HP SURVEY INSTRUMENT CALIBRATION AND SELECTION CHAPTER 3 General Properties of Radiation Detectors January 13 – 15, 2016 TECHNICAL MANAGEMENT SERVICES 1

Chapter 3 – General Properties of Radiation Detectors Gas-Filled Detectors Ionization Chambers Gas Proportional Detectors Geiger-Muller Detectors Scintillation Detectors Photomultiplier Tubes and Photodiodes Semiconductor Detectors Neutron Detectors 2

Detector - produces an observable signal when interacting with radiation. Sensor – monitors the detector and converts detector signal to an electrical signal. Sensor and detector often are same device. Electronics assembly – Supplies operating voltage, processes signal from sensor then sends to readout unit for display. 3

Readout unit – displays instrument reading in rate mode (cps, dpm, mrem/h, etc.. ) and/or scaler mode (counts, mrem, uSv, etc …). Can be an analog meter and/or digital display. Can be integrated with probe or a separate device for use with multiple probes. 4

Gas, liquid and solid media are utilized as radiation detectors Energy deposited in detector is converted directly or indirectly to an electrical signal Instrument calibration converts electrical signal to a quantity of interest e.g. dose rate 5

6 Basic Electrical Quantities Current–A measure of the movement or flow of electrons past a point in a circuit –1 amp = 6.24 x electrons/second =1 coulomb/second Voltage –A measure of electrical potential energy force that causes electron flow –Measured in volts

7 Basic Radiation Measurement System

ALMOST EVERY TYPE OF RADIATION DETECTOR RESPONDS TO ALMOST EVERY TYPE OF RADIATION THERE ARE NO BAD INSTRUMENTS ONLY MISUNDERSTOOD INSTRUMENTS 8

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Ion Chamber in Current Mode 10

Ion Chamber in Pulse Mode 11

Detector Medium / Radiation Type Detected Ionization Chamber / Alpha, Beta, Gamma, Neutron, Tritium LRAD / Alpha, Beta, Gamma, Neutron, Tritium Tritium in Air (Ion Chamber) / Alpha, Beta, Gamma, Neutron, Tritium 12

Eberline RO-3 Ion Chamber / Alpha, Beta, Gamma, Neutron 13

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15 Eberline RO- 20 Ion Chamber / Alpha, Beta, Gamma, Neutron

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RO-20 Beta Dose Response 19

Long Range Alpha Detector – LRAD 20 Ion Chamber / Alpha, Beta, Gamma, Neutron ??

21 The Eberline LRAD (Long Range Alpha Detector) measures the ion pairs created in air from ionizing radiation and then displays the result in terms of activity (dpm). For a 5 MeV alpha particle which is totally attenuated within a few centimeters of air, about 150,000 ion pairs are generated. The LRAD collects the ion pairs generated by ionizing radiation and uses a current-to-frequency converter to provide a numerical output which is converted to dpm.

22 LRAD Counting Chamber: 13” H x 22” W x 16” D Exterior Size: 23” H x 38” W x 20” D Weight: 180 lbs Power 120V, 60 Hz Battery requirements Electrometer: 5 C-cells, 1,200 hours Detector/Static Precipitator: 90 V lithium

Sartrex Tritium in Air Monitor 23

24 The Model 209 is capable of measuring down to 10μCi/m 3. Its single range allows readings of concentrations up to 20,000μCi/m 3. The detector is an 80 cm 3 ionization chamber. Gamma radiation cancellation is provided by an adjacent sealed 80 cm 3 ionization chamber.

25 QUESTIONS How many DPM of Tritium does the Model 209 detect ? What is the current in femto-amps at the stated detection level of the Model 209 ? What is the leakage current in femto-amps of the Model 209 ? How does this leakage current compare to the other ion chamber instruments we have discussed ?

26 Ion pair production -Created when ionizing radiation interacts with the detector gas. Ion pair collection -When a voltage potential is established across the two electrodes the ion pairs will be attracted to the respective electrode with the opposite charge. Analysis -The amount of current flow is representative of the energy and number of radiation events that caused ionization. The readout circuitry analyzes this current and provides an indication of the amount of radiation that has been detected.

27 Specific Ionization in Air at STP

Gas-Proportional (Sealed) / Alpha, Beta, Gamma, Neutron Gas-Proportional (gas flow) / Alpha, Beta, Gamma, Neutron, Tritium 28

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Ludlum Air Proportional Alpha Detector L

31 WINDOW AREA: 154 cm² (23.9 in²) active; 100 cm² (15.5 in²) open WINDOW: 0.4 mg/cm² aluminized mylar OPERATING VOLTAGE: altitude sensitive sea level: 2050 volts m (2000 ft): 2000 volts 1524 m (5000 ft): 1925 volts 2134 m (7000 ft): 1875 volts

32 COUNTER THRESHOLD SETTING: 1.5–2.0 mV WORKING ENVIRONMENT: splashproof shields and desiccant vented chamber for outdoor use SIZE: 10.2 x 8.9 x 23.6 cm (4 x 3.5 x 9.3 in.) (H x W x L) WEIGHT: 0.7 kg (1.5 lb)

33 HV Plateau using 345 Bq Pu-239 Source Select 1960 as operating voltage. 54 cps with a 345 Bq source = 15.7% 4-pi eff.

LND Model 431 Sealed Gas Proportional Detector 34

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GM (thin end window ~ 2.5 mg/cm 2 ) Alpha, Beta, Gamma, Neutron GM Detector (> 7 mg/cm 2 ) Beta, Gamma, Neutron 39

LND 7311 GM Detector 40

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Scintillator (ZnS) / Alpha, Beta ?, Gamma ?, Neutron ? Ludlum Alpha Detector 44

45 Ludlum 43-5 Alpha Detector Alpha, Beta ?, Gamma ?, Neutron ?

Scintillator (ZnS) / Alpha ?, Beta, Gamma ? 46

Scintillator (ZnS) / Alpha, Beta, Gamma Berthold LB124-SCINT Alpha and Beta-Gamma 47

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49 CROSSTALK Discrimination Crosstalk is a phenomenon that occurs on proportional counting systems that employ electronic, pulse-height discrimination, thereby allowing the simultaneous analysis for alpha and beta-gamma activity. Discrimination is accomplished by establishing two thresholds, or windows, which can be set in accordance with the radiation energies of the isotopes of concern.

50 Recall that the pulses generated by alpha radiation will be much larger than those generated by beta or gamma. This makes the discrimination between alpha and beta-gamma possible. This same method of pulse height discrimination is also applicable to sandwich detectors such as ZnS on a plastic scintillator.

51 Numerous materials scintillate -liquids, solids, and gases. A material which scintillates is commonly called a phosphor or a fluor. The scintillations are commonly detected by a photomultiplier tube (PMT).

52 There are four classes of phosphors of interest in applications of scintillation: –Organic crystals –Organic liquids –Inorganic crystals –Inorganic powders

53 Organic crystal phosphors are normally aromatic hydrocarbons that contain benzene rings. The most common organic crystal is anthracene. Anthracene offers a high response to beta radiation and is commonly used in beta phosphors. Some investigation has been done in applying anthracene to a combination neutron/gamma detector.

54 Organic liquid phosphors, called fluors, are comprised of organic material suspended in an organic solvent. The organic liquid phosphor material is called the solute and is the scintillator. The solvent absorbs the radiation and transfers energy to the solute.

55 Inorganic crystals are comprised of inorganic salts, normally halides, which contain small quantities of impurities, called activators. The most commonly used inorganic crystal scintillator is sodium iodide, activated with thallium -NaI(Tl). NaI(Tl) crystals have a high density -3.7 g/cm3, which provides good gamma detection efficiency. NaI(Tl) has a high response to beta particles; however, the need to hermetically seal a NaI(Tl) crystal to prevent deterioration, limits the actual beta response.

56 Zinc Sulfide activated with Silver (ZnS(Ag)) is an inorganic powder which is commonly used as a phosphor in alpha scintillators. ZnS(Ag) scintillators have a high density, 4.1 g/cm 3, and a relatively high response to beta and alpha radiation. The scintillator response to beta and gamma is typically minimized by the use of ZnS(Ag) as a thin film which is within the alpha interaction range, but too thin for that of beta or gamma.

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Scintillator(Plastic) / Alpha, Beta, Gamma, Neutron 60

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Scintillator (NaI, CZT, etc.) / Gamma Scintillator (LSC) / Alpha, Beta, Gamma, Neutron, Tritium 62

PMTs and Photodiodes / Gamma The purpose of the photomultiplier tube is to detect the scintillations and to provide an output signal proportional to the amount of scintillations. In doing this, photomultiplier tubes can provide amplifications of 1E6 and higher. Construction details vary from design to design, however, all photomultipliers have typical components which are the photocathode, dynode assembly, anode, and voltage divider. The photocathode is made of a material that converts the light photons to electrons. 63

64 The PMT itself is a gamma (photon) detector. For this reason ALL detectors that use a PMT to detect light from a source such as a scintillator have a response to external gamma (photon) radiation.

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66 A photodiode is a semiconductor device that converts light into current. The current is generated when photons are absorbed in the photodiode. The output current from a photodiode is typically much less than from a PMT. In some applications a photodiode can be used in place of a PMT by optimizing the match between the light output frequency of the scintillator and the light frequency efficiency of the photodiode.

Semiconductors / Alpha, Beta, Gamma, Neutron Semiconductor Detectors Semiconductor detectors are solid-state devices that operate essentially like ionization chambers. The charge carriers in semiconductors are not electrons and ions, as in the gas counters, but electrons and "holes." Semiconductor detectors are made of silicon, germanium, CdTe, Hgl, and other materials. 67

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70 ORTEC Silicon Detectors

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Neutron detectors (BF 3, 3 He 3, scintillators) / Gamma, Neutron neutron remmeters Gas proportional counters 3 He(n,p)T and 10 B(n,α) 7 Li based instruments Proton recoil counters Activated foil-based instruments Scintillator-based instruments 74

10 B Uses BF 3 gas usually enriched to ~95% 10 B Limited to gas pressures of about 2atm maximum Excellent gamma rejection properties Boron-coated counters also used with a counting gas 75

3 He Often used with high Z gas to promote energy deposition High pressure operation possible Limited availability Very good gamma discrimination Chemically inert (unlike BF 3 ) 76

NE-213 Liquid Scintillator Provides a neutron spectrum Often used with a 10 B detector to provide a thermal neutron response Good gamma discrimination down to about 2 MeV neutron energy Light weight package 77

Solid Plastic Scintillator Could provide a neutron spectrum Often used with a 10 B detector to provide a thermal neutron response Good gamma discrimination to below1.5 MeV neutron energy Light weight package 78

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80 Instruments based on thermal neutron capture

81 Energy response of bare counter (10B or 3He) is not sufficient to produce a true rem response Response is highest in region where dose per neutron is lowest and vice versa Solution is to modify the incident spectrum to improve the instrument response Adding a shell of PE gives a more tissue-like response 9” diameter sphere is typically used Cd metal or a B-loaded PE internal shell often used as well

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87 Pulsed field performance Accelerator fields are frequently pulsed resulting in high instantaneous neutron fluence and dose rates. Neutron rem meters using PE-moderators are somewhat immune due to the time required to thermalize fast neutrons (typically s) – effectively lengthening the pulse. But there are limitations on the effectiveness of the PE moderator and other means of neutron dose measurements are required E.g. ion chambers in current mode, Boron-lined chambers or chambers with TE walls and gas

88 The HPI 2080 (“Albatross”) is designed to operate in pulsed (and static) fields and uses the standard PE-moderator approach But uses two GM tubes, one surrounded by Ag foil the other with an equivalent mass thickness of tin. The Ag foil is activated through TNA Its GM tube counts subsequent beta decays The second GM tube compensates for the gamma background

89 Scintillator-based instruments – PRESCILA Proton Recoil Scintillator Los Alamos Developed at LANL and commercialized by Ludlum Utilizes a dual detector approach (like the FujiNSN3) for fast and thermal neutron detection. Weight = 6lbs

90 Fast detector is 5 EJ-410P scintillators Recoil protons generated generated in lucite excite ZnS(Ag) Light emission collected by lucite lightguide and detected by central PMT Thermal channel is a 6 LiF+ZnS(Ag) scintillator

91 Other scintillators Liquefied rare gases 6 Li-based glasses Cs 2 LiYCl 6 :Ce (CLYC)

92 Calibration of neutron rem meters Require NIST-traceable source or instrument cal. 252 Cf or 241 AmBe most often used Need to characterize neutron field (dose rate vs distance from source) Measurements and/or Monte Carlo calculations Instruments should be calibrated in a field which gives the most conservative calibration factor Function of instrument energy response and the workplace field

93 Points to consider in selecting a neutron rem meter Energy response Angular response Neutron sensitivity Gamma rejection Ergonomics Calibration source Reliability Pulsed field performance Ease of: use, maintenance/repair and calibration

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95 ANSI N323AB Test and Calibration, Portable Survey Instruments ANSI N323D Installed Radiation Protection Instrumentation ANSI N42.33 Portable Radiation Detection Instrumentation IEEE N42.35 Evaluation and Performance of Radiation Detection Portal Monitors IEEE N42.43 Performance Criteria for Mobile and Transportable Radiation Monitors

96 End of Chapter 3 Questions ? Comments ?