Dear Dr Suresh : The quantities and units of radiation dose are inherently more complex than those used in toxicology or pharmacology, and additional complexity has resulted from several changes required by evolving concepts in radiation dosimetry. The first widely used physical quantity of radiation was “exposure,” related to the ability of x or gamma radiation to ionize air; its unit was the roentgen (R). Exposure was limited to photon radiation with energy less than 2.5 MeV. The quantity “absorbed dose” (D) was introduced because it was applicable to all forms of ionizing radiation and absorbing materials. Absorbed dose is energy deposited per unit mass, and its original unit was the rep (roentgen equivalent-physical); 1 rep equaled 93 ergs per g (0.0093 J per kg) of absorbing material. The rep was replaced with the rad (radiation absorbed dose); 1 rad equaled 100 ergs per g (0.01 J per kg). The “dose equivalent” (H) and its unit, the rem (roentgen equivalent-man), were introduced to account for the different biologic effects of the same absorbed dose from different types of radiation; H is the product of D, Q, and N at a point of interest in tissue, where D is absorbed dose, Q is the quality factor, and N is the product of any other modifying factors. The “effective dose equivalent” was introduced to include the different sensitivities of individual tissues and organs, which are important for internal dosimetry: its unit is the same as the unit of “dose equivalent.” Ionizing radiation is radiation that has enough energy to remove electrons from atoms or molecules (groups of atoms) when it passes through or collides with some material. The loss of an electron with its negative charge causes the atom (or molecule) to become positively charged. The loss (or gain) of an electron is called ionization and a charged atom (or molecule) is called an ion.

Absorbed radiation in materials deposits certain amounts of energy through a variety of interactions. A measure of the energy deposited by radiation in a material is called absorbed dose.Absorbed dose (D) is defined as the mean amount of energy dε¯ deposited per unit of mass (dm) as a result of the interaction of ionising radiation and matter.(D=dε¯dm

The unit of absorbed dose is the gray (Gy): 1 Gy = 1 J/kg. An old but still used unit of absorbed dose is rad: 1 rad = 0.01 Gy.

A problem with defining absorbed dose in Gy for radiation protection purposes is that the biological effect of an absorbed dose of 1 Gy in tissue is dependent on the type and energy of the incident radiation. To overcome this difficulty a quantity called equivalent dose is used. To take account of the nature of radiation, a weighting factor called the radiation weighting factor, WR, is used.

Equivalent dose in the tissue (HT) is given by the sum over all types and energy ranges of radiation (R):For more on this please take a look at :

https://www.sciencedirect.com/topics/engineering/absorbed-radiation

Please alos take a look at the following useful links:

https://www.researchgate.net/post/How_to_calculate_UV_energy_doses_in_grays_Gy_for_specific_exposure_times

https://www.researchgate.net/post/How-can-I-calculate-the-UV-dose-in-J-m2

https://www.ncbi.nlm.nih.gov/books/NBK230653/

I hope the input helps. Regards--

(1) Choose a material that absorbs the UV's energy. For example, water.

(2) Find a literature to determine the depth (penetration length) at which the UV is absorbed by the material. This often depends on the wavelength of the UV light.

(3) Calculate the mass of the material in a volume: unit area (cm2) x the depth above (cm) x density of the material (g/cm3) .

(4) Assuming that the material has absorbed all the UV energy, divide this energy by the mass above.

(5) Note that this answer is an averaged one in J/kg.

(6) Consider that UV of short wavelengths can cause ionization, however, in most cases it is only excitation, so we do not usually refer to UV as ionizing radiation. As a unit, it is indeed J/kg=Gy, but what is the point of using a unit related to ionizing radiation?

There is no conversion. The Gy is a volume dose. The UV dose is an area dose. UV dose penetrates no more than a few mm, energy dependent. There is no need to convert to volume as all energy is deposited near the surface.

The concept of the Gy is simple. The Gy is a measure of energy deposited to a volume of tissue. The assignment of Gy to a volume of tissue is complicated. Assigned dose requires knowledge of the energy of the radiation, the depth in tissue, area of the radiation beam, and direction of the beam.

You will encounter a similar problem comparing electron and gamma beams. Conversion are available based on assumptions of depth of penetration, but these are skin doses instead of depth doses. Comparing electron doses to gamma doses is complicated. Electron doses could be compared to UV as both are area doses. Nevertheless, the differences in depth of penetration poses problems.