isotope course manual laboratory1

8/12/2019 Isotope Course Manual Laboratory1

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Laboratory 1

Pulse counting (the GM-tube)

Summary

Intrumentation for radioactivity measurements normally operate as pulse counting equipment .

An ionizing particle emitted from the radioactive preparation (sample) and absorbedin the detector results in an electrical (im)pulse in the electronic measuring circuit.

The number of pulses per unit recording time (the pulse rate), when corrected for the

detector pulse rate in the absence of a radioactive preparation, is taken as a measureof the activity of the preparation. The fraction of radioactive disintegrations in thepreparation that produces a recorded pulse is the counting efficiency .

Each pulse has a nite lifetime, and in part of this time period (the deadtime ) thedetector is unable to record new events. The shorter the deadtime, the higher thepulse rates that can be measured without loss.

The properties of the detector, the sample-detector geometry, the characteristics of the electronic circuit, and the physical properties of the radionuclide work together todetermine the counting efficiency. Only if the counting efficiency is exactly the samefor the different samples in a series can the pulse rate be taken as representative of the relative activities of the samples.

These basic aspects of pulse counting are illustrated in the present laboratory, usinga GM-tube as the detector for measuring the pulse rate from radionuclides emitting radiation.

Radioactivity measurements are normally carried out with the purpose of obtaininga numerical measure of the activity of a radioactive sample. Activity is dened asthe number of disintegrations per unit time . The SI-unit for activity is the becquerel (Bq). By denition, 1 Bq 1 disintegration per second.When a radioactive atom decays, ionizing radiation is emitted. The radiation canbe of several different types. In the present laboratory we will use radionuclides thatin practice are pure emitters, i.e. particles (electrons) are emitted in the decay

Laboratory Course Manual 2012


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38 Pulse counting (the GM-tube)

event. The detector used (a GM-tube) is designed with this particular radiation typein mind. radiation is always emitted with a continuous energy distribution, whichmeans that the individual particle may have an energy between 0 and a maximumvalue (E max ) characteristic of the radionuclide and decay in question.

A radioactive decay may give rise to different types of ionizing radiation, and eachradiation type may be emitted with different energies. In order to illustrate theseradionuclide properties in a simple graphical form, a decay scheme is often used. Thedecay schemes for the radionuclides used in the present laboratory ( 14 C and 36 Cl)are shown in gure 1.8 on page 54.

The physical properties of the radionuclides are often presented in tabular form. Anexcerpt of the isotope table for 14 C and 36 Cl are shown in table 1.3 on page 55.

1.1 Objectives

In this laboratory exercise we introduce the application of the Geiger-Mller detector(GM-tube, or GM-counter) for the detection of radiation. The most importantdetector characteristics and sources of error are described, and some general problemsconcerning detector systems for ionizing radiation are dealt with.

The GM-tube is introduced as the rst detector in this course because of its sim-plicity and ease of use, and because it illustrates some of the fundamental aspects of counting radioactive samples, including important sources of error. The GM-tube iswidely used in instruments for monitoring radiation levels and contamination in thelaboratory. These applications will be dealt with in Laboratory 5.

The GM-tube is used less frequently for analytical purposes, although it still providesan easy and cheap way of quantifying many of the radionuclides used in biologicallaboratories.

The GM-tube is an example of a gas ionization detector . The Addendum 1 (sec-tion 1.8.2 on page 62) contains ia brief overview of gas ionization detectors and theirmost important applications.

1.2 Practical information

With respect to the time for this laboratory exercise, please see the course timeschedule on the website. The participants work pairwise together using the sameequipment, so that the four members of a laboratory subgroup (rapportgruppe) ac-quire data amongst themselves from two measurement stations (instrument setups).

The subgroup writes an essay or laboratory report (rapport) taking into accountthe data from both stations. The report must give a brief overview of the topic of the exercise, and an evaluation of the results obtained. Please refer to the general

1 The addendum attached to some of the chapters in the laboratory manual contains supplemen-tary information about the topic of the particular laboratory. The essay for the laboratory may beprepared without reference to the supplementary information.

Radioactive isotopes and ionizing radiationDepartment of Biology, University of Copenhagen


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1.3 Theory 39

description of the formal requirements for written laboratory essays on page27 ff , andto the specic requirements for the present laboratory on page 55.

All questions about this laboratory should be directed to the teacher in charge

(page 60).

1.3 Theory

1.3.1 About measurement theory

The detectors used for radioactivity measurements are normally designed so that theabsorption of an ionizing particle (e.g. a particle) or photon results in an electricalimpulse (a transient voltage deection, often referred to as a pulse) in the detector

circuit. The pulse is then amplied and recorded in the scaler. If M pulses arerecorded during a time interval t, the average pulse rate is M/ t. This is alsoreferred to as the counting rate, r :

r = M

t (1.1)

Normally the time is measured in seconds (s) or minutes (min), and accordingly theunits for counting rate become ips or ipm (impulses per s, or impulses per min).Occasionally the term count is used rather than impulse, and the correspondingunits are cps and cpm .

The quantity r = M/ t is taken to represent the activity of the radioactive sample,or the amount of radioactive material in the sample.

Counting efficiency

For geometrical or other reasons the detector records only part of the disintegrationsoccurring in the radioactive sample. The fraction of disintegrations leading to arecorded pulse is called the counting efficiency (E )

2 . It is reported either as afraction ( 0 2.25 MeV) are capable of penetrating the 2 mg cm 2 thick counter window. Also for weak emitters ab-sorption in the counter window may be a problem, and the energy should exceed0.05 MeV to permit detection with the GM-tubes of this laboratory. Even for 14 C(E max = 0.156 keV), only about half of the particles emitted will penetrate thewindow. The most energy-weak emitters, such as 3 H, cannot be detected. Thesame applies to other low-energetic electron radiation, typically Auger-electrons.

Ionizing electromagnetic radiation, such as and X-rays, are much more pene-trating, and hence the thin window of the GM-tube poses no problem. However,the and X-ray photons have a much smaller probability of depositing energy inthe counting gas, which represents a very small absorbing mass (as compared tothe detectors normally used for electromagnetic radiation). When electromagneticradiation nevertheless may be detected by the GM-tube, this is due largely to theabsorption of photons in the wall of the tube (the metal cathode), which leads toa release of energetic electrons into the counting gas; it is these secondary electronsthat are detected by the tube (in the same way as are particles).

15 The function of the quenching gas (normally a halogen, e.g. Br 2 ) is to neutralize the energyemission (UV-photons) resulting from the collection by the positive ions of an electron from thetube cathode, a process that would otherwise lead to a series of after-pulses.

16 Also referred to as directly ionizing radiation.



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The efficiency of detection of and Xradiation depends partly on the constructionof the tube wall (type of material and thickness), partly on the energy of the radi-ation. In the GM-tubes of this laboratory, the detection efficiency is considerablylower than for radiation provided the energy is high enough for the particlesto penetrate the window.

We will return in Laboratory 3 and 4 to the processes responsible for the absorptionof radiation and electromagnetic radiation in matter.

1.8.2 Types of gas ionization detectors

Gas ionization detectors may be constructed in many different ways. An importantparameter is the voltage level (voltage difference between anode and cathode), forwhich the detector is designed:

Ion chamber : if the voltage difference is kept signicantly above zero but stillrelatively low, all electrical charges liberated by the primary ionization may wellbe collected on anode and cathode and thus contribute to the current in theexternal circuit. However, the voltage difference will be too low to allow the freeelectrons to produce secondary ionizations. The number of charges collected onanode / cathode is then a measure of the primary ionization produced by theincident ionizing particle. Under these conditions the current strength in theexternal circuit is proportional to the ionization rate produced by the ionizingradiation entering the detector. After a suitable calibration of the instrumentsetup, this principle may be used to check the activity of a radionuclide in agiven preparation (dose calibrator) or for estimating a biologically relevantradiation dose or dose rate. Since the current strength is recorded (and not thepulse rate), this is not pulse counting.

Proportional counter : If the voltage difference between anode and cathode isincreased (to above the level characteristic for the ion chamber), secondary ion-izations occur, i.e. the electrons liberated by the incident ionizing particle gainsufficient kinetic energy in the electrical eld to further ionize the moleculesof the counting gas. The absorption of an ionizing particle or photon in thecounting gas therefore produces an avalanche of electrons and positive ions,leading to an electrical (im)pulse in the external circuit. The number of pulsesper unit measuring time may be taken as a measure of the activity of the ra-dioactive sample. Within a certain voltage interval the pulse size (measuredin volts) is proportional to the magnitude of the primary ionization, i.e. tothe energy deposited in the counting gas by the primary particle or photon. Aproportional counter can therefore in addition to simple pulse counting be used to differentiate between radiation of different energy, or between radi-ations with different ionization density (e.g. radiation and radiation maybe distinguished). Proportional counters have many practical applications, andthey may be calibrated for dosimetric measurements.

GM-counter (GM-tube): If the voltage difference between anode and cathodeis further increased, a very violent secondary ionization occurs. Even a verysmall primary ionization (small energy deposition caused by the ionizing par-ticle or photon) leads to the entire volume of the detector tube being lled

Radioactive isotopes and ionizing radiationDepartment of Biology, University of Copenhagen


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1.8 Addendum 63

with secondary ionizations. A large electrical pulse is generated in the externalcircuit, however its magnitude is independent of the size of the primary ioniza-tion. The GM-tube therefore cannot be used to differentiate between radiationof different energies. The large gas amplication makes the GM-tube easy touse, since an advanced electronic signal amplication is not called for.



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isotope course manual laboratory1

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