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Introduction to Geiger Counters
A Geiger counter (Geiger-Muller tube) is a device used for the detection and measurement of all types of radiation: alpha, beta and gamma radiation. Basically it consists of a pair of electrodes surrounded by a gas. The electrodes have a high voltage across them. The gas used is usually Helium or Argon. When radiation enters the tube it can ionize the gas. The ions (and electrons) are attracted to the electrodes and an electric current is produced. A scaler counts the current pulses, and one obtains a ”count” whenever radiation ionizes the gas. The apparatus consists of two parts, the tube and the (counter + power supply). The Geiger-Mueller tube is usually cylindrical, with a wire down the center. The (counter + power supply) have voltage controls and timer options. A high voltage is established across the cylinder and the wire as shown on the page of figures. When ionizing radiation such as an alpha, beta or gamma particle enters the tube, it can ionize some of the gas molecules in the tube. From these ionized atoms, an electron is knocked out of the atom, and the remaining atom is positively charged. The high voltage in the tube produces an electric field inside the tube. The electrons that were knocked out of the atom are attracted to the positive electrode, and the positively charged ions are attracted to the negative electrode. This produces a pulse of current in the wires connecting the electrodes, and this pulse is counted. After the pulse is counted, the charged ions become neutralized, and the Geiger counter is ready to record another pulse. In order for the Geiger counter tube to restore itself quickly to its original state after radiation has entered, a gas is added to the tube. For proper use of the Geiger counter, one must have the appropriate voltage across the electrodes. If the voltage is too low, the electric field in the tube is too weak to cause a current pulse. If the voltage is too high, the tube will undergo continuous discharge, and the tube can be damaged. Usually the manufacture recommends the correct voltage to use for the tube. Larger tubes require larger voltages to produce the necessary electric fields inside the tube. In class we will do an experiment to determine the proper operating voltage. First we will place a radioactive isotope in from of the Geiger-Mueller tube. Then, we will slowly vary the voltage across the tube and measure the counting rate. On the figures page is a graph of what we might expect to see when the voltage is increased across the tube. For low voltages, no counts are recorded. This is because the electric field is too weak for even one pulse to be recorded. As the voltage is increased, eventually one obtains a counting rate. The voltage at which the G-M tube just begins to count is called the starting potential. The counting rate quickly rises as the voltage is increased. For our equipment, the rise is so fast, that the graph looks like a ”step”
a)They are relatively inexpensive b)They are durable and easily portable c)They can detect all types of radiation
Some of the disadvantages of using a Geiger Counter are:
a)They cannot differentiate which type of radiation is being detected. b)They cannot be used to determine the exact energy of the detected radiation c)They have a very low efficiency
Resolving time (Dead time)
After a count has been recorded, it takes the G-M tube a certain amount of time to reset itself to be ready to record the next count. The resolving time or ”dead time”, T, of a detector is the time it takes for the detector to ”reset” itself. Since the detector is ”not operating” while it is being reset, the measured activity is not the true activity of the sample. If the counting rate is high, then the effect of dead time is very important. We will first discuss how to correct for dead time, and then discuss how one can measure what it is.
Correcting for the Resolving time: We define the following variables: T = the resolving time or dead time of the detector tr = the real time that the detector is operating. This is the actual time that the detector is on. It is our counting time. tr does not depend on the dead time of the detector, but on how long we actually record counts. tl = the live time that the detector is operating. This is the time that the detector is able to record counts. tl depends on the dead time of the detector. C = the total number of counts that we record. n = the measured counting rate, n = C/tr N = the true counting rate, N = C/tl
Note that the ratio n/N is equal to:
n N
C/tr C/ti
tl tr
This means that the fraction of the counts that we record is the ratio of the ”live time” to the ”real time”. This ratio is the fraction of the time that the detector is able to record counts. The key relationship we need is between the real time, live time, and dead time. To a good approximation, the live time is equal to the real time minus C times the dead time T :
tl = tr − CT (3) This is true since CT is the total time that the detector is unable to record counts during the counting time tr. We can solve for N in terms of n and T by combining the two equations above. First divide the second equation by tr:
tl tr
CT
tr
= 1 − nT (4)
From the first equation, we see that the left side is equal to n/N :
n N
= 1 − nT (5)
Solving for N, we obtain the equation:
N =
n 1 − nT
This is the equation we need to determine the true counting rate from the mea- sured one. Notice that N is always larger than n. Also note that the product nT is the key parameter in determining by how much the true counting rate increases from the measured counting rate. For small values of nT , the product nT (unitless) is the fractional increase that N is of n. For values of nT < 0 .01 dead time is not important, and are less than a 1% effect. Dead time changes the measured value for the counting rate by 5% when nT = 0.05. The product nT is small when either the counting rate n is small, or the dead time T is small.
Measuring the Resolving Time We can get an estimate of the resolving time of our detector by performing the following measurement. First we determine the counting rate with one source alone, call this counting rate n 1. Then we add a second source next to the first one and determine the counting rate with both sources together. Call this counting rate n 12. Finally, we take away source 1 and measure the counting rate with source 2 alone. We call this counting rate n 2.
orbiting the nucleus. Two different types of interaction with the electrons can occur: photo-absorption and Compton scattering. We begin by discussing these two types of gamma interactions, then we discuss the operation of the gamma detector. Photo- absorption:
Photo-absorption In photo-absorption, the gamma is absorbed by the electron. The interaction with an electron at rest is shown graphically on the figures page. The gamma particle (photon) enters from the left with a distinct momentum and energy, and the electron is at rest. After the interaction, and gamma particle has been ”absorbed” by the electron which travels off to the right. Since energy and momentum are conserved in the interaction the electron gains the energy and momentum of the gamma photon.
Compton Scattering In Compton scattering, the gamma scatters off the electron. The interaction with an electron at rest is shown graphically on the figures page. The gamma particle (photon) enters from the left and the electron is at rest. In this case, the gamma is not absorbed, but scatters off the electron. The electron has gained some energy, and the gamma photon has lost some. The scattered gamma photon can interact with other electrons in the material. When a photon approaches an electron, one cannot predict exactly will happen. There is a certain probability that photo-absorption will happen, a probability that Compton scattering will occur, and a probability that no interaction will take place at all. The angle that the photon scatters is also probabilistic. Using the principles of quantum mechanics, one can calculate the probabilities for each of these possibilities. As with radioactive decay, probability enters in the physics of the interaction. The probability of each process depends on the energy of the gamma. For photo-absorption the probability decreases rapidly with the energy of the gamma. For higher energies, the probability for Compton scattering is much larger than for photo-absorption.
The NaI Multi-Channel Analyzer (MCA)
The MCA system is used to detect only gamma and X-ray radiation. However, it detects the radiation well, and the MCA can also determine the energy of gamma and X-ray particles. The MCA system consists of 3 main parts: the detector itself, the amplifier/power-supply, and a computer. The detector has two parts: a scintilla- tion crystal (sodium iodide) and a photo-multiplier tube. The computer stores and
crystal changes one high energy photon into many low energy photons. We can count the number of low energy photons with a photomultiplier tube. The nice thing about the crystal is that the number of low energy photons (of visible light) is proportional to the energy of the gamma particle. The low energy (visible) photons enter the photo-multiplier tube at one end. The net effect is that a current pulse is produced. The nice thing about the photomultiplier tube is that the current pulse it produces is proportional to the number of visible photons that enter the tube. The photomultiplier tube requires a high voltage. The value of the voltage is given by the manufacture, and ranges from 550 to 1000 volts for our photomultiplier tubes. Before you turn on the MCA system, be sure that the high voltage is set properly. Once set, we will not change it during the experiment(s). The current pulse from the photomultiplier tube enters an amplifier, which ampli- fies the current. Finally, this amplified current is input into a ”card” in the computer. The ”card” contains a multi-channel analyzer (MCA). The multi-channel analyzer ”bins” the pulse according to its strength. Pulses with larger current get ”binned” in a larger channel number. Changing the amplifier gain changes the scale on the horizontal axis. Although there are many steps to the detector system, the end result is that the channel number that gets ”binned” is proportional to the energy deposited in the crystal. The binned channel is proportional to the current pulse which is proportional to the number of visible photons which is proportional to the energy of the gamma. This approximate proportionality is what makes the crystal an accurate measuring device. We have described the ideal case: the gamma is photo-absorbed by an electron in the crystal. This senario would result in a sharp spike at a channel number cor- responding to the energy of the gamma. However, thermal effects in the NaI crystal broaden the sharp spike into a ”bell-shaped” Gaussian peak. For the Cs^137 example in the figure above, the peak caused by photo-absorption is at channel number 390. We refer to this peak as the photopeak. The channel number of the photopeak is pro- portional to the energy of the gamma particle. If we measure a different isotope which emits a gamma at a different energy, the photopeak will be shifted. The position of the photopeak will also change if we change the amplifier gain. For experiments where calibration is important, we keep the amplifier gain set to a particular value which is useful for all the experiments.
- The gamma particle Compton scatters off an electron in the crystal
A common situation is when the gamma particle scatters off an electron in the crystal. After scattering, the gamma can leave the crystal. In this case, only part of
conversion, a hole in an inner shell is produced. When an orbiting electron fills the hole, an x-ray is emitted. In the spectra, X-rays may be seen at low channel number. In our Cs^137 example, the large peak at channel number 30 is a characteristic X-ray from Barium. The energy of the X-ray will depend on the isotope present, not every spectrum will have these characteristic X-rays present. There is another bump in the spectra of Cs^137 at around channel number 50. This peak is produced by characteristic X-rays from lead. The lead is in the shielding around the detector. When a gamma from the source knocks out an inner electron in the lead shielding, X-rays can be emitted when inner hole is filled. This peak will always be present when lead shielding is used. If we took the spectrum without the lead shielding, the peak would disappear. We have described the main features of the gamma spectrum for a single pho- topeak. If an isotope emits more than one gamma, then each gamma will produce a photopeak, a Compton region, backscattering bump, and maybe characteristic X- rays. Although these patterns will overlap with multiple gamma production, the photopeaks are usually clear enough to distinguish.
High Resolution Germanium Detectors
We also have a high resolution Ge detector in our laboratory. The photopeaks for these detectors are very clear and narrow. One does not need to worry about the Compton region or backscattering peaks.
Liquid Scintillation Detector
The liquid scintillation detector is the detector often used by biologists. The detec- tor is a liquid, or ”fluor”, and the sample is placed in the liquid. The fluor contains a substance that fluoresces when a charged particle is slowed down (or absorbed) by it. The fluorescence that it emitted is in the visible light region. Photo-multiplier tubes are placed around the liquid to detect this emitted light. The detector is designed to detect mainly (or only) beta particles. Basically, the detector works as follows: When a beta particle is emitted, it enters the fluid and excites the ”fluor” as it slows down. The fluor then de-excites and emits photons. The photons are detected by a photo-multiplier tube, which sends a current pulse to an amplifier. The amplified current pulse is then binned via a multi-channel analyzer similar to the gamma spectrometer. The continuous energy spectrum is recorded by the detector. Most liquid scintillation detectors do not print out or display the whole spectrum as is done with the gamma detectors in the lab. For liquid scintillation detectors, the
user usually sets a counting window: the initial and ending channels. The detector prints out and/or displays the sum of all the counts between the initial and final channels. These channels are often refered to as the lower and upper channels. In order to set an appropriate window, the user must know the energy range of the beta that is emitted and the energy/(channel number) for the detector. The energy of the emitted electron in beta decay is not mono-energetic, the elec- tron shares its energy with the neutrino. Hence, the beta spectrum does not have a distinct peak, but rather a continuous spectrum. Thus, the liquid scintillation de- tector is not useful in identifing the isotopes in a sample, but rather in counting the amount of a single known isotope in the sample. The detector is mainly used in radioisotope tracing experiments. The liquid scintillation detector has nice features for biologists:
- Biological samples are often in liquid form, or can be placed in liquids.
- Hydrogen and carbon are common elements in biological samples, and can be tagged using 3 H (tritium) and 14 C. These two isotopes are beta emitters.
- The liquid scintillation detector is very efficient since the sample is placed in the detecting material.
In the next chapter we will discuss the calibration and methods of analysis of the liquid scintillation detector.
