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CHAPTER 3DETECTION DEVICES

ROBERT ESHUNS.L.T DEPARTMENT

ACCRA POLYTECHNIC

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Because human senses cannot sense radiation, instruments that detect radiation are essential tools.

After a nuclear disaster detecting radiation becomes particularly invaluable, as high levels of radiation can become hazardous to life.

Regular monitoring while using radioactive substances are critical to the safety of personnel.

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Detection of radioactivity is necessary to ascertain their presence and Intensity

Detection indirect (based on the effects of radioactivity)• Darkening of photographic plates• Ionization of atoms

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Film badges

Based on the fact that radiation affects photographic films.

Used to monitor the level of radiation that personnel working with radioactive materials or x-rays are exposed to.

Typically worn on the outside of clothing, around the chest or torso. This location monitors exposure of most vital organs.

The badge is worn over the protection to monitor the dose actually received to unprotected parts (parts not covered with lead, etc).

Film periodically removed and developed to measure exposure.

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Film surrounded by case preventing light and moisture from damaging it.

Reacts to radiation and becomes dark.

Other badges placed on the finger as a ring. For detecting ionizing radiation, such as a narrow beam directed toward a work area.

Multiple badges can be used to cover different areas, if radiation exposure is a high risk.

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Ionization chamber

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Uses an air-filled chamber to detect X-ray and gamma rays.

When radioactive particles form ion pairs inside the chamber, the anode collects electrons produced from this process.

The anode then produces a small electric current.

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Electrometer measures this current and this in turn displays and records the level of radiation present.

Typically measures radiations in units of milliroentgens per hour (mR/hr) and roentgen per hour (R/hr).

Construction: Gas filled enclosure between two conducting electrodes, with a voltage across them.

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Electrodes in the form of parallel plates or coaxial cylinders.

Gas atoms in chamber ionized by radiation.

Electrons from the ionisation attracted to the positive voltage plate (anode).

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Positively charged atoms attracted to negative voltage plate (cathode).

Resulting ionization current is measured.

This current is directly proportional to the radiation being detected.

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Geiger-Muller (GM) counter or Geiger Counter (GC)

The G.C. creates an electric pulse when radiation interacts with a gas within it.

It converts and records the electric pulse into a radiation reading.

Radiation is typically measured in Counts per minute (CPM).

G. C.s detect alpha and beta particles and gamma rays.

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Construction: Diode filled with inert gas at low pressure, in which the anode is a metal rod fixed along the axis of a cylindrical cathode.

Anode - insulated wire in the tube connected to the positive terminal of a d.c. Source.

Cathode - metal tube connected to the negative terminal of d.c. source.

High p.d. maintained between anode and cathode.

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Some atoms of gas ionised by radiation passing through thin window.

Released electrons accelerated by the field between the wire and cylinder ionise other atoms.

Avalanche of free electrons released.

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Avalanche creates a pulse of current at the output of the tube.

Radiation is directly proportional to avalanches.

Pulses are amplified, and used to trigger an electronic counter or delivered to speaker which clicks each time a particle enters the detector.

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Proportional counter

Used for quantifying alpha and beta radiation, neutron detection, and x-ray spectroscopy.

Pulses produced larger than those produced by ion chamber.

Pulse size reflects the energy deposited by the incident radiation in the detector gas.

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This makes it possible to differentiate the larger pulses produced by alpha particles from the smaller pulses produced by beta or gamma rays.

Count particles result from ionising radiation and measure their energy.

Construction and operation similar to that of a GM counter, except that it uses a lower operating voltage.

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Made of tube filled with a mixture of an inert gas and a quench gas.

Creates ion pairs, made up of an electron and a positively charged atom.

Electrons attracted to anode, under the influence of an applied field.

Positive ions attracted to the cathode.

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The gas in the counter does not contain electronegative components such as oxygen.

Otherwise, electrons heading towards the anode will combine with the electronegative gas.

If this happens, a negative ion goes to the anode rather than an electron, and unlike the electron, the negative ion will fail to produce an avalanche.

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Scintillation counters

Based on the property of certain chemical compounds of emitting short flashes of light when excited by charged particles or photons of high energy.

Counts flashes of light resulting from phosphor material struck by radiation.

Consists of transparent crystal or organic liquid that fluoresces when struck by ionizing radiation.

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Flash from the material is measured by a Photo Multiplier Tube.

PMT detector is attached to an amplifier and other electronic equipment to count signals

The photons emitted by the scintillator are converted to an electrical signal.

P.M.T. Is made up of several electrodes, known as dynodes, whose potentials are increased in succession along the length of the tube.

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One electron entering the PMT ejects several other electrons creating an avalanche.

A large electric pulse emerges at the output of the P.M.T. which pulse is sent to an electronic counter.

In summary, alpha or beta particles or gamma rays produce a flash of light in a crystal (zinc sulphide and silver for alpha and beta radiation, sodium iodide and tellurium for gamma-rays).

This is detected by a photomultiplier tube and the electrical pulse recorded.

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Other detectors (a) Photographic plates. Though these are

not good for gamma-radiation, as there is insufficient ionization, alpha and beta particles produce visible tracks in the plate where they pass.

(b) Electroscope. The leaf falls, due to ionization of the surrounding air. This is rather a crude method and is not good for gamma radiation.

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(c) Bubble chamber. Radiation creates tracks of bubbles in a superheated liquid such as hydrogen or propane. The bubble chamber is a much more effective detector of radiation than the cloud chamber, because of the much greater number of atoms per unit volume of the liquid in it. This means that there is a much greater chance of a collision occurring between an incoming particle and a nucleus.

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(d) Solid-state detector. This is a reverse biased p-n junction of semiconductor material, and when ionizing radiation falls on it ion pairs are formed at the junction, thus producing a current through it.

(e) Cloud chambers show the tracks of radioactive particles rather than measure the intensity of the radiation. Two types exists: the expansion type and the diffusion type. Their final results are similar but they use different methods to achieve it.

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(f) The spark counter measures the range of alpha-particles. In nuclear research a stack of spark counters is used to show the track of a particle as a line of sparks.