December 1972 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Parts 1 and 2 of this series covered the theory of nuclear physics and laboratory investigation
devices. This final installation talks about commercially available test instruments for detecting
and measuring nuclear radiation levels. My introduction to Geiger counters was in the old
The Adventures of Superman television show
(the originals with George Reeves) where they were used by villains to verify that
their stash of Kryptonite
would be sufficient to disable our superhero. I could not find anywhere whether Kryptonite's
emission type is alpha particles (helium nucleus; i.e., 2 protons and
2 neutrons), beta particles (electron), gamma rays
(electromagnetic waves), or some other form. Multiple designs
of detectors are used based on radiation type and strength to be measured.
Side note: Did you know there are various
colors of Kryptonite, all with unique effects on former dwellers of the
Part 2 appeared in the
issue of Popular Electronics, and will be posted soon. I do not yet own the October
1972 edition with Part 1.
How the Ionization Chamber, Proportional Counter, and Geiger-Muller Tube
Part 3: Radiological Survey Meters
By J. G. Ello, Radiation Measurements and Instrumentation Electronics Division, Argonne
So far, we have discussed the phenomenon of radioactivity and found that the three types
of radiation are alpha and beta particles and gamma rays. Radiation can ionize atoms. Since
the ions have an electrical charge, they can be directed in an electrical field to a positively-charged
anode (collector) where they are neutralized by supplying the positive ions with electrons
from a battery, thus providing a measurable current.
Fig. 1 - Ionization chamber.
To make use of this detector current, a unit of measurement must be established. Radiological
survey meters measure radiation intensity in roentgens per hour (R/hr). A roentgen is the
intensity of gamma radiation that will reduce one electrostatic unit of ions/cu cm of dry
air, or the intensity of radiation that will produce a little over 2 billion ion pairs/cu
cm of dry air. The unit of measurement for detectable radioactivity is called the roentgen,
the order of magnitude most commonly used is the milliroentgen, or mR.
As an example, a radioactive source is emitting an intensity of 2 mR/hr at a point a certain
distance from the source. If you were to stand at this distance for one hour, you would receive
a 2 mR/hr exposure. If the intensity were only 1 mR/hr and exposure time were extended to
two hours, the radiation would be the same since (1 mR/hr) (2 hr) = 2 mR.
Ionization Detector. The construction of an ionization chamber is quite
simple. As shown in Fig. 1, it consists of an insulated central anode enclosed by a shell
of conducting material with an alpha window. The ionization chambers used in many commercial
survey meters are designed for only beta/gamma radiation If alpha particles are to be detected,
a very thin window is incorporated into one end of the detector to allow the particles to
pass into the detector. Most alpha windows are made from 1-mil (0.001") thick Mylar with a
coating of conducting material on both sides. In some ionization survey meters, slide-type
alpha and beta absorbers are used to permit measuring beta particles in the presence of alpha
particles and gamma rays in the presence of beta articles.
Fig. 2 - Two views of ionization survey meter.
Fig. 3 - Schematic of ionization survey meter.
An ionization survey meter is shown in Fig, 2. Through use of its built-in absorbers, it
can detect and measure alpha, beta, and gamma radiation. This is a self-contained unit, powered
by internal batteries. It consists of an ionization chamber, range selector, amplifier, and
dc microammeter. The alpha absorber is a 0.01"-thick cellulose sheet, while the beta absorber
is a 0.102"-thick plate of aluminum. Its range may vary from about 5 mR/hr to 50 R/hr.
A simplified block diagram of the ionization survey meter is shown in Fig. 3. The ionization
detector is maintained at the proper voltage for operation in the ionization-chamber region
of the pulse-size/detector-voltage curve. As ions are neutralized at the collector, a small
ion-chamber current flows through the selected range resistor. The pulse developed across
this resistor is applied to the grid of the amplifier tube or gate of a field-effect transistor.
This minute current is amplified and passed through the meter movement which is calibrated
In use, the ionization chamber is located in the bottom forward position of the instrument
with the window on the bottom surface. To detect beta and gamma radiation simultaneously,
the aluminum absorber is pulled up; to detect alpha, beta, and gamma radiation simultaneously,
both absorber tabs are pulled up.
To check the unit's operation, an alpha or beta source is held close to the Mylar window.
With the survey meter adjusted to a suitable range, a reading should be indicated on the meter.
When making surveys, the unit would be moved across the suspected material at about a 2 in./second
scan and close to the material's surface to obtain a true reading. Whenever a rapid jump of
the meter pointer is observed, radioactivity is most likely present in the material. To determine
if it is gamma radiation, both the alpha and beta absorbers should be in place. To read beta
radiation, the alpha absorber is left in place and the beta absorber is removed. To determine
beta intensity, the gamma reading is subtracted from the new reading. To check for alpha radiation,
both alpha and beta absorbers must be removed. Alpha intensity is determined by subtracting
the reading obtained for beta only from the new reading. If no change is observed, there is
no detectable alpha radiation present.
Proportional Detector. Although the ionization detector is ideal for detecting
the three types of radiation, it needs a very sensitive electrometer tube or field-effect
transistor amplifier stage. To eliminate the need for sensitive amplifiers, detectors in which
internal amplification takes place have been devised. This amplification is referred to as
the proportional region on the pulse-size / detector-voltage curve. In this region, it is
possible to differentiate between alpha-, beta-, and gamma-generated pulses.
The internal amplification within the detector is caused by increasing the detector voltage
and, hence, the electrical field between the anode and cathode which causes the electrons
produced in the primary ionization of the atom to travel at higher velocities. The primary
electrons also have sufficient energy to dislodge other electrons in their path and thereby
generate larger pulses.
Fig. 4 - Air proportional detector.
Fig. 5 - An air proportional survey meter.
Shown in Fig. 4 is a basic sketch of an air proportional detector. It consists of a 1-mil
diameter center anode wire mounted on Teflon insulators. The alpha particle window, fastened
to the shell by an adhesive, is made from 0.25-mil thick Mylar. The detector may consist of
one center wire or as many as ten wires.
A battery-operated count-rate survey meter and its air proportional detector are shown
in Fig. 5. The detector has a metal screen to protect the alpha window against punctures.
It is called air proportional because it contains air instead of a counting gas like that
used in Geiger-Muller detectors. The detector is capable of responding to all three types
of radiation and, through the use of a discriminating control, identify pulses produced by
alpha particles and those produced by beta particles.
The survey meter consists of an adjustable detector, high-voltage power supply, range selector,
amplifiers, discriminator, and count-rate circuit which supplies current to the dc microammeter.
The meter is calibrated in counts/minute instead of mR/hr as in the ionization and Geiger-Muller
The simplified block diagram of the survey meter is shown in Fig. 6. The detector is set
to operate in the proportional region by means of the detector voltage supply. Pulses produced
at the center wire (anode) are coupled to the grid of the first amplifier stage and are then
coupled to the second amplifier through the range selector circuit. At the discriminator control,
pulses of proper size are selected and fed to the count rate circuit which supplies the dc
current for the meter.
The average efficiency of an air proportional detector is about 10-15 percent; the detector,
therefore, is affected by only 10-15 percent of the total disintegration taking place. For
example, if a radioactive source disintegrates at a rate of 1000 disintegrations/minute (d/min),
the detector "sees" about 100-150 d/min.
The surveying technique is the same as for ionization survey meters. The proportional survey
meter is used mainly in the field for alpha detection and measuring. If beta and/or gamma
radiation are to be measured, the discriminator control must be adjusted to pass the smaller
pulses they produce. In addition to gamma pulses, beta and alpha pulses, if present, will
also be indicated under these conditions. Hence, the unwanted reading (in this case, the alpha
reading) must be subtracted from the total reading. Absorbers can also be used as in the ionization
Fig. 6 - A simplified circuit diagram of an air proportional survey meter.
Fig. 7 - (A) The thin-wall Geiger-Muller detector. (B) End-window G-M tube.
Geiger-Muller Detector. Often referred to as the "G-M tube," the Geiger-Muller
detector is the most often used radiation detection instrument. It operates in the Geiger-Muller
region of the pulse-size/detector-voltage curve. The main difference between the proportional
detector and the G-M tube is that in the former, the incident radiation produces an avalanche
of electrons at only one point in the detector, while in the latter, the electron avalanche
spreads along the entire length of the wire anode. The pulse size in the proportional detector
varies with the number of secondary electrons produced. In the G-M tube, electron amplification
is much larger so that the pulse size is practically independent of the number of electrons
produced per nuclear radiation incident. The G-M tube, therefore, cannot discriminate between
types of radiation.
The larger pulse size is due to the high electrical field and the fact that the G-M tube
is filled with a counting gas, such as neon plus halogen, instead of air. This provides an
additional electron amplification factor, referred to as "gas amplification." The main advantage
of gas amplification is that the detector itself requires less external amplification.
Shown in Fig. 7 A is a basic thin-wall G-M tube, while Fig. 7B depicts a basic end-window
G-M tube. The cathode is stainless steel or glass tubing with a conductive coating on the
To detect alpha radiation, the wall of the tube must be very thin. For this application,
the G-M detector is fabricated with a very thin window as shown in Fig. 7B. The cathode material,
usually made from stainless steel, is about 0.003" thick, mainly for strength. The end window
is made from very thin mica. The center wire is supported only at the base end, while the
free end is tipped with a glass bead to prevent spurious internal discharges.
In Fig. 8 are shown two battery-powered G-M survey meters. The end-window G-M survey meter
is at the left; a thin-wall G-M meter is on the right. Both instruments contain a high-voltage
detector supply, amplifier and trigger stages, and a drive circuit for the dc microammeter.
Like the proportional survey meter, an audio jack is provided for aural monitoring.
A simplified block diagram of a G-M survey meter is shown in Fig. 9. The detector is maintained
at a fixed voltage to operate in the Geiger-Muller region of the pulse-size/detector-voltage
curve. Due to the detector gas electron amplification, a pulse amplifier is not required;
detector pulses are coupled directly to a two-tube trigger circuit which modifies the pulse
sizes and feeds them to the metering circuit which is calibrated in mR/hr. The coupling between
the trigger tubes serves as the range selector.
The walls and mica windows of the G-M tube are very thin; so, care must he exercised to
avoid any physical shock to the detector. Unlike an ionization survey meter which can work
if the detector's window is ruptured, G-M detectors become inoperative if the wall or window
Another limitation of G-M detectors is their inability to respond indefinitely to radiation,
which can be done by ionization and air proportional detectors. The normal life of a G-M tube
is about a billion counts, after which efficiency drops off. Many G-M tubes cease to give
any indication and may read zero mR/hr when exposed to a very high radiation intensity.
Fig. 8 - (Left) End-window and (right) thin-wall Geiger-Muller survey meters.
Fig. 9 - Basic diagram of G-M survey meter.
Scintillation Detector. Another type of detector used in surveying radioactivity
is known as the "scintillation detector." It is made from a phosphorescent material which
gives off flashes of light called scintillations, when exposed to nuclear radiation. Modern
scintillation materials are in the form of crystals, liquids, or gases. The most widely used
at the present time is a sodium iodide crystal, optically coupled to a #6292 photomultiplier
When exposed to nuclear radiation, the. scintillator gives off a glow which is converted
into electrical pulses by the photo-multiplier tube. The scintillation detector consists of
an optical coupling system contained inside a light-tight package. To detect alpha and beta
particles, the housing must have a very thin opaque window to exclude all outside light while
passing only the radiation particles.
The magnitude of the electrical pulses generated by the scintillations is proportional
to the energy of the radiation. In most cases, the pulse information is analyzed in a multichannel
pulse-light analyzer to identify nuclear radiation by type. In addition, the scintillation
detector can operate in counting ranges many times higher than the G-M detector.
Solid-State Detector. One of the newer developments in radiation detection
is the solid-state detector. It operates on basically the same principle as the G-M tube and
ionization detector except that it employs a semiconductor material such as silicon instead
of a gas for the counting material. When employed as a detector, the silicon is in a highly
purified state and, like other detectors, is sensitive to and can measure energy levels of
alpha, beta, and gamma radiation.
Silicon diode detectors can be made very small, so small in fact that they can be mounted
in hypodermic needles. Most detectors, however, measure about 1" in diameter by 1/16" thick.
They are used mainly in the health physics field for counting airborne radioactive particles.
Non-electronic Detectors. Radiological survey meters discussed above are electronic instruments
used for measuring radiation intensity at a given moment in time. There are, however, devices
which measure the total accumulated radiation dosage to which an object is exposed over a
period of time (as short as a few minutes or as much as a week or more).
The self-indicating dosimeter is basically a miniature ionization chamber made in the form
of a fountain pen. It contains a capacitor which is originally charged up to a fixed voltage.
When exposed to radiation the ions formed in the chamber remove some of the capacitor's charge
and reduce the voltage. The voltage charge remaining is measured by a movable quartz fiber
built into the unit. An internal scale, when held to the light and viewed through a lens magnifier,
indicates this voltage drop in mR/hr.
Another type of dosimeter employs the radioluminescence phenomenon. Called a thermoluminescent
dosimeter, it works in a manner similar to the scintillation detector. However, external equipment
is needed to "develop" the indicating medium and to count the accumulated radiation intensity
One of the oldest known types of detectors is the film badge. The radiation received on
exposure causes the film to darken in proportion to the amount received. Special film badges
have been developed for low-energy beta, gamma, X-ray, and neutron types of radiation. Detection
of alpha particles is virtually impossible due to the fact that the film must be packaged
in an opaque container. External equipment is needed for developing and interpreting the film's
Posted July 20, 2017