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Home , Counts per minute, Geiger counter

Operation of the Tube and its Associated Equipment

* Ray Lemmerman, Austin High School, Austin, Minnesota

The author outlines an experiment to introduce students to the theory and use of Geiger counters, including setting correct operating votage.

Operating Principleof the Geiger-MuellerTube occurs because the original ions produce ad- Man is not able to detect ionizing radia-

ditional ions. Pulses of ions, rather than Downloaded from http://online.ucpress.edu/abt/article-pdf/27/6/435/21628/4441005.pdf by guest on 01 October 2021 tions by the use of his senses. He must rely simply unstructured currents, become de- upon electronic or chemical devices to per- tectable and these pulses tend to be pro- form this task for him. Since there is a wide portional to the numbers of ions in the range of levels and various physi- originating event. At still higher voltages this cal properties involved in radiation, no one so-called proportional region will (in a suc- instrument is capable of monitoring all situa- cessful counter) give way to one in which tions. Therefore, persons working in the all pulses are of large uniform size, regardless field of radiation must be able to choose of the originating events. This is the Geiger- the proper instrument to evaluate a situation. Mueller region. Any radiation to be detected must reach the detecting element. Alpha particles, which When an ionizing particle passes through a have low penetrating power, would require G. M. tube operating in the Geiger-Mueller region, a a very thin window or a windowless area certain number of original positive and negative ions adjacent to the detector. Beta particles, many result. The negative ions are free of which are also of low penetrating power, usually electrons attracted to the anode. In traveling to the anode also require a thin window usually of mica, they interact aluminum or plastic less than 1/1000 of an with the gas which fills the tube creating inch thick to be effective as a beta detector. additional ions. Since these produce still further the Gamma rays and have high pene- ions, result is a build-up of elec- trating power and may be detected by instru- trons to the point that an easily detectable ments which have thicker coverings over the voltage pulse occurs. This avalanche action radiation sensitive element. is the phenomenon of gas amplification. of The detectors commonly used in high Equality pulse size from ionizing particles of school classrooms are Geiger-Mueller tubes. various energies depends upon space Such tubes are often also called G. M. Count- charge build-up to a self-limiting maximum ers. A G. M. tube is a sensing device in which around the whole anode. ionizing yield electrical impulses. Since the electrons have less mass than the It typically consists of a metal cylinder with positive ions, the negative ions will reach the a wire running coaxially through the center. anode long before the positive ions reach the The wire is the anode, or positive electrode, cathode. Precautions must be taken so that and the surrounding cylinder, often simply a second pulse is not generated as positive the wall of the tube, is the cathode. See ions reach the cathode. This action can be Fig. 1. prevented by filling the tubes with counting Operating characteristics of the tube de- gas (often or ) and a small pend upon the voltage impressed between amount of quenching gas (usually alcohol or anode and cathode. At moderate voltages, halogen). In the alcohol-argon filled tube, the tube can act as an ion chamber. The ions when a positively charged alcohol ion comes created in the enclosed gas by ionizing radia- within 10-7 cm of the cathode, it pulls an tion result in a low current between the elec- electron from the emission field of the cath- trodes. At higher voltages gas amplification ode. The electron neutralizes the ion leaving

435 436 THEAMERICAN BIOLOGY TEACHER

End Window G.M. Tube

Cathode Ahnode Quenching Window Downloaded from http://online.ucpress.edu/abt/article-pdf/27/6/435/21628/4441005.pdf by guest on 01 October 2021

Pulse + Counter

Each ion event may produce 108 electrons to the electrode.

High Voltage

an excited molecule. The molecule later loses nent dissociation and consequently has an its excitation by dissociating into two un- indefinitelife. charged atomic groups resulting in no ion A G. M. tube requiresa finite time to regis- formation. Since argon ions react with al- ter a pulse, during which time anotherevent cohol molecules to yield alcohol ions, all will not be counted. This period, about 200 positive ions become alcohol ions by the microseconds, is referred to as the "dead time they reach the cathode. The positive tiune."In G. M. tubes used in most common ions are thereby neutralized without creation instruments the "dead time" sets the limit of further ions in any kind of avalanche of counting at a maximumrate of approxi- process. mately 50,000 (c/mi). The life of the alcohol-argon filled tube is The output pulses from G. M. tubes are approximately 108 counts or about 100 days generally recorded as measurementsby in- of continuous operation counting 100 c/sec. strumentscalled scalers or rate meters. Each However, the halogen quenched tube per- surge or pulse registered by such a meter forms the quenching action without perma- representsthe detection of a single radiation OPERATIONOF THEGEIGER COUNTER TUBE 437 Downloaded from http://online.ucpress.edu/abt/article-pdf/27/6/435/21628/4441005.pdf by guest on 01 October 2021

S 110 1 } *' _ * - t 1 t t w~~~~

incident within the tube. ments. Gamma rays produce ionizing particles There are many models of radiationmeas- only after much less frequent collision with uring devices available to schools. A scaler atomic electrons. Most gamma rays pass, rate-meteris perhapsthe most common.The therefore, through a G. M. counter without instruments pictured in Figs. 2 and 3 are detection. The counter efficiency for gamma types which are adequate for student use. radiationis very low. The typical G. M. tube These sensitive instruments are generally is most sensitive to beta particles but does quite costly. As with other delicate instru- register some gamma rays. Good explana- ments,it is desirablefor the student to under- tions of variouscounting instrumentsmay be stand the basic operating principles before found in Chapter III of Principlesof Radia- undertakingtheir operation. The principles tion and Contamination Control, Vol. III, of operationfor differentGeiger instruments Information Relating to Nuclear Weapons are basicallythe same although delicate pro- Effects, Bureau of Ships, Navy Department, cedures for operationmay vary. The manual Washington,D. C. of instructionsfor each particularinstrument should be read before using it. OperatingProcedures and Precautionsin the Use of a G. M. Counterand Scaler There are a number of points of general The Geiger counter is the instrumentmost informationto keep in mind regarding the students associate with radiation measure- use of Geiger counters: ments. Students may profitably participate 1. Dependable operation hinges on the first in the use of a scaler rate-meterto de- proper voltage setting for the Geiger- tect radiation quantit4tively. Working with Muellertube. a scaler rate-meterto determinethe physical 2. The high voltage carriedby the G. M. and chemical properties of the different ra- tube is potentially dangerous. Cable dioisotope emissions will provide necessary connectionsfor the G. M. tube should background for later experi- be handled cautiously. 438 THEAMERICAN BIOLOGY TEACHER Downloaded from http://online.ucpress.edu/abt/article-pdf/27/6/435/21628/4441005.pdf by guest on 01 October 2021

3. Students should be cautioned never to the more expensive tubes is usually touch the window of the G. M. tube. 2 mg/cm2 or less. A check of the G. M. The window is easily broken, thus ruin- tube specifications for a given instru- ing the tube for future use. ment will show if alpha particles can 4. The tube may be permanently dam- be detected. aged if operated at too high a voltage. 7. The high energy beta emissions will Operating voltages vary for different pass through a single thickness of card- tubes. Some instruments, such as the board. Most beta radiations can pass one pictured in Fig. 3, have no high through the window material of G. M. voltage control. These counters have tubes to record beta events. the proper operating voltage set at the 8. Gamma radiation is very penetrating factory. and will pass through varying thick- 5. The G. M. tubes with an organic quench nesses of metal. Gamma radiation will have an operating life of approximately 1 x 108 counts. When not counting, the penetrate a G. M. tube even when the high voltage should be switched to low shield is placed across the window. or turned off. will 9. Radiation occurs as random events. It trigger the G. M. tube, thus reducing is advisable to have the C/M figures the life of the 'tube if the voltage is from longer rather than from shorter left on. counting periods. The best results may 6. Alpha particles have a range of about be obtained by taking the average 4 mg/cm2. The window material of count from three or more such periods. OPERATIONOF THEGEIGER COUNTER TUBE 439

Chase, in Radioisotope Methodology, Materialsand Methods: presents a comprehensivetreatment of The following apparatuswill be required radiation counting statistics. to carry out this investigation. 10. When counting a number of unknown samples, one should always work with 1. A scaler rate-meterinstrument with a sources in the same relative position to side window probe, or end window the tube. This will avoid errors which tube and audible speaker. result from not duplicating counting 2. A radiumD & E standardsource. geometry. 3. A beta source. 11. A record of the material used and the 4. A gammasource. shelf numberof the sample holder may 5. A luminouswatch dial. have value for future reference. 6. A sample holder. 12. Naturalradiation known as background Downloaded from http://online.ucpress.edu/abt/article-pdf/27/6/435/21628/4441005.pdf by guest on 01 October 2021 radiationshould be counted each day. A scaler rate-metercomes eqcuippedwith a All counts sholuldbe adjusted by sub- G. M. tube installed in a probe or equipped tracting background C/M from those with an end window tube. The probe has a recorded for the sample. shield which can open or close the window. 13. When making counts over an extended Closing the shield will cut out all alpha and period it is wise to make daily checks beta radiation. However, gamma radiation of the instrumentwith a standardsam- can penetrate the shield. Opening the shield ple such as a Radium D & E source. will allow most beta particles and a few Comparing standard counts from day more gamma rays to penetrate the tube. to day will show if the instrumentis The coveringover the windows of all but the operatingconsistently. Chase, in a sec- most expensive G. M. tubes will stop alpha tion titled Instrument Efficiency, dis- particles. cusses adjustmentswhich can be made Each G. M. tube has a different optimal to help correctcounting problems. operatingvoltage. The best operatingvoltage 14. If a radiation source is not available can be determinedby plotting an operating one can be made using uraniumoxide curve.The determinationof this cuirveshould (1U308). One milligram of uranium be carriedout with new tubes or before using oxide undergoesapproximately 724 dis- an unfamiliarinstrument. integrationsper minute. Place 50 milli- Procedureto determineoperating voltage: grams of uraniumoxide in a soda bot- 1. Have the students read the manual to tle cap. Add a drop of thin model become familiar with the controls. If airplanecement. Mix and spread a thin there are any questionsthey should ask coat over the bottom of the cap. Cover the instructorbefore proceeding. with aluminum foil to screen out the 2. With the voltage control turned down alpha and weak beta particles.If uran- the student should turn on the meter ium oxide is not ni- available, uranyl and allow it to warm up for 2-3 min- trate be substituted. might utes. They should adjust the meter to Experiment:How to Determine the Correct zero, then switch on the high voltage. of a Tube OperatingVoltage Geiger Note: The instrumentdial, in Fig. 2, of the Purpose Geiger CountingExperiment: has two scales. The rate in C/M is in 1. Students will have the opportunityto black figures.The high voltage scale is learn how to operate a radiation in red figures. counter. 2. They will learn how to determine the 3. The next step is to check the manual best operating voltage for their par- for the approximateoperating voltage. ticular counter. With the high voltage on, students set 3. They will learn how to interpret the the control to read voltage about half characteristicsof the G. M. tube curve. way up the scale. 4. They will develop understandings 4. With a standardradiation source in the which will later aid radiobiology ex- sample holder, students will use what- periments. ever scale which seems appropriatefor 440 THEAMERICAN BIOLOGY TEACHER

their sample.The auditoryspeaker will creases. This is the breakdownvoltage. also give them an indicationof the level 8. Students should be cautioned against of radiationbeing used. raising the voltage any higher or ever 5. They are now ready to determine the keeping it that high. On recognizing a best operating voltage for their G. M. clear increasein count rate they should tube: They turn the high voltage down immediately drop the voltage back to until the meter no longer records any prevent damage to the G. M. tube. It counts; slowly increase the voltage un- is better to estimate the count rate at til the counting begins; record counts such voltage than to risk ruin of the for five minute periods and record the tube by attempting a measurement. C/M figures. 9. The C/M data should be plotted on 6. The G. M. tube voltage is now in- graph paper. Data should approximate creased by 50 volt incrementsand the the graph curve in Fig. 4. The Geiger Downloaded from http://online.ucpress.edu/abt/article-pdf/27/6/435/21628/4441005.pdf by guest on 01 October 2021 increased C/M data for each voltage thresholdand the operatingplateau of change is recorded. the graph should be noted. Operating 7. With the first few hundred volts in- voltage should be set about 50-75 volts crease there will be a rapid C/M above the Geigerthreshold. This should increase. Following this initial increase be approximately3/ the distance across there will be a voltage range where the the plateau. C/M number will increase very little 10. With the operatingvoltage determined, with increasedvoltage. Finally a point students now record a counting period will be reached where there will be a with no near the rapid increase in C/M with voltage in- counter. The backgroundrate is sub-

Breakdow 0 Voltage 10, 00

Geiger Threshold oo 1 Operating Voltage 9,00

p - _ ~~~PlateauG. M. Region

8,00

.7, 00

0

6,00

Starting Potential

5 , 0 004-I| 600 700 800 900 1,000 1,100 1,200 Applied Voltage

G. M. Characteristic Curve (end window tube) OPERATIONOF THEGEIGER COUNTER TUBE 441

tracted from the measured total rate uel, U. S. Atomic Energy Commission, July 1958, for any sample. 57 pages. Supt. of Documents, U. S. Government Printing Office, Washington, D. C. 11. With the standardsample in the holder, Preparation,Maintenance and Application of Stand- count with the G. M. shield opened and ards of Radioactivity, National Bureau of Stand- closed. The differencein counts should ards Circular, #594, Supt. of Documents, U. S. be noted. The student should be able Government Printing Office, Washington, D. C. to determine the number of gamma Principles of Radiation and Contamination Control, Vol. III, Technical Informationrelating to Nuclear counts and the number of beta counts. Weapons, Bureau of Ships, Navy Department. 12. Finally, check familiar objects which R. A. Sulit, E. J. Leahy, and A. L. Baietti, Supt. you think may be radioactive,such as of Documents, U. S. Government Printing Office, luminous watch dials, orange glazed Washington, D. C. pottery, and mineral samples. Downloaded from http://online.ucpress.edu/abt/article-pdf/27/6/435/21628/4441005.pdf by guest on 01 October 2021 References The author wishes to express his gratitude Principles of Radioisotope Methodology, Chase, to Dr. Donald J. Fluke of The Florida State Grafton D. and J. L. Robinowitz, Burgess Pub- University for his careful review of and lishing Co., 1959, 286 pages. LaboratoryExperiments with Radioisotopesfor High thoughtful comments regarding the prepara- School Science Demonstrations, Schenberg, Sam- tion of this manuscript.

PhysicalHalf-Life of a * Clyde M. Senger,Western Washington State University, Bellingham,Washington

The author discusses methods of determining the half-life of materials and techniques for handling these materials in the classroom.

The concept of half-life, the time required and since this number decreases due to de- for one half of a material to change or to be cay during the time interval, the succeeding lost, is an important one, both in radiation time interval should have a somewhat lower physics and in biology. decay rate. The rate of decay can be ex- Radioisotopes or are readily pressed mathematicallyby R = R0e-t or in detectable because they emit ionizing radia- the log form, log R = log R. -0.4343 At tions which are relatively penetrating. With where R is the rate at a time, t, after an each emission or series of emissions the nu- initial rate R,, A is the decay constant for cleus of that particular nuclide changes to the particularradionuclide and e is the base that of another nuclide. Thus, each atom can of the natural or Napierian logarithms undergo a particular decay with emission of (1, 2, 3, 4). the characteristic radiation only once. With this formula and a value for the de- Mathematical analysis of radionuclide de- cay constant, one can calculate the time cay indicates that there is a certain probabil- interval required for the decay rate to de- ity that any particular atom will decay in crease to any specific fraction of the initial some short unit of time. The expected decay rate. Traditionallyscientists have used the rate is the product of the number of atoms of time for the rate to fall to one-half the initial the radionuclide present and this probability rate. This value is called the half-life of the of decay. The actual decay rate in any par- radionuclide and TY2is the typical symbol ticular time interval may vary from the ex- used for it. Using the log form of the basic pected rate due to chance, but the rates for decay formula,after one half-life R RO/2 a number of appropriately short time inter- and t T . Substitutingwe have log RI/2 = vals will average to the expected rate. log R0-0.4343 AT?,or log Ro - log 2 = log Since the rate at any particular time is Ro- 0.4343 XTv,log 2 - 0.4343 XT,, T, = based on the number of atoms at the start 0.6931/A or A = 0.6931/Ty2. Substitutingin