What is Internal Dosimetry – Definition

Internal dosimetry assessment relies on a variety of monitoring, bio-assay or radiation imaging techniques. Internal Dosimetry – Internal Exposure
ionizing radiation - hazard symbol
Ionizing radiation – hazard symbol

Radiation dosimetry is the measurement, calculation and assessment of the absorbed doses and assigning those doses to individuals. It is the science and practice that attempts to quantitatively relate specific measures made in a radiation field to chemical and/or biological changes that the radiation would produce in a target.

Internal Dosimetry

If the source of radiation is inside our body, we say, it is internal exposure. The intake of radioactive material can occur through various pathways such as ingestion of radioactive contamination in food or liquids. Protection from internal exposure is more complicated. Most radionuclides will give you much more radiation dose if they can somehow enter your body, than they would if they remained outside. Internal dosimetry assessment relies on a variety of monitoring, bio-assay or radiation imaging techniques.

See also: Internal Dose Uptake

Medical Dosimetry

Medical dosimetry is the calculation of absorbed dose and optimization of dose delivery in medical examinations and treatments. In general, radiation exposures from medical diagnostic examinations are low (especially in diagnostic uses). Doses may be also high (only for therapeutic uses), but in each case, they must be always justified by the benefits of accurate diagnosis of possible disease conditions or by benefits of accurate treatment. These doses include contributions from medical and dental diagnostic radiology (diagnostic X-rays), clinical nuclear medicine and radiation therapy. Medical dosimetry is often performed by a professional health physicist with specialized training in that field. In order to plan the delivery of radiation therapy, the radiation produced by the sources is usually characterized with percentage depth dose curves and dose profiles measured by a medical physicist.

The medical use of ionizing radiation remains a rapidly changing field. In any case, usefulness of ionizing radiation must be balanced with its hazards. Nowadays a compromise was found and most of uses of radiation are optimized. Today it is almost unbelievable that x-rays was, at one time, used to find the right pair of shoes (i.e. shoe-fitting fluoroscopy). Measurements made in recent years indicate that the doses to the feet were in the range 0.07 – 0.14 Gy for a 20 second exposure. This practice was halted when the risks of ionizing radiation were better understood.

See also: Medical Exposures

Whole-Body Counting

A whole-body counter is an instrument that measures the amounts of gamma-emitting radionuclides in the body (i.e. it is a gamma spectrometer). In nuclear facilities, these counters are used for measurement of radioactivity within the human body, that means, for internal contamination measurements. This must not be confused with a “whole body monitor” which used for personnel exit monitoring, which is the term used in radiation protection for checking for external contamination of a whole body of a person leaving a radioactive contamination controlled area. Whole-body counters are very sensitive devices and therefore they are often surrounded by large quantities of lead shielding to reduce the background radiation. A whole-body counter consists, for example, of a stand-up booth with two large-area NaI scintillation detectors. The upper detector monitors lungs, the lower detector monitors the gastrointestinal tract.

It must be noted, all people also have some radioactive isotopes inside their bodies from birth. These isotopes are especially potassium-40, carbon-14 and also the isotopes of uranium and thorium. The average annual radiation dose to a person from internal radioactive materials other than radon is about 0.3 mSv/year of which:

  • 2 mSv/year comes from potassium-40,
  • 12 mSv/year comes from the uranium and thorium series,
  • 12 μSv/year comes from carbon-40.

The variation in radiation dose from one person to another is not as great, but it is detected also by a whole-body counter.

Gamma Spectroscopy

As was written, the study and analysis of gamma ray spectra for scientific and technical use is called gamma spectroscopy, and gamma ray spectrometers are the instruments which observe and collect such data. A gamma ray spectrometer (GRS) is a sophisticated device for measuring the energy distribution of gamma radiation. For the measurement of gamma rays above several hundred keV, there are two detector categories of major importance, inorganic scintillators as NaI(Tl) and semiconductor detectors. In the previous articles, we described the gamma spectroscopy using scintillation detector, which consists of a suitable scintillator crystal, a photomultiplier tube, and a circuit for measuring the height of the pulses produced by the photomultiplier. The advantages of a scintillation counter are its efficiency (large size and high density) and the high precision and counting rates that are possible. Due to the high atomic number of iodine, a large number of all interactions will result in complete absorption of gamma-ray energy, so the photo fraction will be high.

HPGe Detector - Germanium
HPGe detector with LN2 cryostat Source: canberra.com

But if a perfect energy resolution is required, we have to use germanium-based detector, such as the HPGe detector. Germanium-based semiconductor detectors are most commonly used where a very good energy resolution is required, especially for gamma spectroscopy, as well as x-ray spectroscopy. In gamma spectroscopy, germanium is preferred due to its atomic number being much higher than silicon and which increases the probability of gamma ray interaction. Moreover, germanium has lower average energy necessary to create an electron-hole pair, which is 3.6 eV for silicon and 2.9 eV for germanium. This also provides the latter a better resolution in energy. The FWHM (full width at half maximum) for germanium detectors is a function of energy. For a 1.3 MeV photon, the FWHM is 2.1 keV, which is very low.

References:

Radiation Protection:

  1. Knoll, Glenn F., Radiation Detection and Measurement 4th Edition, Wiley, 8/2010. ISBN-13: 978-0470131480.
  2. Stabin, Michael G., Radiation Protection and Dosimetry: An Introduction to Health Physics, Springer, 10/2010. ISBN-13: 978-1441923912.
  3. Martin, James E., Physics for Radiation Protection 3rd Edition, Wiley-VCH, 4/2013. ISBN-13: 978-3527411764.
  4. U.S.NRC, NUCLEAR REACTOR CONCEPTS
  5. U.S. Department of Energy, Instrumantation and Control. DOE Fundamentals Handbook, Volume 2 of 2. June 1992.

Nuclear and Reactor Physics:

  1. J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
  2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
  3. W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
  4. Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
  5. W.S.C. Williams. Nuclear and Particle Physics. Clarendon Press; 1 edition, 1991, ISBN: 978-0198520467
  6. G.R.Keepin. Physics of Nuclear Kinetics. Addison-Wesley Pub. Co; 1st edition, 1965
  7. Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
  8. U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
  9. Paul Reuss, Neutron Physics. EDP Sciences, 2008. ISBN: 978-2759800414.

See also:

Radiation Dosimetry

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