6. Radiation Detection Gas Filled Detectors Air or Other Gas Incident Ionizing Radiation Electrical Current Measuring Device + - Cathode - Anode + + + + - - - + - Voltage Source
26. Use of high activity sealed sources to examine structural components such as beams or pipes
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61. N(t 0 ), A (t 0 ) are the initial number of radionuclides and initial activity, respectively. The half life t 1/2 of a radionuclide is the time by which the number of radionuclides has reduced to 50%. This shows a direct correlation between half life and decay constant for each radionuclide. The lifetime r of a nucleus is defined by: Quite often the expression “lifetime” can be found for radionuclides. This means that after a period corresponding to the “lifetime” of a radioactive nucleus the initial abundance has decreased to 36.8% of its initial value, of a nucleus can be found!
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64. Unit for exposure E is the Roentgen [R] which is defined by the ionization between EM-radiation and air. 1 Roentgen is the amount of EM-radiation which produces in 1 gram of air 2.58 10 -7 C at normal temperature (22°C) and pressure (760 Torr) conditions. Dosimetry Units Due to the interaction between radiation and material ionization occurs in the radiated material! (Energy transfer from the high energetic radiation photons or particles to atomic electrons.) The ionization can be used as measure for the amount of exposure which the material had to radiation. 1 R = 2.58 10 -4 C/kg
65. When interacting with matter EM-radiation shows particle like behavior. The 'particles' are called photons. The energy of the photon and the frequency (or wavelength ) of the EM-radiation are determined by the Planck constant h: h=6.62 -34 J s = 4.12 10 -21 MeV s The photon energy for X-rays and -rays is in the eV to MeV range.
66. X-rays originate either from characteristic deexcitation processes in the atoms (K , K transitions) (Characteristic X-rays). The photon energy corresponds to the difference in binding energy of the electrons in the excited levels to the K-level.
67. X-rays also originate from energy loss of high energy charged particles (e.g. electrons) due to interaction with the atomic nucleus ( bremsstrahlung )
68. The exposure rate ER (= ionization/time) can be related to the activity A of a source (in units mCi) via : F is the exposure constant in units [ (R cm 2 ) / (h mCi) ] , and d is the distance between source and material in units [cm]. The exposure constant is characteristical for the radiation source:
69. The absorbed dose D of radiation in any kind of material depends on the typical ionization energy of the particular material. The absorbed dose is defined in terms of the absorbed radiation energy per mass W 1P . It therefore clearly depends on the energy loss behavior of the various kinds of radiation. The unit for the absorbed dose is : 1 Gray = 1Gy = 1 J/kg = 10 4 erg/kg = 100 rad The average ionization energy for air is W 1P 34 eV/ion. With 1 eV = 1.6022 10 -19 J and the charge per ion is 1.6 10 -19 , this yields for the absorbed dose in air D for 1 R exposure of EM radiation: D = 1 R • 34 J/C = 2.58 10 -4 C/kg 34 J/C = 8.8 10 -3 J/kg = 8.8 10 -3 Gy = 0.88 rad
71. There is an empirical relation between the amount of ionization in air and the absorbed dose for a given photon energy and absorber (body tissue). The absorbed dose in rads per roentgen of exposure is known as the roentgen-to-rad conversion factor C C is approximately equal to one for soft body tissue in the energy range of diagnostic radiology. The increase for bone material is due to higher photoelectric absorption cross section for low energy photons.
72. Dose (rad) = Exposure (R) x R to Rad Conversion factor
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75. Exposure, exposure rate and absorbed dose are independent of the nature of radiation. Biological damage depends mainly on the energy loss of the radiation to the body material. These energy losses differ considerably for the various kinds of radiation. To assess the biological effects of the different kind of radiations better, as new empirical unit the dose equivalent H is introduced: DOSE EQUIVALENT with the quality factor Q which depends strongly on the ionization power of the various kinds of radiation per path length. In first approximation Q Z of radiation particles, Q( , X, ) 1. As higher Q as higher the damage the radiation does!
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77. EFFECTIVE DOSE The various body organs have different response to radiation. To determine the specific sensitivity to radiation exposure a tissue specific organ weighting factor w T has been established to assign a particular organ or tissue T a certain exposure risk. The given weighting factors in the table imply for example that an equivalent dose of 1 mSv to the lung entails the same probability of damaging effects as an equivalent dose to the liver of (0.12/0.05) 1 mSv = 2.4 mSv The sum of the products of the equivalent dose to the organ H T and the weighting factor w T for each organ irradiated is called the effective dose H : Like H T , H is expressed in units Sv or rem!.
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81. or Natural Decay Law The rate of the decay process is determined by the activity A (number of decay processes per second) of the radioactive sample. The activity is proportional to the number of radioactive nuclei (radionuclide) is the decay constant! Differential equation for N(t) can be solved
Bremsstrahlung (means braking radiation) Incident Beta (or e-) changes direction due to radial acceleration. In accordance with classical theory, beta loses energy by EM radiation Produce spectrum of photon NRG (Max is same NRG as incident beta) The probability of Brem production increases with the energy of the Beta and the atomic number of the absorber (larger nucleus is bigger target with bigger sphere of influence)
Manufacturers of X-ray for diagnostic regulated by FDA Typically have 3 basic controls - kVp, time, & mA (discuss each) Amount of filtration required depends on kVp 30 kVp = 0.3 mm Al equiv 70 kVp = 1.5 mm 120 kVp = 3.2 mm
Almost a century ago in 1895, Roentgen discovered the first example of ionizing radiation, x-rays. The key to Roentgens discovery was a device called a Crooke’s tube, which was a glass envelope under high vacuum, with a wire element at one end forming the cathode, and a heavy copper target at the other end forming the anode. When a high voltage was applied to the electrodes, electrons formed at the cathode would be pulled towards the anode and strike the copper with very high energy. Roentgen discovered that very penetrating radiations were produced from the anode, which he called x-rays. X-ray production whenever electrons of high energy strike a heavy metal target, like tungsten or copper. When electrons hit this material, some of the electrons will approach the nucleus of the metal atoms where they are deflected because of there opposite charges (electrons are negative and the nucleus is positive, so the electrons are attracted to the nucleus). This deflection causes the energy of the electron to decrease, and this decrease in energy then results in forming an x-ray. Medical x-ray machines in hospitals use the same principle as the Crooke’s Tube to produce x-rays. The most common x-ray machines use tungsten as there cathode, and have very precise electronics so the amount and energy of the x-ray produced is optimum for making images of bones and tissues in the body.
Since we cannot see, smell or taste radiation, we are dependent on instruments to indicate the presence of ionizing radiation. The most common type of instrument is a gas filled radiation detector. This instrument works on the principle that as radiation passes through air or a specific gas, ionization of the molecules in the air occur. When a high voltage is placed between two areas of the gas filled space, the positive ions will be attracted to the negative side of the detector (the cathode) and the free electrons will travel to the positive side (the anode). These charges are collected by the anode and cathode which then form a very small current in the wires going to the detector. By placing a very sensitive current measuring device between the wires from the cathode and anode, the small current measured and displayed as a signal. The more radiation which enters the chamber, the more current displayed by the instrument. Many types of gas-filled detectors exist, but the two most common are the ion chamber used for measuring large amounts of radiation and the Geiger-Muller or GM detector used to measure very small amounts of radiation. Demonstration of Geiger-Muller Detector.
The second most common type of radiation detecting instrument is the scintillation detector. The basic principle behind this instrument is the use of a special material which glows or “scintillates” when radiation interacts with it. The most common type of material is a type of salt called sodium-iodide. The light produced from the scintillation process is reflected through a clear window where it interacts with device called a photomultiplier tube. The first part of the photomultiplier tube is made of another special material called a photocathode. The photocathode has the unique characteristic of producing electrons when light strikes its surface. These electrons are then pulled towards a series of plates called dynodes through the application of a positive high voltage. When electrons from the photocathode hit the first dynode, several electrons are produced for each initial electron hitting its surface. This “bunch” of electrons is then pulled towards the next dynode, where more electron “multiplication” occurs. The sequence continues until the last dynode is reached, where the electron pulse is now millions of times larger then it was at the beginning of the tube. At this point the electrons are collected by an anode at the end of the tube forming an electronic pulse. The pulse is then detected and displayed by a special instrument. Scintillation detectors are very sensitive radiation instruments and are used for special environmental surveys and as laboratory instruments. Demonstration of NaI detector
The incident fast moving electron is interacted upon by the nuclear fields from the atom or molecule. The electron is then slowed down and deflected. As it slows down it gives of bremsstrahlung radiation.
Ch 15 RHHB: Ionizing Radiation Bioeffects and Risks
Perform Example on Board: If the exposure rate at 2 feet = 1000 mR/hr What is the exposure rate at 10 feet How much distance from the source in # 1 should a barrier be placed so the exposure rate is 2 mR/hr?
multiply gamma constant in (mSv/hr)/MBq by 3.7 to get (mrem/hr)/uCi RHHB p.3-11 has 6CEN rule of thumb RHHB p. 6-7 to 6-14 has gamma constants
RHHB p. 6-15 RHHB p. 6-61
RHHB p. 52, Table 3.1(3-2 - 3-4) Beta rules of thumb Range in air 12 ft per MeV Ave beta NRG is 1/3 Max NRG at 1 cm Dose rate (rad/hr) = 300*mCi at 1 foot Dose rate (rad/hr) = 300*Ci
RHHB p. 409:(11-14 through 11-23) Good Work Practices for surveys, instrument use, hoods, use of sealed sources, use of unsealed sources, use of radioluminous materials
Ch 15 RHHB: Ionizing Radiation Bioeffects and Risks
Response check - check source, did it pass Selecting proper scale - ALWAYS work from lowest to highest scale. Response time - Slow versus Fast in high radiation area
NARM not in agreement but most AS regulate
Units roentgen (R) or Coulomb/kg (C/kg) 1 C/Kg = 3876 R Applies only to X or Gamma field in air Essentially a measure of the amount of ionization in air by X or gamma
Unit is rad (radiation absorbed dose) or gray 1 gray = 100 rad Applies to all types of radiation For X or gamma in human tissue 1 rad = 1 R
Unit is rem or sievert 1 Sv = 100 rem Applies only to living humans Puts all radiation on an equivalent risk basis An administrative concept not intended for acute doses
Units roentgen (R) or Coulomb/kg (C/kg) 1 C/Kg = 3876 R Applies only to X or Gamma field in air Essentially a measure of the amount of ionization in air by X or gamma
Unit is rad (radiation absorbed dose) or gray 1 gray = 100 rad Applies to all types of radiation For X or gamma in human tissue 1 rad = 1 R
Unit is rem or sievert 1 Sv = 100 rem Applies only to living humans Puts all radiation on an equivalent risk basis An administrative concept not intended for acute doses
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