Are millirems the same as milliroentgens ? When I search google I found many helpful conversion charts and they all say the following. 1 Rad = 1 Rem but .86956 millirem is 1 milliroentgen. How is this possible? Assume 1 apple = 1 orange then 1/1000 of an apple = 1/1000 of an orange.( Let's not worry about the apple stem.) One of my dosimeters is measured in milliroentgen's and the exposure charts are in millirems. Should I convert or use tit for tat?
The Roentgen (R) is a measure of air ionization (“exposure”): This measurement is easily accomplished using a GM tube (an ion chamber operated at a voltage putting its operation in the Geiger region) and survey meter as long as the energy field being measured is uniform. It can also be measured with an electrometer attached to an ionization chamber. The primary advantage of this quantity is the ease of measuring it electronically. This was the method pioneered by the Curies, and it is still in use. SI units are coulombs / kg air. Customary unit is the Roentgen (R), which equals 0.00026 C/kg, and its subdivisions (such as mR). Importantly, this measure is only valid for ionizing electromagnetic radiation (photons), not ionizing particulate radiations (e.g., alpha, beta, neutron, etc.).
Absorbed dose: Dose, in ionizing radiation measurements, means the amount of energy deposited locally by radiation in a given mass. This can be measured directly in some instances, such as by calorimetry, but it is more often calculated from ionization current and electronic properties of the detection medium. The SI units are joules / kg, which is given the special name of Gray (Gy). A common customary unit is the rad (100 ergs / g or 0.01 Gy).
Here is where you may be getting confused, as for convenience, it is commonly assumed that exposure and absorbed dose are the same when expressed in traditional units (i.e., 1 R = 1 rad).
I don't know where you get your information, but here some quotes and sources.
First of all, US Nuclear Regulation Commission puts it this way:
...dose equivalent (in rems) is equal to the absorbed dose (in rads) multiplied by the quality factor of the type of radiation
This link shows difference between Rem unit and Roentgen unit. Namely, Rem is unit "biologically effective dose" and Roentgen is unit of "Intensity" of the source.
The roentgen (R) is a measure of radiation intensity of xrays or gamma rays. It is formally defined as the radiation intensity required to produce and ionization charge of 0.000258 coulombs per kilogram of air
And (emphasis mine)
The biologically effective dose in rems is the radiation dose in rads multiplied by a "quality factor" which is an assessment of the effectiveness of that particular type and energy of radiation. For alpha particles the relative biological effectiveness (rbe) may be as high as 20, so that one rad is equivalent to 20 rems. However, for x-rays and gamma rays, the rbe is taken as one so that the rad and rem are equivalent for those radiation sources.
...1 roentgen actually deposits about 0.96 rem in soft biological tissue, when all weighting factors equal unity. Older units of rem following other definitions are up to 17% smaller than the modern rem.
I think main confusion is between biological effect (chance of cancer development) and matter ionization by radiation. They are linked but used to be calculated differently.