MASTER Lab Manual Student - Department Of Physics & Astronomy
Spectrum TechniquesLab ManualStudent VersionRevised, March 2014
Table of ContentsStudent Usage of this Lab Manual . 3What is Radiation? . 4Introduction to Geiger-Müller Counters . 8Good Graphing Techniques . g a Geiger Plateau . 12Statistics of Counting . 20Background . 26Resolving Time . 30Geiger Tube Efficiency . 37Shelf Ratios . 43Backscattering. 48Inverse Square Law . 57Range of Alpha Particles. 62Absorption of Beta Particles . 69Beta Decay Energy . 74Absorption of Gamma Rays . 80Half-Life of Ba-137m . 88AppendicesA.B.C.D.E.F.SI Units . 99Common Radioactive Sources. 101Statistics . 102Radiation Passing Through Matter. 109Suggested References. 113NRC Regulations . 116Spectrum Techniques Student Lab Manual2
Student Usage of Lab ManualThis manual is written to help students learn as much as possible about radiationand some of the concepts key to nuclear and particle physics. This manual in particularis written to guide you through a laboratory experiment set-up. The lab manual has thefollowing layout: Detailed background material on radiation, the Geiger-Müller counter and itsoperation, and radiation interaction with matter. Thirteen laboratory experiments with instructions, data sheets, and analysisinstruction.A piece of standalone equipment from Spectrum Techniques may not be entirelyequipped for the laboratory environment. Additional resources and recommendationsare made in the teacher’s notes of the experimental write-ups for schools that wish torun the specific experiments. Also, schools operate on different class schedules,varying from 42-minute periods to 3-hour lab sessions. Thus, the labs are written withflexibility to combine them in different manners (our suggestions are listed below).The lab manual is not intended to be a “recipe” book but a guide on how to obtainthe data and analyze it to answer certain questions. What this means is that explicitdirections as to every single button to push are not given, but the student will haveguidance where this can be inferred.NOTE: All directions in this laboratory manual assume the use of a PC computerwith Microsoft Excel used for the experiments. Any manual operation has theappropriate directions given in the product manual. All operations listed in the directionsbelow may be carried out on the screen of the Spectrum Techniques equipment. Also,all instructions use the ST-360 model Geiger-Müller counter, but the other models, ST160 and ST-260, have similar functions available.Spectrum Techniques Student Lab Manual3
What is Radiation?This section will give you some of the basic information from a quick guide of thehistory of radiation to some basic information to ease your mind about working withradioactive sources. More information is contained in the introduction parts of thelaboratory experiments in this manual.Historical BackgroundRadiation was discovered in the late 1800s. Wilhelm Röntgen observedundeveloped photographic plates became exposed while he worked with high voltagearcs in gas tubes, similar to a fluorescent light. Unable to identify the energy, he calledthem “X” rays. The following year, 1896, Henri Becquerel observed that while workingwith uranium salts and photographic plates, the uranium seemed to emit a penetratingradiation similar to Röntgen’s X-rays. Madam Curie called this phenomenon“radioactivity”. Further investigations by her and others showed that this property ofemitting radiation is specific to a given element or isotope of an element. It was alsofound that atoms producing these radiations are unstable and emit radiation atcharacteristic rates to form new atoms.Atoms are the smallest unit of matter that retains the properties of an element(such as hydrogen, carbon, or lead). The central core of the atom, called the nucleus, ismade up of protons (positive charge) and neutrons (no charge). The third part of theatom is the electron (negative charge), which orbits the nucleus. In general, each atomhas an equal amount of protons and electrons so that the atom is electrically neutral.The atom is made of mostly empty space. The atom’s size is on the order of anangstrom (1 Å), which is equivalent to 1x10-10 m while the nucleus has a diameter of afew fermis, or femtometers, which is equivalent to 1x10-15 m. This means that thenucleus only occupies approximately 1/10,000 of the atom’s size. Yet, the nucleuscontrols the atom’s behavior with respect to radiation. (The electrons control thechemical behavior of the atom.)Spectrum Techniques Student Lab Manual4
RadioactivityRadioactivity is a property of certain atoms to spontaneously emit particles orelectromagnetic wave energy. The nuclei of some atoms are unstable, and eventuallyadjust to a more stable form by emission of radiation. These unstable atoms are calledradioactive atoms or isotopes. Radiation is energy emitted from radioactive atoms,either as electromagnetic (EM) waves or as particles. When radioactive (or unstable)atoms adjust, it is called radioactive decay or disintegration. A material containing alarge number of radioactive atoms is called either a radioactive material or a radioactivesource. Radioactivity, or the activity of a radioactive source, is measured in unitsequivalent to the number of disintegrations per second (dps) or disintegrations perminute (dpm). One unit of measure commonly used to denote the activity of aradioactive source is the Curie (Ci) where one Curie equals thirty seven billiondisintegrations per second.1 Ci 3.7x1010 dps 2.2x1012 dpmThe SI unit for activity is called the Becquerel (Bq) and one Becquerel is equal to onedisintegration per second.1 Bq 1 dps 60 dpmOrigins of RadiationRadioactive materials that we find as naturally occurring were created by:1. Formation of the universe, producing some very long lived radioactive elements,such as uranium and thorium.2. The decay of some of these long-lived materials into other radioactive materials likeradium and radon.3. Fission products and their progeny (decay products), such as xenon, krypton, andiodine.Man-made radioactive materials are most commonly made as fission products orfrom the decays of previously radioactive materials. Another method to manufactureSpectrum Techniques Student Lab Manual5
radioactive materials is activation of non-radioactive materials when they arebombarded with neutrons, protons, other high-energy particles, or high-energyelectromagnetic waves.Exposure to RadiationEveryone on the face of the Earth receives background radiation from naturaland man-made sources. The major natural sources include radon gas, cosmicradiation, terrestrial sources, and internal sources. The major man-made sources aremedical/dental sources, consumer products, and other (nuclear bomb and disastersources).Radon gas is produced from the decay of uranium in the soil. The gas migratesup through the soil, attaches to dust particles, and is breathed into our lungs. Theaverage yearly dose in the United States is about 200 mrem/yr. Cosmic rays arereceived from outer space and our sun. The amount of radiation depends on where youlive; lower elevations receive less ( 25 mrem/yr) while higher elevations receive more( 50 mrem/yr). The average yearly dose in the United States is about 28 mrem/yr.Terrestrial sources are sources that have been present from the formation of the Earth,like radium, uranium, and thorium. These sources are in the ground, rock, and buildingmaterials all around us. The average yearly dose from these sources in the UnitedStates is about 28 mrem/yr. The last naturally occurring background radiation source isdue to the various chemicals in our own bodies. Potassium (40K) is the majorcontributor and the average yearly dose in the United States is about 40 mrem/yr.Background radiation can also be received from man-made sources. The mostcommon is the radiation from medical and dental x-rays. There is also radiation used totreat cancer patients. The average yearly dose in the United States is about 54mrem/yr. There are small amounts of radiation in consumer products, such as smokedetectors, some luminous dial watches, and ceramic dishes (with an orange glaze). Theaverage yearly dose in the United States is about 10 mrem/yr. The other man-madesources are fallout from nuclear bomb testing and usage, and from accidents such asChernobyl. That average yearly dose in the United States is about 3 mrem/yr.Spectrum Techniques Student Lab Manual6
Adding up the naturally occurring and man-made sources, we receive onaverage about 360 mrem/yr of radioactivity exposure. What significance does thisnumber have since millirems have not been discussed yet? Without overloading youwith too much information, the government states the safety level for radiation exposure5,000 mrem/yr. (This is the Department of Energy’s Annual Limit.) This is three timesbelow the level of exposure for biological damage to occur. So just living another year(celebrating your birthday), you receive about 7% of the government regulated radiationexposure. If you have any more questions, please ask your teacher.Spectrum Techniques Student Lab Manual7
The Geiger-Müller CounterGeiger-Müller (GM) counters were invented by H. Geiger and E.W. Müller in1928, and are used to detect radioactive particles ( and ) and rays ( and x). A GMtube usually consists of an airtight metal cylinder closed at both ends and filled with agas that is easily ionized (usually neon, argon, and halogen). One end consists of a“window” which is a thin material, mica, allowing the entrance of alpha particles. (Theseparticles can be shielded easily.) A wire, which runs lengthwise down the center of thetube, is positively charged with a relatively high voltage and acts as an anode. The tubeacts as the cathode. The anode and cathode are connected to an electric circuit thatmaintains the high voltage between them.When radiation enters the GM tube, it will ionize some of the atoms of the gas*.Due to the large electric field created between the anode and cathode, the resultingpositive ions and negative electrons accelerate toward the cathode and anode,respectively. Electrons move or drift through the gas at a speed of about 104 m/s, whichis about 104 times faster than the positive ions move. The electrons are collected a fewmicroseconds after they are created, while the positive ions would take a fewmilliseconds to travel to the cathode. As the electrons travel toward the anode theyionize other atoms, which produces a cascade of electrons called gas multiplication or a(Townsend) avalanche. The multiplication factor is typically 106 to 108. The resultingdischarge current causes the voltage between the anode and cathode to drop. Thecounter (electric circuit) detects this voltage drop and recognizes it as a signal of aparticle’s presence. There are additional discharges triggered by UV photons liberatedin the ionization process that start avalanches away from the original ionization site.These discharges are called Geiger-Müller discharges. These do not effect theperformance as they are short-lived.Now, once you start an avalanche of electrons how do you stop or quench it?The positive ions may still have enough energy to start a new cascade. One (early)method was external quenching, which was done electronically by quickly rampingdown the voltage in the GM tube after a particle was detected. This means any moreSpectrum Techniques Student Lab Manual8
electrons or positive ions created will not be accelerated towards the anode or cathode,respectively. The electrons and ions would recombine and no more signals would beproduced.The modern method is called internal quenching. A small concentration of apolyatomic gas (organic or halogen) is added to the gas in the GM tube. The quenchinggas is selected to have a lower ionization potential ( 10 eV) than the fill gas (26.4 eV).When the positive ions collide with the quenching gas’s molecules, they are slowed orabsorbed by giving its energy to the quenching molecule. They break down the gasmolecules in the process (dissociation) instead of ionizing the molecule. Any quenchingmolecule that may be accelerated to the cathode dissociates upon impact producing nosignal. If organic molecules are used, GM tubes must be replaced as they permanentlybreak down over time (after about one billion counts). However, the GM tubes includedin Spectrum Techniques set-ups use a halogen molecule, which naturally recombinesafter breaking apart.For any more specific details, we will refer the reader to literature such as G.F.Knoll’s Radiation Detection and Measurement (John Wiley & Sons) or to Appendix E ofthis lab manual.A -ray interacts with the wall of the GM tube (by Compton scattering or photoelectric effect) to producean electron that passes to the interior of the tube. This electron ionizes the gas in the GM tube.*Spectrum Techniques Student Lab Manual9
Physics LabGood Graphing TechniquesVery often, the data you take in the physics lab will require graphing. The following are a fewgeneral instructions that you will find useful in creating good, readable, and usable graphs.Further information on data analysis are given within the laboratory write-ups and in theappendices.1. Each graph MUST have a TITLE.2. Make the graph fairly large – use a full sheet of graph paper for each graph. Byusing this method, your accuracy will be better, but never more accurate that thedata originally taken.3. Draw the coordinate axes using a STRAIGHT EDGE. Each coordinate is to belabeled including units of the measurement.4. The NUMERICAL VALUE on each coordinate MUST INCREASE in the directionaway from the origin.Choose a value scale for each coordinate that is easy to work with. The range of thevalues should be appropriate for the range of your data.It is NOT necessary to write the numerical value at each division on the coordinate.It is sufficient to number only a few of the divisions. DO NOT CLUTTER THEGRAPH.5. Circle each data point that you plot to indicate the uncertainty in the datameasurement.Spectrum Techniques Student Lab Manual10
6. CONNECT THE DATA POINTS WITH A BEST-FIT SMOOTH CURVE unless anabrupt change in the slope is JUSTIFIABLY indicated by the data.DO NOT PLAY CONNECT-THE-DOTS with your data! All data has someuncertainty. Do NOT over-emphasize that uncertainty by connecting each point.7. Determine the slope of your curve:(a) Draw a slope triangle – use a dashed line.(b) Your slope triangle should NOT intersect any data points, just the best-fit curve.(c) Show your slope calculations right on the graph, e.g.,slope y y 2 y1 answer x x 2 x1BE CERTAIN TO INCLUDE THE UNITS IN YOUR SLOPE CALCULATIONS.8. You may use pencil to draw the graph if you wish.9. Remember: NEATNESS COUNTS.Spectrum Techniques Student Lab Manual11
Lab #1: Plotting a GM PlateauObjective:In this experiment, you will determine the plateau and optimal operating voltageof a Geiger-Müller counter.Pre-lab Questions:1. What will your graph look like (what does the plateau look like)?2. Read the introduction section on GM tube operation. How does electric potentialeffect a GM tube’s operation?Introduction:All Geiger-Müller (GM) counters do not operate in the exact same way becauseof differences in their construction. Consequently, each GM counter has a different highvoltage that must be applied to obtain optimal performance from the instrument.If a radioactive sample is positioned beneath a tube and the voltage of the GMtube is ramped up (slowly increased by small intervals) from zero, the tube does notstart counting right away. The tube must reach the starting voltage where the electron“avalanche” can begin to produce a signal. As the voltage is increased beyond thatpoint, the counting rate increases quickly before it stabilizes. Where the stabilizationbegins is a region commonly referred to as the knee, or threshold value. Past the knee,increases in the voltage only produce small increases in the count rate. This region isthe plateau we are seeking. Determining the optimal operating voltage starts withidentifying the plateau first. The end of the plateau is found when increasing the voltageproduces a second large rise in count rate. This last region is called the dischargeregion.To help preserve the life of the tube, the operating voltage should be selectednear the middle but towards the lower half of the plateau (closer to the knee). If the GMtube operates too closely to the discharge region, and there is a change in theSpectrum Techniques Student Lab Manual12
performance of the tube. Then you could possibly operate the tube in a “continuousdischarge” mode, which can damage the tube.Geiger 80085090095010001050110011500High Voltage (Volts)Figure 1: A classical plateau graph for a Geiger-Müller counter.By the end of this experiment, you will make a graph similar to the one in Figure 1,which shows a typical plateau shape.Equipment Set-up for ST-360 Counter with GM Tube and stand (Counter box, powersupply – transformer, GM Tube, shelf stand, USB cable, and a source holderfor the stand) as shown in Figure 2. Radioactive Source (e.g., Cs-137, Sr-90, or Co-60) – One of the orange, blue,or green sources shown above in Figure 2.Spectrum Techniques Student Lab Manual13
Figure 2: ST360 setup with sources and absorber kit.Procedure:1. Plug in the transformer/power supply into any normal electricity outlet and intothe back of the ST-360 box. Next, remove the red or black end cap from the GMtube VERY CAREFULLY. (Do NOT touch the thin window!) Place the GMtube into the top of the shelf stand with the window down and BNC connector up.Next, attach the BNC cable to the GM tube and the GM input on the ST-360.Finally, attach the USB cable to the ST-360 and a USB port on your PC (if youare using one).2. Turn the power switch on the back of the ST-360 to the ON position, and doubleclick the STX software icon to start the program. You should then see the bluecontrol panel appear on your screen.3. Go to the Setup menu and select the HV Setting option. In the High Voltage(HV) window, start with 700 Volts. In the Step Voltage window, enter 20. UnderSpectrum Techniques Student Lab Manual14
Enable Step Voltage, select On (the default selection is off). Finally, selectOkay.4. Go under the Preset option and select Time. Enter 30 for the number ofseconds and choose OK. Then also under the Preset option choose Number ofRuns. In the window, enter 26 for the number of runs to make.5. You should see a screen with a large window for the number of Counts andData for all the runs on the left half of the screen. On the right half, you shouldsee a window for the Preset
Spectrum Techniques Student Lab Manual 3 Student Usage of Lab Manual This manual is written to help students learn as much as possible about radiation and some of the concepts key to nuclear and particle physics. This manual in particular is written to guide you through a laboratory experiment set-up. The lab manual has the following layout:
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Each week you will have pre-lab assignments and post-lab assignments. The pre-lab assignments will be due at 8:00am the day of your scheduled lab period. All other lab-related assignments are due by 11:59 pm the day of your scheduled lab period. Pre-lab assignments cannot be completed late for any credit. For best performance, use only Firefox or
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Lab Safety & IR Spectroscopy Reading: Handbook for Organic Chemistry Lab, section on Lab Safety (Chapter 1) and IR Spectroscopy (Chapter 16). Organic Chemistry by Marc Loudon, 6th ed., pp. 569-591 (12.1-12.5). There is no prelab or lab report for today’s experiment. During today’s lab, you will check into a lab drawer.
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