Chapter 14 Radiation Surveys

Transcription

1 Chapter 14 Radiation Surveys Radiation surveys consist of radiation measurements and observations of radiation protection controls. Measurements may consist of direct radiation readings and/or analysis of samples taken from the radiation environment. Observations of radiation protection controls may include monitoring work practices such as use of gloves and lab coats, and verifying that control devices such as signs, labels and interlocks are in place. When results of radiation surveys are compared against appropriate standards, they can be used to determine the effectiveness of the radiation safety program, and/or to determine whether regulatory requirements have been satisfied. From the workers point of view, surveys are needed to determine how the work they are performing could affect themselves and other individuals who could be exposed. Surveys are also needed to determine whether their work falls within the radiation safety guidelines established for the project. For these reasons each radiation worker is responsible for performing radiation surveys at meaningful times. This chapter describes instrumentation and measurement techniques for determining what levels of external radiation are present from radiation sources, how to interpret the results of a survey, and, when necessary, what corrective actions should be taken Compliance Assessment by Project Directors All project directors should assess their state of compliance with regulatory requirements at least once each year. This assessment should be a comprehensive review of radiation safety practices in the lab in comparison with the requirements established in project authorization documents. A self-inspection checklist has been developed to help project personnel determine the compliance status of the project. This checklist is available from the Radiation Safety Section (RSS) Proper Use of Survey Instruments Personnel who conduct radiation surveys must understand the capabilities and limitations of the survey instruments being used. The following sections briefly describe the instruments frequently used in radiation protection and discuss their proper use. Explanations of operating principles may be found in a wide variety of textbooks. Two excellent references on this matter are the latest editions of Jacob Shapiro's book entitled Radiation Protection - A Guide for Scientists and Physicians published by Harvard University Press, Cambridge Massachusetts, and Glenn F. Knoll's Radiation Detection and Measurement published by John Wiley & Sons, New York Ionization Chambers Ionization chambers are the simplest of the gas filled detectors. Portable ion chambers utilizing current measurement circuitry are commonly used as survey instruments for radiation monitoring purposes. Dose measurement instruments such as pocket dosimeters and condenser r-meters are also available which operate on the principal of reduction of a charge Page 1 of 15

2 within the chamber. Some ion chambers that are designed to measure the quantity of radioactive material in a sample allow the sample to be placed within the chamber. Ion chambers are generally used for x- and gamma-ray measurements but can also be used for beta particle or electron measurements if equipped with a suitably thin wall or window. Most ion chamber survey meters have a reasonably linear response over a wide range of x- and gammaray energies. Many are equipped with a cap or sliding door that should be removed or opened when measuring energies below about 50 kev. If the walls and gas filling of an ion chamber consist of materials with atomic numbers not too different from the atomic numbers of the components of air, then the response of the ion chamber is not very energy dependent. This means that the response of the ion chamber will be reasonably correct over a wide range of x-ray or gamma-ray energies. Ionization chamber type survey meters are frequently used when measuring external radiation fields from x-ray units and larger quantities of radioactive material. Because most of them have a higher detection threshold than Geiger counters and are more expensive, ion chamber survey instruments are infrequently used when working with tracer quantities of radioactive materials Geiger-Mueller Counters The Geiger-Mueller tube is one of the oldest and most useful radiation detectors in existence. Tubes are filled with a counting gas and may be constructed in a variety of shapes and sizes, usually cylindrical. Geiger detectors or probes usually consist of a Geiger tube encased in a protective housing. Depending upon the design, Geiger detectors can have good sensitivity to alpha, beta, x, and gamma radiations. Some probes have a side window that can be opened to permit detection of higher energy beta particles and low energy x or gamma radiation. When open, side window tubes are approximately mg/cm 2 in thickness. When closed they are 1000 mg/cm 2 thick or more. Some Geiger tubes are constructed with a thin end window made of mica that is typically no more than 2 mg/cm 2 in thickness. End window probes are useful when working with sources of lower energy beta particles, gamma, or x rays. Geiger detectors are usually connected to rate meters or scalers. Those utilizing a rate meter are typically calibrated in units of counts per minute, counts per second, milliroentgen per hour, or roentgens per hour. Most portable Geiger counters used for radiation monitoring purposes are of this variety. Some portable units and many laboratory instruments utilize scalers which record each individual Geiger tube pulse during a selectable time interval. These instruments are primarily used to determine the quantity of radioactive material in a sample rather than to determine exposure rates. Most Geiger tubes have a reasonably flat response to photon energies between 200 kev and 1.25 MeV. Significant under-response or over-response is possible, particularly at photon energies below about 200 kev. When measuring gamma or x rays below about 200 kev, the open side window or the thin end window should be facing the source. Detectors without windows should not be used for measurements of photons below about 200 kev. Low energy gamma and x rays on the order of 30 kev and below may be greatly attenuated by the walls of the tube, drastically reducing the tube's sensitivity at these energies. Caution should be used when monitoring low energy sources because the Geiger tube may not respond Page 2 of 15

3 even though a hazard exists. Radiation from an analytical x-ray unit or an I-125 source will probably not be detected with a side window tube or through the side of an end window tube. It is imperative that a thin end window tube be used at these energies with the thin window facing the source. The minimum energy necessary for an alpha or beta particle to be detected by a Geiger tube is dependent upon the density thickness of the tube's walls and holder in which it is mounted. Side window detectors cannot detect alpha particles; have a cut-off energy of about 200 kev for betas when the window is open, and a cutoff of over 2 MeV for betas when the window is closed. These properties make them poorly suited for use in the typical biomedical research laboratory. Through the end window of an end window tube, a survey meter can detect beta particles greater than about 35 kev and alpha particles greater than about 4 MeV. This includes virtually every radionuclide in use at UIC except H-3 and Fe-55. Thin end window Geiger tubes cannot detect H-3 because the beta particles (E max = 18.6 kev) do not possess enough energy to pass through the thin end window. They cannot detect Fe-55 with the necessary sensitivity because the low energy x-rays (0.64, 5.887, 5.899, and 6.49 kev) are almost completely attenuated by the window. A thin end window Geiger counter can also be used to estimate the skin dose from a beta emitter point source deposited on the skin. While most end window Geiger counter survey instruments are calibrated in mr/h, a unit that is only defined for gamma and x rays, readings can be converted into a good estimate of the dose rate in rads per hour. A one microcurie beta particle point source placed very close to the window of a typical end window Geiger counter will cause a response of about 150 mr/h. Assuming the dead layer of skin has a density thickness of 7 mg/cm 2, a one microcurie point source of a typical beta emitter will deliver a dose of about 9000 rad/h to the tiny volume of living tissue immediately adjacent. (9,000 rad/h/ mrad/rad) == 60,000 mrad/h to the tissue 150 mr/h/mci mr/h on the GM survey meter In other words, the Geiger counter reading in mr/h should be multiplied by 60,000 to obtain the maximum dose rate to the skin in mrad/h. Geiger detectors should always be switched on before approaching an intense source of radiation because of an effect known as jamming, tube paralyzation, or tube saturation. Jamming occurs when the detector is placed in a very strong radiation field in which the tube is unable to recover from one detected radiation before responding to the next. This causes continuous discharge in the tube and the meter reads zero. Geiger counters should not be used to measure radiation from sources that produce strong electromagnetic fields, such as a generator, or that emit pulsed radiation fields, such as a linear accelerator because this may result in extremely erroneous readings Solid Scintillation Detectors Scintillators are materials that absorb and convert radiation energy to photons of light. Detectors are made by optically coupling the scintillant to a photomultiplier tube and enclosing Page 3 of 15

4 the assembly in a light tight container. The photomultiplier tube converts the flashes of light into electrical pulses that can be counted and analyzed. Portable and laboratory equipment may utilize rate meters or scalers to measure the output pulses of solid scintillation detectors. A wide variety of solid scintillation detectors are available that can detect alpha, beta, x, gamma, and neutron radiations. Most scintillation phosphors are inorganic crystals with a small but precisely controlled impurity. The most commonly used solid scintillator is sodium iodide doped with a thallium impurity, NaI(Tl). Scintillation phosphors are optically coupled to photomultiplier tubes and inserted into sleeves or cases to seal out ambient light. Table 14.1 lists several solid scintillators that are commercially available. Analytical laboratory instruments designed for detection of photons from a variety of gamma emitting radionuclides such as auto-gamma counters, single channel analyzers, and multichannel gamma spectrometers usually use fairly large (2" x 2" or 3" x 3") NaI(Tl) detectors. Because large crystals detect radiation with high efficiency, several inches of lead shielding is usually employed to reduce the background radiation. These instruments can detect very small quantities of radioactive material. They are commonly used in radiation protection to count smear or wipe test samples, leak test samples, and bioassay samples. TABLE SOME COMMERCIALLY AVAILABLE SOLID SCINTILLATION PHOSPHORS RADIATION DETECTED Alpha Particles Beta Particles X and Gamma Rays Thermal Neutrons Fast Neutrons Neutrons TYPE OF SCINTILLATOR Zn(Ag) Plastic, CaF 2 (Eu), Anthracene NaI(Tl), CsI(Na) 6 Li dispersed in ZnS (Ag) ZnS(Ag) 6 LiI(Eu) Portable survey meters that use NaI(Tl) crystals for detection of x and gamma rays are commonly available but cost significantly more than Geiger counters. Some portable detectors do not provide adequate shock absorption for their fragile and expensive photomultiplier tubes and crystals, rendering them vulnerable to breakage when improperly handled. Thicker crystals suffer from high background counts because sufficient shielding cannot usually be employed. Detectors with thin NaI(Tl) crystals are the most useful in portable equipment because they detect fewer background counts while efficiently detecting lower energy x and gamma rays such as those emitted by I-125. Some detector designs employ the use of a thin aluminized Mylar window which also permits the detection of energetic beta particles such as those of P Liquid Scintillation Counters Page 4 of 15

5 Liquid scintillation counters are large analytical instruments capable of detecting very small quantities of alpha, beta, and gamma emitting radionuclides. Samples are prepared by mixing them with a fluid detector called liquid scintillation fluid, which consists of a solvent to dissolve both the radioactive sample and a scintillator. Radiation energy emitted by the sample is absorbed by the solvent molecules, transferred to the scintillator, and released in the form of light photons. The sample and fluid are usually placed into small vials that are counted in the a dark chamber within the liquid scintillation counter. Background radiation is reduced by using several inches of lead shielding. Dual photomultiplier tubes and coincidence counting circuitry is employed in most units to reduce photomultiplier tube noise and further reduce background. Scintillation counters can be equipped with several counting channels or with a spectrometer. Because the fluid detector is in direct contact with the sample, low energy beta emitters such as tritium, and low energy x-ray emitters such as Fe-55 can be readily detected. Extremely high counting efficiencies can be obtained if samples are properly prepared and the correct type of cocktail is used. In order to obtain the highest possible counting efficiency, some samples must be digested with a solubilizer to create a homogeneous counting mixture. Quenching and chemiluminescence can cause interference with detection of radioactivity in samples. Scintillation counters are commonly used in radiation protection to count smear or wipe test samples, leak test samples, and bioassay samples. Sample preparation is not necessary when analyzing smears Acquisition and Calibration of Portable Survey Instruments Projects that use radioactive materials other than tritium (H-3) or Fe-55 are usually required to possess and use a properly operating survey meter. The need for obtaining a survey meter for users of radiation producing machines is evaluated on a case by case basis. The most common radiation survey meter in use at UIC for detection of radioactive materials is a portable battery operated Geiger counter equipped with a thin end window detector and rate meter calibrated in milliroentgen per hour, mr/h. Sturdy reliable units of this variety are readily available at a reasonable cost. Care should be taken when selecting a survey meter because some are very poorly constructed and cannot withstand normal daily use. Others have rate meters that do not respond in a linear fashion and cannot be calibrated to the standards required by the license issued to UIC. Before purchasing a portable survey meter consult with the RSS for advice. All survey instruments are calibrated at least annually and checked at least quarterly for proper operation by the Radiation Safety Office Radioactive Material Surveys by Project Personnel Radiation protection surveys are performed to measure contamination and external radiation levels. Survey results are used to maintain control over the radiological environment. If surveys are conducted at meaningful times, they will be effective in preventing the spread of contamination and in minimizing personnel exposures. Since project personnel know when, where, and how radioactive material work has been performed in their laboratories, they should know the best times to conduct surveys. Page 5 of 15

6 Contamination Monitoring Whenever radioactive material in unsealed form is used, it is possible to create contamination in the work area. Use of unsealed radioactive material can also cause personnel contamination. Project personnel should frequently survey the work area for contamination, promptly clean any contamination that is detected, and conduct additional surveys to determine the effectiveness of the decontamination effort. Because there is no practical way to prevent contamination from spreading onto objects or into areas where contamination is not permitted, decontamination should continue until no contamination remains. The following guidelines should be used to determine when contamination surveys should be conducted. Hands, clothing, shoes, counter tops, floors, and other objects should be surveyed for contamination at the following meaningful times: During and after each handling or use of radioactive material; Immediately after dispensing radioactive material from a stock solution; Immediately after performing any procedure that has a significant probability of causing contamination; Immediately after performing new or altered procedures; Whenever there is reason to suspect contamination; and Before leaving the laboratory for breaks and meals, and at the end of each work day Contamination Survey Methods Radionuclides Other Than H-3 and Fe-55 Contamination from radionuclides other than H-3 and Fe-55 may be detected by carefully monitoring the work areas and the floor in the work areas using a thin end window Geiger counter or pancake probe. Care should be taken to distinguish contamination from external radiation levels arising from nearby sources. The end window of the detector should be moved slowly past the surface being monitored. The detection sensitivity decreases rapidly as the distance from the surface being surveyed increases. Therefore, it is important to place the thin end window of the Geiger detector close (about 1 cm) to the surface being monitored. Care should be taken to avoid contaminating the detector. Surface wipes must be taken to detect contamination in high background areas, when a Geiger counter is not available, or when additional sensitivity is required. Using moderate pressure, wipe about 100 square centimeters of the surface being monitored with a cotton tipped applicator or piece of filter paper. While a Geiger counter can be used to check wipes for contamination, samples should be counted using a properly adjusted liquid scintillation counter or auto-gamma counter because they have superior sensitivity. H-3 and Fe-55 The extremely low energy of the beta particle emitted by tritium (H-3) prevents its detection with Page 6 of 15

7 a Geiger counter survey instrument. Similarly, the low energies of radiations emitted by Fe-55 prevent efficient detection. Tritium and Fe-55 contamination should be monitored by taking surface wipes. Using moderate pressure, wipe about 100 square centimeters of the surface being monitored with a cotton tipped applicator or piece of filter paper. Samples should be counted using a properly adjusted liquid scintillation counter Area Radiation Measurements External radiation levels can be caused by gamma emitters and high energy beta emitters. Radiation levels for such radionuclides can be monitored in and around the laboratory using a Geiger counter or ionization chamber calibrated in units of milliroentgen per hour (mr/h). The following guidelines should be used to determine when radiation level surveys should be conducted: While radioactive material is in use, radiation levels within the radionuclide laboratory should be kept as low as reasonably achievable and must not cause overexposure of laboratory personnel. Project personnel should be in the habit of frequently monitoring the work areas for excessive radiation levels. Radioactive material should be stored using adequate shielding so that the radiation levels in the immediate surrounding laboratory areas do not cause unnecessary exposure of laboratory personnel. When placing radioactive material into storage that could cause unnecessary exposure, a survey should be conducted. When use of radioactive material could create radiation levels above background in adjacent areas that could be occupied, a survey should be conducted. If any radiation level above background is detected during the survey, determine whether it is within the limits established in Chapter 11. If the limit for unrestricted areas is exceeded, take immediate corrective action. Assistance can be obtained from the RSS if needed Radioactive Material Surveys by the Radiation Safety Section (RSS) Routine radiation surveys of radioisotope laboratories are performed by the RSS to ensure that contamination and radiation levels have not gone unchecked. Quarterly and biweekly frequencies have been established. A laboratory assigned to a quarterly survey frequency will be surveyed at some time during each calendar quarter. The database program used to schedule surveys examines prior survey results and the type, frequency, and quantity of radioactive material that has been received. This has resulted in Radiation Safety Office surveys that are performed at more meaningful times Survey Methods The survey methods used by the RSS are similar to those described previously in this chapter. Contamination monitoring and external radiation measurements are made with a thin end window Geiger counter. Removable surface contamination is monitored by taking surface smears and counting them using a liquid scintillation counter. In addition to radiation Page 7 of 15

8 monitoring, surveys include inspection of the laboratory for the proper postings, and checking that personnel are adhering to the conditions of authorization, Rules for Radioisotope Labs, etc Survey Reports and Corrective Actions Survey reports are completed on the day of the survey. Smear test results and Geiger counter readings are listed directly on the lab sketch in the approximate location taken. Smear results are placed within circles and are reported in disintegrations per minute. Geiger counter readings are reported in mr/h. Deficiencies regarding postings and adherence to the Rules for Radioisotope Laboratories are indicated on the back of the form. The front of the survey will be stamped "ALL OK" in green ink or "CORRECTIVE ACTION NEEDED" in red ink as appropriate. Copies of the reports are sent via campus mail to each project that is authorized to use the room. Survey reports should be promptly reviewed and placed in the three ring binder so that all project personnel have an opportunity to review the results on a timely basis. If contamination, an excessive radiation level, or an item of noncompliance is identified during a survey, the technician performing the survey will inform project personnel at the conclusion of the survey or will call by telephone the same day the survey was conducted. In this way the project personnel are promptly informed about radiation safety problems so that prompt corrective actions can be taken. When a problem is found in a shared lab, project personnel must determine who will take the required corrective action Patient Related Brachytherapy and Radiopharmaceutical Surveys The University of Illinois Hospital (UIH) provides patients with various forms of radiation therapy. These treatments are given by experienced physicians and technologists in the Radiation Therapy Section and Nuclear Medicine Section. Two basic types of treatments are provided: brachytherapy, and radiopharmaceutical therapy. When radiation therapies involve the use of radioactive material in patient rooms, regulations require performance of important radiation protection surveys including surveys of the surrounding areas during the therapy and prior to releasing the patient's room for further use after the therapy has ended. Brachytherapy Brachytherapy involves placing sealed radioactive material within, on the surface of, or in the extremely close proximity of the patient. Sealed sources are made by placing the radioactive material within a strong metal capsule that is sealed to prevent the radioactive material from escaping. The walls of the capsule are thin enough to allow most of the penetrating radiations to escape. Depending upon the types and energies of the radiations emitted from the source, a significant amount of radiation may pass through the patient, creating an external field of radiation. Treatments may last for minutes, hours, or days. Some sources are permanently implanted. After removal of temporary implants, the patient does not contain radioactive material or emit radiation. Permanent implants are normally performed using I-125 seeds, which do not normally create a hazardous radiation field around the patient. Cs-137 Cs-137 brachytherapy is most commonly used for gynecological therapy. Because a significant fraction of the radiation emitted by the sources escapes from the patient and Cs-137 has a long half-life, Cs-137 sources are used only for temporary implants. This penetrating radiation is Page 8 of 15

9 actually emitted by Cs-137's daughter, Ba-137m which has a half life of min. An applicator with no sources is positioned within the patient in a procedure room or surgical suite and the sources are afterloaded into the applicator in the patient's room. Patients are housed in shielded hospital rooms and are cared for by specially trained nursing staff. A patient room survey is conducted after the sources are loaded, and a patient release survey and room clearance survey after the sources are removed. Patients may not be released from hospitalization until the patient release survey has been completed. Ir-192 Ir-192 brachytherapy sources are referred to as seeds because of their small physical size. They are used for a wide variety of treatments and are usually implanted using afterloading techniques. A significant fraction of the radiation from Ir-192 is not absorbed in the patient, therefore Ir-192 seeds are usually used only for temporary implants. Patients are housed in shielded hospital rooms and are cared for by specially trained nursing staff. A patient room survey is performed at the start of the treatment and after the sources have been removed. In addition, a patient release survey is conducted after the sources are removed. Patients may not be released from hospitalization until the patient release survey has been completed. I-125 I-125 brachytherapy sources are also referred to as seeds because of their small physical size. Since most of the radiation emitted from I-125 is easily absorbed by the patient and no significant radiation field is created outside the patient, I-125 seeds are frequently used for a wide variety of permanent implants. I-125 seeds may also be placed in a holder and temporarily attached to the surface of the body. Sources are implanted or attached to the patient in a surgical suite or procedure room. Surveys are performed in the surgical suite whenever sources are being applied or removed and in the patient's room during the treatment and after the treatment is concluded. In addition, a patient release survey must be conducted after the removal of temporary implants to verify all sources were removed. Patients may not be released from hospitalization until this survey has been performed. Radiopharmaceutical Therapy Therapy with radiopharmaceuticals involves the introduction of radioactive material into the body in unsealed form. The distribution of radioactive material in the body and rate of elimination from the patient's body is dependent upon the chemical form of the material, the condition of the patient, and the method and site of administration. I-131 I-131 is usually administered orally in the form of a sodium iodide solution for treatment of the thyroid gland. I-131 therapies may be performed in conjunction with surgical removal of part or the entire thyroid. Administration is usually performed in the Nuclear Medicine Section but may also be performed in the patient's room. Patients receiving more than 30 millicuries must remain hospitalized until the activity is below 30 millicuries. Up to a few hundred millicuries may be administered to hospitalized patients. I-131 is taken up rapidly in functional thyroid tissue. I-131 that remains in circulation is rapidly excreted from the body. Blood, urine, saliva, and perspiration all contain significant quantities of Page 9 of 15

10 I-131 in the first few days following the treatment. Consequently, the room in which the patient is housed can become contaminated with radioactive material. Precautions are taken prior to administration to prevent contamination of the floor and other objects in the patient's room. The patient is given instructions regarding how contamination can be minimized. A survey of the patient room is conducted soon after the material is administered, a daily survey is conducted to determine the approximate quantity of I-131 remaining in the patient, and a room clearance survey is conducted after the patient is released from hospitalization Surveys of Radiation Producing Machines Radiation producing machines must be surveyed annually in accordance with guidelines established by the Illinois Emergency Management Agency (IEMA). The results of these surveys are submitted to the IDNS. Radiation surveys are also performed when radiation producing equipment is installed or modified Medical X-Ray Equipment Inspection of medical X-ray equipment includes: Measurement of the radiation output; Determination of the X-ray beam quality; Tests of operating controls; Tests of the collimators and other beam limiting devices; Tests of beam location and size indicators in relation to the actual X-ray beam; and Tests of interlocks, warning lights, and other safety devices. Further details are not presented in this manual because these tests should only be performed by a qualified medical or health physicist. Contact Radiation Safety for further information Analytical X-Ray Equipment Figure 14.1 shows spectral distributions of X- rays from a tube with 1 mm Be inherent filtration operated at 100 kev and with increasing thicknesses of aluminum filters. Note the two main features of the spectrum representing the two mechanisms by which x rays are produced, bremsstrahlung and electron displacement. Bremsstrahlung, or breaking radiation, is created when the electrons that are bombarding the target in the x-ray tube decelerate in the electric field close to the nucleus of the target atoms. The kinetic energy lost in this deceleration is converted into x rays, the energies of which are dependent upon the charge on the electron, which is fixed, and the strength of the electric field, which is dependent on the distance of the electron from the nucleus. The bremsstrahlung X-rays are represented by the smooth portion of the x-ray energy spectrum. Page 10 of 15

11 Kinetic energy of the electrons bombarding the x-ray tube target can also remove electrons from target atoms. The energy needed to free the electron from the shell is called the binding energy. This produces an atom with excess energy. Additional kinetic energy may also be imparted to the electron that is freed from the target atom. When the vacancy in the inner shell is filled, the binding energy is released, creating a characteristic x ray with an energy equal to the binding energy. Characteristic X- rays are monoenergetic and are represented by the sharp peaks in the x- ray energy spectrum. From the spectrum, notice that as aluminum filtration is added, the lower energy X- rays are preferentially filtered from the beam, increasing the average x- ray energy penetrating the filter. This is sometimes referred to as beam hardening; however, even with significant beam hardening the average x-ray energy is significantly lower than the maximum energy. A formula to approximate the exposure rate from analytical x-ray equipment, Equation 14.2, developed by Bo Lindell, was published in Health Physics, Volume 15, pp This equation provides accuracy of ±25% for tubes with tungsten targets and 1 mm beryllium windows operated at voltages between 25 and 80 kv. Other filtrations may change the exposure rate by a factor of two. X == (50) (V) (I) (Z) (r 2 ) (74) Where X_ = Exposure rate in Roentgens/sec V = Voltage applied to x-ray tube in kv Z = Atomic Number of Target I = Current supplied to tube in ma r = Distance from x-ray tube focal spot in cm Measurement of the radiation fields emitted by analytical x-ray equipment can be very difficult because of the energies of radiation emitted, the intensity of the beams and the very small angles the primary beams frequently subtend. Measurement of scattered radiation can also be difficult. Geiger Counters Page 11 of 15

12 Two types of portable radiation survey instruments are commonly used to perform radiation surveys around radiation producing machines, Geiger counters and ion chambers. Geiger- Mueller, G-M, survey meters are more sensitive than ion chambers, responding to fields as low as 0.01 mr/h. The G-M detector is filled with a counting gas other than air, frequently a mixture of helium and a halogen. Radiation creates ions in the counting gas, causing an electron avalanche and discharge of the tube. The resulting pulse is counted by a rate meter with a scale calibrated in units of exposure, most frequently milliroentgen per hour, or counts per minute. Since G-M survey meters register pulses, they are frequently equipped with a speakers that help the surveyor to hear each event that is detected. Further theory of operation can be found earlier in this chapter. The Geiger tube is significantly smaller than the ion chamber, allowing it to be positioned in smaller spaces. Geiger tubes can also be equipped with thin end windows made of mica that are only about 1.7 mg/cm 2 thick, which allow them to detect low energy X- rays. When measuring low energy X- rays the end window should be pointed toward the source of radiation. Figure 14.3 shows the energy response through the end window of a Ludlum Model 44-7 end window detector, which is commonly used at UIC. Notice that this detector overresponds by a factor of about 3 at kev and under-responds by a factor of 10 at 10 kev. Since the spectra of x-ray energies around analytical x-ray equipment are quite variable, it is not possible to predict the overall energy response of the Geiger counter in such fields. Thus, Geiger counters may be used as a sensitive tool to detect X-ray fields around analytical X-ray equipment, but should not be used to accurately determine exposure rates. A serious disadvantage of the Geiger counter is paralysis or saturation, explained below. During saturation the meter will read zero or near zero in a radiation field that is above the highest radiation field that the meter was designed to measure. This situation can be very dangerous because the surveyor thinks there is no radiation field when the field could be dangerously high. Saturation is readily encountered if a Geiger detector is placed in the primary beam of most analytical X-ray units. For this reason the G-M survey meter should be switched "on" in an area with a low radiation field, checked for proper operation, and moved slowly toward the area being surveyed. If a high reading is immediately followed by a reading of zero, the tube has probably saturated and appropriate precautions should be taken to avoid exposure. (e.g.: Switch off the source of radiation by closing shutters, covering with shielding, removing power from the tube, etc). Page 12 of 15

13 When a Geiger tube discharges, the tube cannot respond again for a brief period of time. This duration is on the order of a hundred milliseconds and is called dead time. Any radiation that interacts in the Geiger tube during the dead time cannot create another pulse and therefore is not detected. During the next hundred or so milliseconds, called the recovery time, the high voltage circuit must re-charge the anode. During the recovery time the Geiger tube will detect radiation events, but will produce weak pulses that are not counted by the rate meter. As shown in Figure 14.4 the dead time plus the recovery time is called the resolving time of the meter. As the exposure rate increases, the number of radiation events that are missed also increases. For example, a Ludlum Model 3 equipped with a Model 44-7 detector has resolving time losses greater than 50% at 200 mr/h, the top of the highest scale. When pulses occur so rapidly that the Geiger tube never gets a chance to fully recover, there are no or almost no pulses strong enough to be counted by the rate meter circuit. This results in a zero or near zero reading on the meter. In general, saturation usually occurs well above the highest exposure rate that a given Geiger counter is designed to measure. For the Ludlum Model 3/44-7 meter, designed to measure exposure rates up to 200 mr/h, saturation occurs above 10,000 mr/h. Another serious disadvantage of the G-M survey meter occurs when measuring radiation from pulsed sources. Pulsed machine sources can emit a great number of x rays in an interval that is shorter than the resolving time of the Geiger tube. In this case the tube can only create one pulse no matter how many x rays interact in the detector. If the next pulse of radiation is emitted after the tube recovers, it is detected. In effect, the meter is measuring the pulse rate of the source. Since almost all of the x rays are not detected under these circumstances, the meter reading in mr/h can be very much lower than the actual exposure rate. Since the source emits pulses at very regular intervals, detection of pulsed fields can usually be recognized by an extremely steady meter reading. Pulsed fields can be accurately measured using an ion chamber as long as there is no significant radio-frequency interference and the instantaneous exposure rate during the pulse is not excessive (see instrument manual or consult manufacturer). Ion Chambers In air filled ion chamber instruments ions created in the sensitive volume of the chamber are collected by a positively charged anode, usually a wire or rod that runs through the center of the chamber, and the negatively charged chamber walls. This causes a very small flow of electrical current that is measured with a sensitive electrometer circuit. Ion chambers are frequently used to measure exposure rates from low energy X- rays because they respond accurately to a wide range of X-ray energies, a property called energy independence. They can also be equipped with very thin (1-2 mg/cm 2 ) windows, allowing fairly accurate (±10%) measurement of radiation exposures from X- rays with energies as low as 6 kev. The energy response curve for a Victoreen Model 471 ionization chamber, taken from the instrument manual, is provided below in Figure Page 13 of 15

14 Many ion chambers can also be operated in the integrate mode, allowing measurement of total exposure over a fairly long time period. The primary disadvantages of ion chamber survey meters are that radiation fields lower than about 0.1 mr/h cannot be measured accurately and the sensitive volume of the chamber is frequently physically larger than the beam being measured, a problem discussed below. Ion chambers will produce accurate measurements only when the entire chamber is in a radiation field of uniform intensity. The radiation field gradient across a detector when the source is too close is shown in Figure If the detector in figure was 3 inches in length, the distance from the source to the near end of the detector would be 0.5 inches (based on the exposure rates shown). Notice that the radiation field at the end of the detector closest to the source is 50 times greater than the exposure rate at the far end (100/2 = 50). If the distance from the source to the detector were changed to one inch, the radiation field near the end of the detector would be 16 times greater than the exposure rate at the far end. At some point, usually taken as 3 times the size of the largest dimension of the detector, the field is sufficiently uniform to make an accurate measurement. In this example, the minimum distance needed to obtain an accurate reading would be 3 x 3 = 9 inches. Page 14 of 15

15 Small leaks in the shielding around analytical x-ray equipment may produce beams of primary or secondary radiation that are very much smaller than the size of the typical ion chamber. Even if the beam has uniform intensity, the entire window of the ion chamber must be in the beam for the meter to produce an accurate exposure rate measurement. If only part of the window is in the beam, the meter will produce a reading that is lower than the actual exposure rate. This occurs because all of the air in the sensitive volume is not being ionized. Corrections for this problem can be made if the total area of the chamber window and the area of the chamber window that is in the beam are known. For example, the area of the window of a Victoreen 471 ion chamber is about 62 cm 2. As shown in Figure 14.7, suppose a beam comes from a point source 2 cm behind the shielding through a 1 mm hole and impinges on the chamber 10 cm from the shielding. The area of the beam that strikes the chamber is 0.28 cm 2. The correction factor that should be applied to the reading on the meter is given by dividing 62 cm 2 by 0.28 cm 2 or 221. Multiply the meter reading by 221 to obtain the correct exposure rate. While use of this method will provide the correct exposure rate, measuring the area of the beam that strikes the window is usually not easy, practical, or safe. Referring to Figure 14.8, one way to avoid the uncertainty in the estimation of the beam area would be to move the chamber further away until the beam covers the entire window. Using the exposure rate at the farther location, X_ 2, a good estimate of the exposure rate at the closer location, X_ 1, can be obtained by applying the inverse square law given in Equation X 1 = X 2 [d 2 /d 1 ] 2 Equation 14.3 Where: X_ 1 = Exposure rate at d 1 X_ 2 = Exposure rate at d 2 d 1 = Distance 1 d 2 = Distance 2 When using an ion chamber to measure radiation fields from analytical x-ray equipment, remember to remove the equilibrium cap from the thin window and point the window toward the source being measured. Exert caution when the cap is off because the window is easily punctured. Page 15 of 15