Radiation Detectors

While Becquerel is given credit for the discovery of radioactivity, the isotopes responsible for this event had been emitting radiation forever. Radioisotopes look like their stable siblings; the only difference between them is that, besides atomic mass, radioisotopes give off ionizing radiation.It is this feature we wish to exploit in detection.

In the table that follows, we compare the ability to ionize matter with the ability to penetrate matter


 Ionization capability


 alpha particles



 beta particles



 gamma rays



Alpha radiation turns out to be a doubly-ionized helium nucleus with a charge of +2. This property makes it easy to be a good ionizer , stealing electrons from matter and leaving a trail of ionized flotsam and jetsom in its wake. Alphas are poor penetrators because their +2 charge causes them to be repelled by target nuclei. A few sheets of paper will stop alpha particles which means that alpha detectors must have detection chambers thin enough to allow alphas to penetrate.

Gamma radiation penetrates best ( needs lots of matter to absorb them) but is not a very good ionizer. This is probably the reason we have life as we know it on this planet. A good ionizer that is a good penetrator would have probably squelched any life form in the primordial soup phase our Earth's development.
Beta particles are in between. A few thicknesses of aluminum foil will stop most of them.

To detect these and other particles, let us exploit the fact that these particles ionize matter.


Photographic Plates 

Photographis plates are coated with a light-sensitive emulsion that is easily ionized when light or charged partivles are incident on them. Plates were first used by Becquerel who discovered radioactivity with them. Remember that he was looking for X-rays which were known to fog photographic plates. We opened this unit with a quote fron Louis Pasteur "Chance favors the prepared mind. " After Roentgen published an account of his discovery, a man named Smith in England announced that he had seen his supply of photographic plates become fogged. He attributed this calamity to the proximity of the plate storage area to his cathode ray tube. He had his assistant move the storage area.

Cloud chamber

 The cloud chamber was developed over the first quarter of the twentieth century by Nobel laureate C.T.R.(Charles Thompson Reese) Wilson. When an air mass containing water vapor undergoes an abrupt drop in pressure, the temperature of the air goes down and the vapor condenses. Preferentially, droplets tend to form on ionized particles. If an ionizing agent is present, its radiation creates a track of ions upon which visible droplets are formed.

Spark chamber


At left is an array of thin metal plates stacked together so that they are electrically isolated from one-another. A high voltage is placed between adjacant plates. Gamma rays passing through the array ionize the air molecules they contact, leaving behind a trail of ions. The high voltage between the plates causes a small visible arc of light to form.

Bubble Chamber

The bubble chamber was a very useful device in the the 1950s through 1980s for witnessing various sub-atomic interactions. It was conceived by Donald Glaser while contemplating a glass of beer. Glaser knew that carbon dioxide comes out of solution to cause bubbles and that this process occurs most readily in the presence of ions. Glaser used liquid hydrogen very near its boiling point.If some ionizing event occurs in this space, the ion trail left behind is made visible by the bubbles that build on them.




 A dosimeter is a small unpretentious device that allows the bearer to monitor cumulative exposure to X-rays and Gamma rays.The unit, which is the size of a fountain pen, is given a charge and then worn on a lapel, a collar or in a pocket protector. A radiation source in the area will ionize the air creating mobilized ions that will discharge the dosimeter over the course of time. The unit is read by observing the scale at one end of apparatus. (See the scale at left.) An employee checks out a unit upon arrival and returns it to dosimetry at the end of the day.

Workers in the nuclear industry install a dosimeter at the beginning of a shift and return it to the radiation health team whose job it is to monitor levels of exposure to radiation.

Geiger - Mueller Counter 


 A (Hans) Geiger- (Walter) Mueller tube consists of a closed cylindrical tube filled with an inert gas and having a wire running along the axis of the tube. A high voltage is maintained between the wire and the side of the tube. When a charged particle enters the tube, it ionizes the gas which creates a place where electrons can flow from cathode to anode. This current can be made audible as a click from a speaker.

See also http://en.wikipedia.org/wiki/Geiger-M%C3%BCller_tube

last edited 12/29/05