Session II.3.3 Part II Quantities and Measurements Module 3 Principles of Radiation Detection and Measurement Session 3 Scintillation Detectors IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources

Scintillation Detectors Upon completion of this section, the student will be able to: Explain the scintillation process Explain how scintillation detectors may be used to determine the radionuclide and associated activity Describe how scintillation detectors are used to detect alpha, beta, and gamma radiation

Scintillation Theory Scintillation is a means of detecting the presence of ionizing radiation Ionizing radiation interacts with a scintillator which produces a pulse of light This light interacts with a photocathode which results in the production of an electron The electron is multiplied in a photomultiplier tube that has a series of focused dynodes with increasing potential voltage which results in an electrical signal

Scintillation Detector (gamma) P H O T E L C R N S D Y I A G U M B F ( V . )

Scintillation Theory The number of counts is dependent on the activity that is present The energy of the electron, and consequently the associated current is proportional to the incident energy of the ionizing radiation By analyzing the energy and corresponding number of counts, the nuclide and activity may be determined

Scintillators Scintillators may be used to detect various types of radiation They are made of either inorganic or organic materials (plastic) Inorganic materials have a higher light output, but have a slow response Organic scintillators have a lower light output but have a faster response

Scintillators The scintillator is optically connected to a photomultiplier (PM) tube The height of the output pulse of the PM tube is analyzed by a mulitchannel analyzer Alpha particles may be detected by placing a thin layer of a scintillator, zinc sulfide (ZnS) Photons interactions are detected using a scintillator made of sodium iodide (NaI) or of plastic

Scintillation Detector (alpha) The alpha scintillator is typically zinc sulfide.

Alpha Scintillation Detector The photomultiplier tube is located in the handle.

Scintillation Detection (photon) This is a review of an earlier slide showing the photocathode and photomultiplier tube. This slide shows the use of the scintillation detector (typically this would be sodium-iodide, NaI) in a shielded counting system – this configuration is used in counting low levels of activity, such as environmental monitoring. In addition, it shows how lead X-rays end up creating a signal that is detected (although the graphic shows the lead X-ray heading for the PM tube). Lead x-ray interference may be reduced using a graded shield-the inside of the lead walls are shielded with cadmium (Cd), and next with a layer of copper (Cu). The cadmium absorbs the lead x-ray, while the copper absorbs any cadmium X-rays produced. The figure also shows how Compton backscattered photons are produced.

Typical Gamma Spectrum Shape Note the lead X-ray and the scatter peak that were depicted in the previous slide. If the gamma photon has an energy of 1.02 MeV or greater, then pair production is possible. In this case their would be a “double escape peak” at E - 1.02 MeV for cases when both annihilation photons are escape(2 x 0.511 MeV = 1.02 MeV). There may also be a “single escape peak” at h - 0.511 MeV. The Compton Edge is located at [(h)/(1 + 2 h/m0c2)] where h is the peak energy of the decay photon, and c is the speed of light, and m0 is the rest mass of an electron. The “double escape peak” is then at h - 2(m0c2), or h - 1.02 MeV. If h < 1.02 MeV, there will not be a double escape peak in the spectrum. For photons with h > 1.02, it is likely that there will be a peak at 0.511 MeV – this is an “annihilation peak” created when the positron created due to pair production is annihilated, but only one of the annihilation photons (0.511 MeV) is captured by the detector. In addition, it is less probable, but still likely that a peak will be present at 1.02 MeV, which is a signal from capture of both annihilation photons (2 X 0.511 MeV).

Gamma Spectrum E = h 0.511 1.02 (E - 0.511) (E - 1.02) Energy (MeV) For an isotope with a gamma photon of energy E (with E>1.02 MeV), the single and double escape peaks are represented (E-0.511 and E-1.02) as well as the annihilation peak (0.51 and 1.02). All energies are in MeV. Energy (MeV)

Spectral Analysis Scintillation detectors, when used with a multichannel analyzer (MCA) provide information on the energy of a photon that has interacted with the detector as well as the activity present The spectra can be analyzed to determine which isotopes are present

Single Channel Analyzer (SCA) Schematic Detector PM Tube HV Power Supply Preamp Amplifier Single Channel Analyzer Scaler

Multichannel Analyzer (MCA) Schematic Detector PM Tube HV Power Supply Preamp Amplifier Multi Channel Analyzer

Resolution The resolution of a detector is the ability to distinguish between peaks (for an MCA). The resolution is measured as the width of the peak, called the full width at half maximum (FWHM). Resolution, R, is expressed as a percentage (%): R = x 100% Typical values for a NaI detectors are 7% to 9% The resolution may be expressed in terms of channel number or keV since the ratio is dimensionless. EFWHM EMAX

Resolution 200 400 600 800 1000 1200 340 360 380 420 440 Channel Number Counts 390 405 375 FWHM = E = 405-375 E(MAX) = 390 R = (100) = 8% 405-375 390 For a semiconductor detector (to be discussed in another chapter), the FWHM is only a couple of keV (or channel numbers), i.e., the peak is much narrower. Consequently, the resolution value is much smaller. This means that the peaks look like “needles” on the spectrum, and it is much easier for the analytical software to identify the peaks. Also, this means that it is easier to distinguish between peaks that are separated by only a couple of keV. If the samples to be counted will have only one nuclide, or nuclides with energies that are quite different, then a NaI scintillation counter is preferable because the initial cost is less expensive and they don’t require liquid nitrogen to cool the detector (which is required by germanium semiconductor detectors).

Examples of Radionuclides Analyzed by Spectroscopy Peak Energy (MeV) Source Sodium-24 2.75, 1.37 Activation Potassium-40 1.46 Natural Cobalt-58 0.81 Cobalt-60 1.17, 1.33 Iron-59 1.10, 1.29 Iodine-131 0.365 Fission Cesium-137 0.662 Zinc-65 1.12 Peak energies are approximate

Spectrum Analysis Confirmation of the presence of a nuclide requires identification of a peak for that nuclide Selecting peaks for this determination requires: that the peaks be unique for that isotope that peaks be adequately separated so they may be identified that there is sufficient abundance for that peak The 0.511 MeV peak is not desirable for identifying isotopes since it is an annihilation peak caused by pair production The resolution is smaller for a germanium (Ge) semiconductor detector, which enables peaks to be identified that are much closer together than for a typical NaI scintillation detector.

Sample Scintillation Instruments The following slides provide a sample of scintillation instruments commercially available in the United States More detailed information concerning these and other instruments may be obtained by visiting the websites of some of the major manufacturers such as Ludlum, Eberline and Bicron

Bicron Micro Rem Meter This lightweight, low level instrument uses a tissue equivalent scintillation detector to make rapid measurements of absorbed dose rate down to background levels. High sensitivity and true dose response sets them apart from other GM or NaI based survey meters and makes them ideal for confirming the boundaries of radiation zones wherever radiation fields occur. Low energy and extended detector options are available. 1 rem = 0.01 Sv Five linear ranges 0-20 rem/h to 0-200 mrem/h; 90% response time <15 sec Energy response 17 keV to 1.3 MeV with low density window option

Eberline Model PM7 “Portal Monitor” FEATURES: Microcomputer controlled Automatic background subtraction RS-232 serial communications port Six large plastic scintillator detectors SENSITIVITY: RDS for a 0.4 second walk through is approximately 100 nCi RDA for a 10 second stop and count is approximately 0.4 nCi Annunciator Lights for Contamination alarm, Ready, Recount, Out of Service Human Silhouette to indicate contamination area SIZE: 228.6 x 101.6 x 50.8 cm WEIGHT: 400 kg excluding lead shielding [800 kg with lead shielding ] POWER: 105 to 125 Va, 47 to 63 Hz, 1 A BATTERY BACKUP: 8 hours operation OPERATING TEMP: 0 to 50 C 1 nCi = 37 Bq

Ludlum Model 44-3 25mm X 1 mm NaI(Tl) Gamma Scintillator for low energy gamma (LEG) INDICATED USE: I125 and x-ray survey SCINTILLATOR: 1" (2.5 cm) diameter X 1 mm thick (NaI)Tl scintillator ENTRY WINDOW: 15 mg/cm2 WINDOW AREA: 5 cm2 active and open RECOMMENDED ENERGY RANGE: Approximately 10 - 60 keV BACKGROUND: Typically 40 cpm/microR/hr SENSITIVITY: Typically 675 cpm/microR/hr (I125) EFFICIENCY(4pi geometry): Typically 19% - I125 TUBE: 3.8cm diameter magnetically shielded photomultiplier OPERATING VOLTAGE: Typically 500 - 1200 volts CONSTRUCTION: Aluminum housing with beige polyurethane enamel paint TEMPERATURE RANGE: (-20oC) to (50oC) SIZE: (5.1 cm) diameter X (17.8 cm)L WEIGHT: 0.5kg

Eberline LEG-1 Application: Low Energy Gamma optimized for 125I Detector Type: 1" diameter by 0.04" thick NaI(Tl) Window: 75.4 mg/cm2 aluminum window Sensitive Area: 0.79 inch2 Operating Voltage: 1,000 V nominal Dead Time: 8 s nominal Background Sensitivity: ~ 60 cpm/mR/h (137Cs) Energy Response/Photon: ~ 95% 125I Energy Range: ~ 15 to 200 kev Operating Temp: -30o to +60o C Housing: Aluminum body Size: 4.2 x 20.1 cm Weight: 340 g 1 inch (1”) = 2.54 cm

3 Scintillator Models Specs 380A 380B 380AB Application Alpha Beta Detector ZnS Beta Scintillator Dual Phosphor Efficiency (4) 21% (239Pu) 22% (90Sr/Y) 9% (99Tc) 18% (239Pu) (see Beta) (Remaining specifications common to all) Voltage 600 V Area 100 cm2 Thickness 0.87 mg/cm2 Sensitivity 12,000 cpm/mR/hr Temperature -40 to +60 oC Housing Al Connector MHV Size 29.2 x 7 x 8.3 cm Weight 0.59 kg

Eberline FCM-4 “floor monitor” The FCM4 floor contamination monitoring system performs continuous surveying of flat terrain areas in a one person operation. Using thin plastic scintillation detectors, this highly sensitive monitor exhibits low gamma background response and does not require the maintenance of gas proportional systems. The FCM4 features four 8" x 6" dual phosphor detectors for alpha/beta detection. A laptop displays activity and survey speed in digital and bar graph forms. Survey speed is integrated, so that count time is adjusted automatically in maximum sensitivity mode. In minimum count time mode, an alarm sounds when the monitor moves too fast for an acceptable count time. In stop-and-count mode, the display indicates when the FCM4 has traveled one detector length, i.e., positioned for the next count. 1” = 2.54 cm

“floor monitor” (cont) Eberline FCM-4 “floor monitor” (cont) Detectors: 4 dual-phosphor scintillators, ZnS and plastic Area: 8" L x 24" W total area (1240 cm2) Thickness: 0.75 mg/cm2 window (screen 83% open) Sensitivity: 500 dpm alpha & 2,000 dpm beta 137Cs, depending on speed Interface: Laptop computer mounted - semi-automatic calibration, independent detector control, data logging. Count Modes: Maximum sensitivity, minimum count time, stop-n-count, background Detector Height: Adjustable to 5" in 10 log steps Power: Rechargeable batteries, approximately, 8 hours operation Dimensions: 35" L x 27" W (excluding handle) Weight: 68 kg

Ludlum Model 3 Survey Meter COMPATIBLE DETECTORS: GM, scintillation METER DIAL: 0 - 2 mR/hr, or 0 - 5k cpm, BAT TEST MULTIPLIERS: X0.1, X1, X10, X100 LINEARITY: Reading within 10% of true value HIGH VOLTAGE: Adjustable from 200 - 1500 volts RESPONSE: fast (4 sec) or slow (22 sec) from 10 to 90% of final reading BATTERY LIFE: 2000 hours with alkaline batteries TEMPERATURE RANGE: -20 to 50oC SIZE: 16.5 cm H X 8.9 cm W X 21.6 cm L WEIGHT: 1.6 kg including batteries 14C 1 mR/hr = 2.6 x 10-7 C/kg/hr MULTIPLIERS: X0.1, X1, X10, X100, X1000 INTERNAL DETECTOR: Energy compensated GM (used with X1000 scale only) ENERGY RESPONSE: 15% 0.06-3 MeV (internal detector) HIGH VOLTAGE: 900 volts BATTERY LIFE: 600 hours with alkaline batteries

Where to Get More Information Cember, H., Introduction to Health Physics, 3rd Edition, McGraw-Hill, New York (2000) Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Table of Isotopes (8th Edition, 1999 update), Wiley, New York (1999) International Atomic Energy Agency, The Safe Use of Radiation Sources, Training Course Series No. 6, IAEA, Vienna (1995)