f. Region of Limited Proportionality (1) Region IV on the gas amplification curve. (2) In this region the voltage is increased above the proportional region and the output is no longer proportional to the input. Objective B.4 (3) The strong field causes increased electron velocity, which results in excited states of higher energies capable of releasing more ion pairs. The positive ions remain near where they were originated and reduce the electric field to a point where further avalanche is impossible. (4) The small individual avalanches which occur start to interfere with each other. (5) is no direct proportionality between the incident radiation and the response. (6) This region cannot be used for radiation detection. g. Geiger-Mueller Region (1) The Geiger-Mueller Region is Region V on the gas amplification curve. Objective B.4 (2) The initial energy deposited by the radiation causes an avalanche, just as with the proportional counter; however, when the avalanche reaches the collecting anode, the energy density is so high that light photons are emitted from the electrode. Objective B.5.c (3) These, in turn, interact with the fill gas or the tube walls to produce photoelectrons. The photoelectrons start another avalanche at some other location on the electrode. This process repeats until the anode is completed enveloped by ions. (4) The voltage has been increased to a point where a single ion pair is enough to cause complete discharge. (5) In the GM region, any radiation event with sufficient energy to create the first ion pair can cause a large pulse. This explains the high sensitivity of detectors. Objective B.5.c
(3) This amplification continues through 10 to 14 stages, the last dynode is reached. (4) At the anode, a current pulse is formed and sent to the circuits. A multiplication factor of over 1 million is possible. 7. Other components of a scintillation system are: a. Linear amplifier – provides additional amplification for the pulse and shapes the pulse. b. Pulse height analyzer – correlates pulse height to radiation energy. c. Readout device – accepts pulses whose heights fall within a given range from the upper and lower level discriminators. 8. Scintillation Outputs a. Efficiency is nearly 100% for alpha or beta that enters the detector. Efficiency is much less for gamma. b. The advantage to using solid scintillation crystals for gamma counting is that the output pulse is directly proportional to the energy of the incident gamma. This makes energy differentiation and nuclide identification possible. 9. In addition to sodium iodide, zinc sulfide and organic scintillators are used. a. Sodium Iodide (Thallium Activated), NaI(Th) – used for gamma counting because of its density. b. Silver Activated Zinc Sulfide, ZnS(Ag) – has a powdered coating on a transparent material, such as mylar, and is used for alpha counting. c. Organic scintillators, both liquid and solid, are used for beta counting. 10. The advantages of scintillation detectors include: a. high sensitivity. b. high efficiency for gamma detection. Objective B.9 c. capacity to handle high counting rates. d. can detect different types and energies. e. can measure the energy spectrum in emitters. Objective B.9 11. The disadvantages of scintillation detectors include: a. detector crystal can be ruined by moisture. b. expensive and fragile. c. poor low energy gamma response. d. can be affected by temperature. e. must have a highly regulated power supply.
e. Humidity and moisture can cause erroneous readings in some detectors. f. RF interference from two-way radios, microwaves, etc. can affect detectors. g. High radiation fields can saturation in some detectors. Attachment 5 h. Some instruments are energy dependent and can give false readings if high or low energy radiations are present. i. Light can influence some detectors. A tiny hole in the window can result in erroneous readings. j. Contamination on the surface of the probe or in some cases, Xenon gas, can give false readings. 3. Returning the Instrument a. Survey the instrument for contamination. b. Use the HIS-20 system to return the instrument. c. Report any problems with the instrument supervision. L. Radiation Survey Instruments 1. The most commonly used dose rate instrument at TVA and the preferred instrument for setting personnel dose rates is the ionization chamber. 2. GM instruments, are sometimes preferred for special applications, such as the teletector for radioactive material shipments. 3. The Microanalyst, a scintillation detector, is required for release surveys of bulk materials, such as clean trash. 4. Portable area radiation monitors usually have GM detectors. 5. Neutrons must be detected with special neutron detection equipment, such as the Ludlum 12.4. Objective B.18 Explain tissue equivalence. State reasons: quick response, sensitive, reach, and audible. M. Contamination Survey Instruments 1. Friskers can be used to count smears for certain applications. 2. Friskers can also be used to direct survey items for low levels of contamination. 3. contamination survey instruments may be portable alpha survey instruments such as the Surveyor M. 4. Laboratory counting instruments will be set up for Beta/Gamma and a separate instrument for alpha. Objective B.19 N. Instrument Efficiency 1. Instrument efficiency is a measure of how effective an instrument is at measuring all of the radiation present. Objective B.20
d. Resolution check – allows the user to more easily distinguish between peaks of similar energies. The known peak centroid and the expected peak energy must fall within a certain energy range. % Resolution = FWHM (in channel or kev) E (centroid channel) Performed monthly. e. Source check – shows if the peaks of interest have been reported, shows the % gain of the peaks of interest, and shows the activity of the peaks. The peaks of interest must be reported, the % gain must be within +/- 2%, and the activity of the peaks must be within +/- 3 standard deviations. Performed once per shift. Objective B.23 Gain shifts the peaks. f. Background check – shows if the normal background of the area has changed. Performed once per day. 8. Conditions that affect spectroscopy equipment response. a. High humidity. b. Abnormal background. c. Electronic d. Extreme temperature. e. Sample geometry. f. RF interference. Objective B.24 P. Instrument Models and Detector Types 1. Ionization instrument models used at TVA include: a. RO-2A. b. RO-7. c. RSO-5. d. RSO-50. e. RSO-50E. f. RSO-500. Objective B.25 Slide 5 Slide 6 Attachment 6 Slide 7 2. Instrument models used at TVA that incorporate proportional detectors include: a. PC-5. b. PCM-1B. c. PCM-2 d. PNR-4 Slide 8 Attachment 7 Slide 9 Slide 10 3. Geiger-Mueller survey instruments used at TVA include: a. BC-4. b. E530-N. c. Lud-14C. 11