24 V Zeners. Wrap the stack with a layer of Kapton^™ tape, and then add C15 to the back side of the board. Wrap the assembly with Kapton tape. Mount the Zener diode regulator board onto the CD V-700 PCB. Check your progress: if you power the instrument, the voltage across C15 should now be around 1,200 V, and approximately 900 V if you short the cathode of D17 to ground.

Now add the biasing circuit comprising R14, R15, R16, and C9. Continue by modifying the metering circuit:

• Remove CR7 (D7). Connect the emitters of the metering transistors in parallel and route them to the – 3 V line.
• Replace L1 with an 18-kΩ, 1/2W resistor (R18 in the CDV700 Pro schematic).
• Modify the circuit to insert a 10-Ω, 1/2-W resistor between the anode of D4 and the “– 15 V” line feeding the metering circuit.
• Replace C6 with a 47-μF, 60-V electrolytic capacitor.
• Replace C1 with two 100-μF capacitors. Leave the negative terminal of one of these capacitors open so that it can be connected to the front-panel, time-constant selection switch.

Next, you will need to modify the CD V-700’s front panel. Start by installing a BNC connector for the probe. Remove and discard the sealing nut through which the GM probe cable passed. Tap this hole using a ³/₈-in.-diameter 32 tpi pitch tap (McMaster-Carr 25705A64 ). Mount a nonisolated bulkhead BNC connector (e.g., Jameco 71589) on this tapped hole. Drill the front panel (use casting marks on the back side) to accommodate the extra switches, connectors, piezo speaker, and LED, as shown in Figure 57a. Use a nibbling tool to cut a 68-mm × 33-mm rectangular hole on the enclosure bottom to accommodate the Veeder-Root A103-000 totalizer, as shown in Figure 57b.

Once you are done with the mechanical changes, wire the instrument as shown in Figure 60, using clean, new cable with insulation for the appropriate voltage rating. Route cables next to the enclosure and keep connections as short, direct, and clean as possible. Use high-voltage test lead wire between the PCB and the center terminal of the probe BNC. Use good heat-shrink tubing to dress all switch connections. Keeping things tidy will really pay off later. Once the PMT preamplifier is added, noise will creep into the system if you don’t pay attention to your wiring habits.

This is a good point to test your “almost LENi”. The voltage at the BNC connector measured with the high-impedance × 10 probe should read around 1,200 V with the probe selector switch in the “Scintillator” position. The flashing LED should light up and blink. Flipping the probe selector switch to the GM position should cause the LED to turn off, and the voltage at the connector should drop to approximately 900 V.

Turn the probe selector switch to the “GM” position. Connect a GM probe (e.g., the original CD V-700 probe to which you have installed a male BNC connector). The unit should produce background clicks and be able to detect the radiation emitted by the operational check source under the CD V-700’s original label. Next, connect the totalizer module to the circuit and verify that the counter advances once for every “click.”

Build the preamplifier circuit on a small piece of prototyping board. Keep wires short and the circuit neat and organized. Unsolder C5. Mount the preamplifier board directly onto the CD V-700 PCB, as shown in Figure 57c. Add the ground wire and bypass capacitors C16 and C17.

Add a two-D cell plastic battery holder to make space for the preamplifier circuit. Drill some mounting holes on the PCB and mount the battery holder using 1/4in. nylon spacers. CONGRATULATIONS! You have completed modifying your surplus ENi CD V-700 model 6b into a CDV-700 Pro!

The CD V-700 Pro is compatible with virtually any GM tube that operates on 900 V. We recommend that you purchase a tube with a mica window (sensitive to alpha radiation) besides the probe that came with your CD V-700, which is sensitive only to beta- and gamma-rays. A suitable tube is the LND 7311 “pancake” GM tube, or a probe made with this tube, such as the GeoElectronics GEO-210.

You can also use the CDV-700 Pro with sensitive scintillation probes. Scintillators are materials that produce a short flash of light when hit by ionizing radiation. We already met one such scintillator when we used zinc-selenide as a phosphor that emits light when struck by electrons inside a CRT. Other purpose-grown inorganic crystals have been developed and are commonly used in physics labs. The most widely used is Nal(Tl) (sodium iodide doped with thallium). In addition, many organic liquids and solids exhibit scintillation. Easy-to-use plastic scintillators have been developed by incorporating organic scintillating substances within a transparent plastic. Plastic scintillators are widely used by experimenters, because they are relatively inexpensive and are very easily shaped.

Detection of the scintillation from a scintillator crystal or plastic is commonly done with a PMT. For example, a cylindrical piece of plastic scintillator can be coupled to the PMT probe we built (Figure 30). However, for the CDV-700 Pro, the PMT has to be wired in a slightly different way than we did in Figure 29. Figure 61 shows the schematic diagram for a scintillation probe powered directly by the CDV-700 Pro. Note that the resistors are of much larger value than those we had used for the probe of Figure 29. This is because the CDV-700 Pro’s power supply cannot provide more than just a couple of microamperes. We used a spare Photonis XP2102, but almost any 2- to 3-in. PMT should work equally well if you follow the general circuit and adapt it to the pinout and number of dynodes of your PMT.

Figure 61 A crystal or plastic scintillator coupled to a PMT can be used as a very sensitive radiation probe for the CDV-700 Pro. (a) Our probe is self-contained, and is powered directly via the CDV-700 Pro’s probe connector. (b) The complete assembly must be built inside a light-proof enclosure. We used a thick cardboard shipping tube and matching tin caps to enclose the probe.

The scintillator should be coupled to the PMT’s face with index-coupling grease. This doesn’t have to be the expensive gel sold for premium optical systems (e.g., Dow Corning Q2-3067 optical coupling compound at over $250 for a 4-oz jar), but rather any low-cost, high-purity silicone grease such as Dow Corning 4. Use a tiny bead and let it squeeze between the PMT and the scintillator to form the thinnest possible interface between the two components. If you prefer not to use index-coupling grease, you may omit it, but this omission will result in a 10–20% drop in probe sensitivity.

Lastly, package the complete assembly within a light-tight enclosure. We used a cardboard shipping tube with 2-in. ID and its matching tin caps to shield the PMT/scintillator assembly from light, but any other light-tight enclosure will work well. Radiation-survey aficionado Charlie Thompson built a ground-survey probe by cleverly enclosing a PMT coupled to a square block of plastic scintillator inside a Home Depot^® paint can.¹⁹

α, β, AND γ

Thomson’s discovery of the electron demonstrated that the phenomenon of ionizing radiation was not a single process. However, the connection between Thomson’s electrons, Roentgen’s X-rays, and the Becquerel “uranium radiation” was a complete mystery.

In 1898, Pierre and Marie Curie discovered that thorium (a metal represented by the symbol Th and atomic number 90) gives off “uranium rays,” which Marie renamed radioactivity. Later that year, the Curies discovered the elements polonium and radium. The Curies realized that radioactivity must be some new property of the atoms themselves, since the long series of chemical processes used to isolate them from radioactive rocks didn’t change their radioactive emissions.

British/New Zealander physicist Ernest Rutherford—then a research student at the Cavendish Laboratory—showed that X-rays and radioactivity were capable of ionizing gases in essentially the same way. A year later, Rutherford noticed that uranium salts produced two different types of radiation—one, which he termed alpha (α) radiation, was highly ionizing but unable to penetrate even thin pieces of paper; the other type of radiation, which he termed beta (β) radiation, exhibited lower ionizing capability but was of higher penetrating power. In 1903, Rutherford realized that a type of radiation from radium discovered (but not named) by French chemist Paul Villard in 1900 must be different from alpha rays and beta rays, due to its much greater penetrating power. Rutherford named it gamma (γ) radiation (Figure 62).

Figure 62 Ernest Rutherford classified radiation based on its penetrating power. He called the radiation that is highly ionizing, but which cannot penetrate a sheet of paper, alpha (α). He called radiation that had lower ionizing power, but higher penetrating power beta (β). Rutherford named the most highly penetrating rays gamma (γ) radiation.

Let’s evaluate the penetrating power of alpha, beta, and gamma radiation for our first experiment with the CDV-700 Pro Geiger counter. You will need a GM tube such as the LND 7311 “pancake” that is sensitive to all three types of radiation. If you prefer, you could purchase a ready-made probe made with this tube, such as the GeoElectronics GEO-210. Please note that the mica window in these GM tubes is extremely fragile, so be very careful to avoid puncturing it and ruining your detector!

Next, you will need three radioactive sources, each with a dominant emission. Table 4 and Figure 63 show some of the radioactive sources available to the experimenter. Safe sources of known activity may be purchased from a number of suppliers including Spectrum Techniques, Images Scientific Instruments, and United Nuclear. These are sources encased in a plastic disk or metallized onto some other substrate. They are exempt from licensing, and require no special handling, storage, or disposal. We used Polonium-210 (²¹⁰Po), Strontium-90 (⁹⁰Sr), and Cobalt-60 (⁶⁰Co) exempt, plastic-disk sources to experiment with the penetrating power of alpha, beta, and gamma.

Figure 63 There are many radioactive sources available to experimenters. Although the best sources for our purpose are sealed disks that contain exempt quantities of known radioactive materials, there are everyday objects such as smoke detectors, old lantern mantles, and watches with radium-painted dials that emit detectable levels of alpha, beta, and gamma radiation.

TABLE 4 Some Radioactive Sources that may Be Used for Experimenting with the Penetrating Power of Alpha, Beta, and Gamma Rays^*

In addition to the prepackaged sources, there are a number of everyday objects such as ionization-type smoke detectors, old lantern mantles, and watches with radium-painted dials that emit detectable levels of alpha, beta, and gamma radiation. Most of these items are no longer manufactured, as exposure to the radioactive material was a health threat to the employees who made them. However, they can be found at online auctions, garage sales, and antique shops. These items were not designed for instructional or experimental use, and may therefore be hazardous when used for other purposes than originally intended. For this reason, our recommendation is to use the purpose-made, encased plastic-disk, exempt sources whenever possible.

The experiment is simple—place each of the available sources, one at a time, close to the window of the GM tube. Pay attention to the clicking rate. Put paper, acrylic, aluminum, and lead absorbers one at a time between the source and the tube. Just a sheet of paper stops alpha radiation, a sheet of aluminum or acrylic stops beta radiation, while gamma is stopped only by a sufficiently thick sheet of lead. You can also try moving each source away from the detector. Alpha has a very short range and quickly deposits all of its energy in the air between the source and the detector. Beta has a range of about 10 cm in air, and gamma gets weaker with distance but doesn’t come to a stop at any particular distance.