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A helium nucleus produced from the radioactive decay of heavy metals and some nuclear reactions
Alpha particle
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A mechanism in which data is represented by continuously variable physical quantities
Analog
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An ordinary electron or positron ejected from the nucleus of a beta-unstable radioactive atom
Beta particle
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An elastic collision between an electron and a photon. In this case, the photon has more energy than is required to eject the electron from orbit, or it cannot give up all of its energy in a collision with a free electron
Compton scattering
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Rate circuits are important in the source and intermediate ranges
Decades per minute (DPM)
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Discrimination in radiation detection circuits refers to the process of distinguishing between different types of radiation on the basis of pulse height.
Discriminator
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The most likely interaction between fast neutrons and low atomic mass number absorbers. The interaction is sometimes referred to as the “billiard ball effect.”
Elastic scattering
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Process for uranium (U-235 or U-238) is a nuclear reaction whereby a neutron is absorbed by the uranium nucleus to form the intermediate (compound) uranium nucleus (U-236 or U-239).
Fission
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A photon of electromagnetic radiation with a very short wavelength and high energy
Gamma ray
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A radiochemistry measurement method that determines the energy and count rate of gamma rays emitted by radioactive substances
Gamma Spectroscopy
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Gas filled radiation detectors used to detect incident radiation. Detectors, used for the most part, to measure alpha and beta particles, neutrons, and gamma rays
Gas Filled Detector
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A radiation detector that operates in Region V, or G-M region of the gas filled detector characteristic curve
Geiger-Müller detector
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The process of removing one or more electrons from a neutral atom
Ionization
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Electrical devices that detect radiation when voltage is adjusted so that conditions correspond to the ionization region of the gas filled detector characteristic curve.
Ionization chamber
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Causes some of the neutron’s kinetic energy to be transferred to the target nucleus in the form of kinetic energy and some internal energy
Inelastic scattering
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The exponent that indicates the power to which a number is raised to produce a given number (i.e., the logarithm of 100 to the base 10 is 2).
Logarithm
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Occurs when a low energy gamma strikes an orbital electron. The total energy of the gamma is expended in ejecting the electron from its orbit
Photoelectric effect
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A radiation detector that operates in the proportional region of a gas filled a detector characteristic curve
Proportional counter
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Takes place when a neutron is absorbed to produce an excited nucleus. The excited nucleus regains stability by emitting a gamma ray
Radiative capture
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The amount of time, normally in seconds, required for neutron flux (power) to change by a factor of e, or 2.718
Reactor period
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Refers to a measurement or quantity that is capable of being represented on a scale (i.e., neutron flux on source range, intermediate range, and power range meters).
Scalar
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A solid-state radiation detector that uses a scintillation crystal (phosphor) to detect radiation and produce light pulses
Scintillation detector
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The ratio of the electrical output signal to the electrical noise generated in a cable run or the instrumentation
Signal-to-noise ratio
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The number of ion pairs produced per centimeter of travel through matter
Specific ionization
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The energy lost per unit path length. Also called linear energy transfer (LET).
Stopping power
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GAS FILLED DETECTOR CHARACTERISTIC CURVE
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Concerning interaction in radiation detectors, DESCRIBE how the following types of radiation interact with matter
Alpha (α)
Beta (β)
Gamma (γ)
Neutron (n)
Alpha - Large They have the least penetrating radiation. the specific ionization of an alpha particle is very high - Beta - The specific ionization of a beta particle is low. Size of an Electron
- Gamma - very high penetrating power.No Mass penetrates everything
- Neutron - Because of its relatively large size and lack of charge, the neutron is fairly difficult to stop, and has a relatively high penetrating power
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DESCRIBE the principles of operation of a gas-filled detector to include
How the electric field affects ion pairs
How gas amplification occurs
 - Gases are used in radiation detectors since their ionized particles can travel more freely than those of a liquid or a solid. Typical gases used in detectors are argon and helium, although boron-trifluoride is utilized when the detector is to be used to measure neutrons.
- The voltage applied to the anode and cathode determines the electric field and its strength. As detector voltage is increased, the electric field has more influence upon electrons produced. Sufficient voltage causes a cascade effect that releases more electrons from the cathode. The ion pairs initially formed by the incident radiation attain a great enough velocity to cause secondary ionization of other atoms or molecules in the gas. The resultant electrons cause further ionizations. This multiplication of electrons is termed gas amplification
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DESCRIBE the operation of an ionization chamber to include
Types of radiation detected and measured
Voltage variations
Gamma sensitivity reduction
Ionization chambers have two distinct disadvantages when compared to proportional counters: they are less sensitive, and they have a slower response time
Gamma rays have very little trouble in penetrating the metal walls of the chamber. The metal wall, however, stops alpha particles and beta particles
However, a window of almost any thickness will prevent an alpha particle from entering the chamber - If the inner surface of the ionization chamber is coated with a thin coat of boron, the following reaction can take place
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DESCRIBE the operation of a proportional counter
As voltage is increased into the proportional region, the primary ions acquire enough energy to cause secondary ionizations (gas amplification) The proportional counter can be used to detect alpha, beta, gamma, or neutron radiation in mixed radiation fields
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DESCRIBE the operation of a Geiger-Müller (G-M) detector
Discrimination is not possible, since the pulse height is independent of the type of radiation - The number of electrons produced is independent of applied voltage and the number of electrons collected is independent of the number of electrons produced by the initial radiation
- The G-M counter produces many more electrons than does a proportional counter; therefore, it is a much more sensitive device. It is often used in the detection of low-level gamma rays and beta particles for this reason
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DESCRIBE the operation of a scintillation counter
- Scintillation counters are constructed by coupling a suitable scintillation phosphor to a light-sensitive photomultiplier tube. Radition is used on a crystal to excite an electron which gives up its energy to a photon that emits light. There are three classes of solid-state scintillation phosphors:
- •organic crystals
- •inorganic crystals
- •plastic phosphors
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DESCRIBE the operation of a gamma spectrometer
- A gamma spectrometer uses a scintillation counter, normally NaI.
- A multichannel analyzer separates the pulses based on pulse height
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Fission chambers use neutron-induced fission to detect neutrons. The chamber is usually similar in construction to that of an ionization chamber, except that the coating material is highly enriched U-235. The neutrons interact with the U-235, causing fission. One advantage of using U-235 coating rather than boron is that the fission fragment has a much higher energy level than the alpha particle from a boron reaction.
Wide Range Fission Chamber
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As voltage increases to V0, the pulse height increases until it reaches a saturation value. At voltages less than V0, ions move slowly toward the electrodes, and the ions tend to recombine to form neutral atoms or molecules. Gas ionization instruments are, therefore, not operated in this region of response. The response of the detector is determined by the applied voltage.
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There is no appreciable increase in the pulse height(strength). The field strength is more than adequate to ensure collection of all ions produced; however, it is insufficient to cause any increase in ion pairs formed due to gas amplification. The response of the detector is constant in respect to voltage. The number of ions collected stays essentially the same. This region provides the most accurate indication of the NIS radiation field.
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The voltage is sufficient to impart a very high velocity to the electrons produced through ionization of the gas by charged (radiation) particles All the ions produced by incident radiation are collected. The velocity of these electrons is sufficient to cause ionization of other atoms or molecules in the gas. This multiplication of electrons is called gas amplification and is referred to as Townsend avalanche. Gas amplification is proportional to applied voltage. The source range channels use this region to obtain a larger pulse from each ionizing event.
Proportional Region (III)
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The strong field causes increased electron velocity, which releases more electrons from the cathode. These events cause the Townsend avalanche to spread along the anode. The positive ions remain near where they were originated and reduce the electric field to a point where further avalanches are impossible. For this reason, Region IV is called the limited proportional region, and it is not used for detector operation.
Limited Proportional Region (IV)
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The pulse height obtained is on the order of several volts. The field strength is so great that the discharge, once ignited, continues to spread until amplification cannot occur, due to a dense positive ion sheath surrounding the central wire (anode). . This is where the number of ion pairs level off and remain relatively independent of the applied voltage. This leveling off is called the Geiger plateau that extends over a region of 200 to 300 volts. In the G-M region, the gas amplification factor depends on the specific ionization of the radiation to be detected. This region is used extensively in radiation monitoring, but not used by the NIS
Geiger-Mueller Region (V)
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The applied voltage is so high that, once ionization takes place in the gas, there is a continuous discharge of electricity, so that the detector cannot be used for radiation detection.
Continuous Discharge Region (VI)
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LIST the type of detector used in each of the following nuclear instruments:
a.Source range
b.Intermediate range
c.Power range
The source range normally uses a proportional counter, while the intermediate and power ranges use ionization chambers. A compensated ion chamber is used for the intermediate range. The power range uses an uncompensated ion chamber.
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STATE the reason gamma compensation is NOT required in the power range
Uncompensated ion chambers are utilized in the power range because gamma compensation is unnecessary; the neutron-to-gamma flux ratio is high. Having a high neutron-to-gamma flux ratio means that the number of gammas is insignificant compared to the number of neutrons.
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DESCRIBE how a wide range fission chamber is used to detect neutrons
Fission chambers use neutron-induced fission to detect neutrons. The chamber is usually similar in construction to that of an ionization chamber, except that the coating material is highly enriched U-235. The neutrons interact with the U-235, causing fission. One advantage of using U-235 coating rather than boron is that the fission fragment has a much higher energy level than the alpha particle from a boron reaction. - Neutron-induced fission fragments produce many more ionizations in the chamber per interaction than do the neutron-induced alpha particles from boron. This allows the fission chambers to operate in higher gamma fields than an uncompensated ion chamber with boron lining. Fission chambers are often used as current indicating devices and pulse devices simultaneously. Because of the fission chamber’s dual use, it is often used in “wide range” channels in nuclear instrumentation systems. Fission chambers are also capable of operating over the source and intermediate ranges of neutron levels.
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An Alpha particle
consists of two neutrons and two protons
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In respect to a gas filled detector
sufficient voltage creates a cascade effect, releasing more electrons
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When radiation enters a proportional counter
the gas becomes ionized
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A gamma spectrometer uses a
scintillation counter
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What does the neutron interact with in a wide range fission chamber?
The U-235 coated detector wall
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