What is a Geiger counter?
A Geiger counter is a metal cylinder filled with low-pressure gas sealed in by
a plastic or ceramic window at one end. Running down the center of the tube there's a thin metal
wire made of tungsten. The wire is connected to a high, positive voltage so there's a strong electric
field between it and the outside tube. When radiation enters the tube, it causes ionization, splitting
gas molecules into ions and electrons. The electrons, being negatively charged, are instantly attracted
by the high-voltage positive wire and as they zoom through the tube collide with more gas molecules
and produce further ionization. The result is that lots of electrons suddenly arrive at the wire,
producing a pulse of electricity that can be measured on a meter and (if the counter is connected to
an amplifier and loudspeaker) heard as a "click." The ions and electrons are quickly absorbed among
the billions of gas molecules in the tube so the counter effectively resets itself in a fraction of a
second, ready to detect more radiation. Geiger counters can detect alpha, beta, and gamma radiation.
How a Geiger counter works
In summary then, here's what happens when a Geiger counter detects some radiation:
1. Radiation (dark blue) is moving about randomly outside the detector tube.
2. Some of the radiation enters the window (gray) at the end of the tube.
3. When radiation (dark blue) collides with gas molecules in the tube (orange), it causes
ionization: some of the gas molecules are turned into positive ions (red) and electrons
(yellow).
4. The positive ions are attracted to the outside of the tube (light blue).
5. The electrons are attracted to a metal wire (red) running down the inside of the tube
maintained at a high positive voltage.
6. Many electrons travel down the wire making a burst of current in a circuit connected to it.
7. The electrons make a meter needle deflect and, if a loudspeaker is connected, you can hear a
loud click every time particles are detected. The number of clicks you hear gives a rough
indication of how much radiation is present (the meter gives you a much more accurate idea).
Who invented the Geiger counter?
Geiger counters are the most familiar of various ionizing radiation detectors that work in broadly the
same way. German physicist Hans Geiger (18821945) developed the idea in 1912 while working
with Ernest Rutherford, the New-Zealand-born physicist who "split the atom" (proved
experimentally that atoms consisted of other, smaller particles). Back in Germany, sixteen years
later, Geiger greatly improved the instrument with the help of a colleague named Walter Mller,
which is why Geiger counters are often called Geiger-Mller counters (or Geiger-Mller tubes).
Photographic plates were also an important tool in early high-energy physics, as they get blackened
by ionizing radiation. For example, in the 1910s, Victor Franz Hessdiscovered cosmic radiation as it
left traces on stacks of photographic plates, which he left for that purpose on high mountains or sent
into the even higher atmosphere usingballoons.
Medical imaging[edit]
The sensitivity of certain types of photographic plates to ionizing radiation (usually X-rays) is also
useful in medical imaging and material science applications, although they have been largely
replaced with reusable and computer readable image plate detectors and other types of X-ray
detectors.
Decline[edit]
The earliest flexible films of the late 1880s were sold for amateur use in medium-format cameras.
The plastic was not of very high optical quality and tended to curl and otherwise not provide as
desirably flat a support surface as a sheet of glass. Initially, a transparent plastic base was more
expensive to produce than glass. Quality was eventually improved, manufacturing costs came down,
and most amateurs gladly abandoned plates for films. After large-format high quality cut films for
professional photographers were introduced in the late 1910s, the use of plates for ordinary
photography of any kind became increasingly rare.
The persistent use of plates in astronomical and other scientific applications started to decline in the
early 1980s as they were gradually replaced by charge-coupled devices(CCDs), which also provide
outstanding dimensional stability. CCD cameras have several advantages over glass plates, including
high efficiency, linear light response, and simplified image acquisition and processing. However,
even the largest CCD formats (e.g., 8192x8192 pixels) still do not have the detecting area
and resolution of most photographic plates, which has forced modern survey cameras to use large
CCD arrays to obtain the same coverage.
The manufacture of photographic plates has been discontinued by Kodak, Agfa and other widely-
known traditional makers. Eastern European sources have subsequently catered to the minimal
remaining demand, practically all of it for use in holography, which requires a recording medium
with a large surface area and a submicroscopic level of resolution that currently (2014) available
electronic image sensors cannot provide. In the realm of traditional photography, a small number of
historical process enthusiasts make their own wet or dry plates from raw materials and use them in
vintage large-format cameras.
Preservation[edit]
Several institutions have established archives to preserve photographic plates and prevent their
valuable historical information from being lost. This is a particular need in astronomy, where
changes often occur slowly and the plates represent irreplaceable records of the sky and astronomical
objects that extend back over 100 years. An example of an astronomical plate archive is the
Astronomical Photographic Data Archive (APDA) at the Pisgah Astronomical Research
Institute (PARI). APDA was created in response to recommendations of a group of international
scientists who gathered in 2007 to discuss how to best preserve astronomical plates. The discussions
revealed that some observatories no longer could maintain their plate collections and needed a place
to archive them. APDA is dedicated to housing and cataloging unwanted plates, with the goal to
eventually catalog the plates and create a database of images that can be accessed via the Internet by
the global community of scientists, researchers and students. APDA now has a collection of more
than 200,000 photographic images from over 40 observatories that are housed in a secure building
with environmental control. The facility possesses several plate scanners, including two high-
precision ones, GAMMA I and GAMMA II, built for NASA and the Space Telescope Science
Institute (STScI) and used by a team under the leadership of the late Dr. Barry Lasker to develop the
Guide Star Catalog and Digitized Sky Survey that are used to guide and direct the Hubble Space
Telescope. APDA's networked storage system can store and analyze more than 100 terabytes of data.
A scintillation counter is an instrument for detecting and measuring ionizing radiation by using the
excitation effect of incident radiation on a scintillator material, and detecting the resultant light
pulses.
It consists of a scintillator which generates photons in response to incident radiation, a
sensitive photomultiplier tube which converts the light to an electrical signal and electronics to
process this signal.
Scintillation counters are widely used in radiation protection, assay of radioactive materials and
physics research because they can be made inexpensively yet with good quantum efficiency, and can
measure both the intensity and the energy of incident radiation.
When a charged particle strikes the scintillator, its atoms are excited and photons are emitted. These
are directed at the photomultiplier tube's photocathode, which emits electrons by the photoelectric
effect. These electrons are electrostatically accelerated and focused by an electrical potential so that
they strike the first dynode of the tube. The impact of a single electron on the dynode releases a
number of secondary electrons which are in turn accelerated to strike the second dynode. Each
subsequent dynode impact releases further electrons, and so there is a current amplifying effect at
each dynode stage. Each stage is at a higher potential than the previous to provide the accelerating
field. The resultant output signal at the anode is in the form of a measurable pulse for each photon
detected at the photocathode, and is passed to the processing electronics. The pulse carries
information about the energy of the original incident radiation on the scintillator. Thus both intensity
and energy of the radiation can be measured.
The scintillator must be in complete darkness so that visible light photons do not swamp the
individual photon events caused by incident ionising radiation. To achieve this a thin opaque foil,
such as aluminised mylar, is often used, though it must have a low enough mass to prevent undue
attenuation of the incident radiation being measured.
Background radiation is the ubiquitous ionizing radiation that people on the planet Earth are exposed
to, including natural and artificial sources.
Both natural and artificial background radiation varies depending on location and altitude.
Radioactive material is found throughout nature. Detectable amounts occur naturally in soil, rocks,
water, air, and vegetation, from which it is inhaled and ingested into the body. In addition to
this internal exposure, humans also receive external exposure from radioactive materials that remain
outside the body and from cosmic radiation from space.
The biggest source of natural background radiation is airborne radon, a radioactive gas that emanates
from the ground.
The Earth and all living things on it are constantly bombarded by radiation from outer space. This
radiation primarily consists of positively charged ions from protons to iron and larger nuclei derived
sources outside our solar system. This radiation interacts with atoms in the atmosphere to create
an air shower of secondary radiation, including X-rays, muons, protons, alpha
particles, pions, electrons, andneutrons.
Some of the essential elements that make up the human body, mainly potassium and carbon, have
radioactive isotopes that add significantly to our background radiation dose. An average human
contains about 30 milligrams of potassium-40 (40K) and about 10 nanograms (108 g) of carbon-
14 (14C),[citation needed] which has a decay half-life of 5,730 years. Excluding internal contamination by
external radioactive material, the largest component of internal radiation exposure from biologically
functional components of the human body is from potassium-40. The decay of about 4,000 nuclei
of 40K per second[11] makes potassium the largest source of radiation in terms of number of decaying
atoms. The energy of beta particles produced by 40K is also about 10 times that from the beta
particles from 14C decay. 14C is present in the human body at a level of 3700 Bq with abiological
half-life of 40 days.[12] There are about 1,200 beta particles per second produced by the decay of 14C.
However, a 14C atom is in the genetic information of about half the cells, while potassium is not a
component of DNA. The decay of a 14C atom inside DNA in one person happens about 50 times per
second, changing a carbon atom to one ofnitrogen.[13] The global average internal dose from
radionuclides other than radon and its decay products is 0.29 mSv/a, of which 0.17 mSv/a comes
from 40K, 0.12 mSv/a comes from the uranium and thorium series, and 12 Sv/a comes from 14C.[1]
