1. Definitions and units

Ionizing radiation, among which are X-rays, and its potential adverse biological effects on the human body are described quantitatively by established quantities and units.

Exposure

Exposure determines the quantity of photon radiation (it is only applied to gamma-radiation and X-rays) by their ability to ionize air molecules (it can only be measured for air - not just any material). It is the charge of all ions of the same sign produced in dry air by the secondary electrons and positrons generated by the radiation in a particular mass of air when their kinetic energy is lost completely. Exposure measured under conditions of secondary electronic equilibrium can be converted into air Kerma (see below). The SI unit of exposure is coulomb per kilogram (C/kg). The historical off-system unit is Roentgen (R). The units are in the ratio:

	1 R = 2,58*10-4 C/kg
	1 C/kg = 3,88*103 R

Air Kerma

Air Kerma (Kinetic Energy Released in Material or per unit MAss) is amount of the initial kinetic energy transferred (from photons) to all secondary charged particles in air, per unit mass. The SI unit of Kerma is Gray (1 Gy = 1 J/kg). The earlier off-system unit is rad ("radiation adsorbed dose"). The units are in the ratio:

	1 Gy = 100 rad
	1 rad = 0,01 Gy

Air Kerma is the energy equivalent of the Exposure under the condition that all radiation-generated secondary charged particles lose their energy for air ionization only. In fact some portion of initial transferred energy is re-radiated with Bremsstrahlung photons ("Braking radiation"). However this effect is very small and can be neglected for low and medium photon energies. Therefore, the ratio of Kerma to Exposure is nearly constant:

	1 Gy = 114 R
	1 R = 0,00877 Gy

Absorbed dose

Absorbed dose is the energy imparted to the medium as a result of irradiation, per unit mass. Absorbed dose is related to Kerma and has the same unit (Gray) but is not the same quantity. Kerma is the energy that was transferred to the medium and absorbed dose is the energy that actually remained in the medium. The difference between two quantities is due to the fact that some secondary charged particles can go out of or come into the medium volume under consideration thereby changing the energy balance. That is why Kerma decreases with depth in the absorbing material because the magnitude of the primary ionizing radiation is decreasing, where as, absorbed dose initially rises to a peak before falling away with depth. Ultimately it is the absorbed dose that determines the radiation effect on the human body.

Dose equivalent

Dose equivalent is a quantity that takes into account "radiation quality" which relates to the degree in which a type of ionizing radiation will produce biological damage. Not all radiation has the same biological effect, even for the same amount of absorbed dose. The Dose equivalent is obtained by multiplying the Absorbed dose by a Quality Factor. The resulting quantity can then be expressed numerically in Sieverts (Sv, 1 Sv = 1 J/kg) if Absorbed dose is measured in Gray. The old unit is Rem when Absorbed dose is measured in Rads. For gamma radiation and X-rays Dose equivalent is considered to be equal to the Adsorbed dose:

	1 Sv = 1 Gy
	1 Sv = 100 rem
	1 mSv = 1*10-3 Sv
	1 µSv = 1*10-6 Sv

Effective dose

Different tissues (or body parts) are not equally sensitive to a kind of radiation. That is, if the entire body were irradiated with uniform beam of a single type of radiation, some parts of the body would react more dramatically than others. The effective dose to an individual is calculated as a weighted average of the dose equivalent to different body tissues by using Tissue Weighting Factors designed to take into account different tissue contributions to overall effective biological damage to the body. Effective dose is used as a measure of the likelihood of stochastic effects of radiation exposure: carcinogenesis and hereditary effects. The SI unit of Effective dose is Sievert.
The effective dose is important to estimate the biological effect of irradiation but it cannot be measured directly with any instrument. A sophisticated thorough study using an anthropomorphic human body phantom was performed to estimate the typical effective doses received by the individuals from DRS SecureScan. The results of this study were described in the report (Department of Radiology of Long Island Jewish Medical Center (New York), report "Radiation Evaluation for the CONPASS Body Scanner", 06/21/2002). All data related to the DRS SecureScan effective doses are based on the above mentioned report.

2. Dose comparison from different sources of ionizing radiation

The typical effective dose obtained by an individual during one scan is less than 2 Sv that is of no danger for human health at all. Such a low level of irradiation allows up to 150 scans per year per individual that is only about 30% of the annual effective dose from natural and man-caused sources. DRS SecureScan typical doses are much lower than those from medical radiological examinations and are comparable with daily irradiation levels from natural sources. The dose caused by cosmic radiation during any air flight is much higher than that received from one scan with DRS SecureScan.
The comparative data summarized in the following tables give excellent evidence of extremely low level of irradiation for DRS SecureScan examinations.

Dose rise	Radiation source		Effective dose, µSv
		X-ray tomography of head	50,000 (max.)* 10,000(typical)
		Dental X-ray radiography	5,000 (max.)* 1,000 (typical)
		Chest X-ray radiography	400 (max.)* 100 (typical)
		Air flight Duration, h ** London - Los Angeles 10,5 61,6 Los Angeles - Tokyo 11,7 43,3 Paris - Washington 8,0 41,0 London - New York 7,30 37,0 Lisbon - New York 6,5 29,0 London - Rome 2,3 12,3 London - Frankfurt 1,35 7,2 Moscow -Minsk 1,05 5,5
		natural radiation background (24 h)	2,7-13*** on average - 8
		SecuryScan scanner study (1 scan)	<2
		radiation leakage dose measured at 0.1 m from any outer surface of SecuryScan (1 hour)	<1

* Maximum permissible doses for typical medical diagnostic studies were taken from the international standard "International basic safety standards for protection against ionizing radiation and for the safety of radiation sources", Safety Series №115-I, International Atomic Energy Agency, 1994.
** Typical doses for air flights were taken from the US Federal Aviation Administration (FAA) and National Research Center for Environment and Health (GSF, Germany) reports. Some missing data were interpolated by using dose per hour values for similar air flights from the above mentioned reports. Cosmic radiation dose depends upon departure and destination geographical locations, flight time, flying path and typical altitudes along path.
*** Natural radiation background varies with geographical location on the Earth. Natural background includes contributions from different sources as considered below.

Average annual doses of ionizing radiation from natural sources
received by a member of the population in the USA *

Source of radiation	Effective dose, (µSv/year)
Cosmic radiation (uniform whole-body exposure)	270
Radioactive material in the ground (uniform whole-body exposure)	280
Radioactive material in body tissues (tissue doses vary)	400
Inhaled radon (primarily to bronchial epithelium)	2000
	Total = 2950

*Source of information: FAA Radiobiology Research Team report.

3. DRS SecureScan detection ability versus effective dose comparison

DRS SecureScan is currently able to perform detection of suspected items in the following range of doses per scan per inspected person. The dose choice depends on the required detection ability.

4. Additional sources of information