Radiation doses and specimen heating

It has been remarked above that electron microscopy produces a large radiation dose in the specimen. The standard unit of radiation dosage used from 1959 until recently, was the rad. This is defined as the absorption of 100 erg of energy from ionizing radiation per gram of material irradiated. This unit of absorbed dose is used because usually the chemical effect of radiation depends only on the absorbed energy per unit mass. It is independent of the type of radiation or its rate of application. 

Uniform radiation fields can be measured absolutely in such absorbed energy units by 

This energy deposition rate, 4 GGy s corresponds to a temperature increase of ten million degrees per second for a thermally isolated material of specific heat 0.4 J g K Irradiation of a large piece of material at a dose rate of 1 GGy s would result in its vaporization in a fraction of a second. Apart from transmission electron microscopes, only nearby nuclear explosions and devices that give short pulses of irradiation for the study of fast radiation processes [96] produce such high dose rates. It is important to realize that continuous irradiation in the electron microscope, even at such dose rates, does not have to cause a large rise in the temperature of the specimen. This is because a very small object like the illuminated area in a TEM has a large surface area per unit volume, and so it is efficiently cooled by thermal conduction into the rest of the specimen. 

There have been many calculations of the equilibrium temperature rise of a specimen in the beam of a TEM, usually for the ideal case of perfect thermal contact with the surroundings [7, 97, 98]. The temperature rise is approximately proportional to the beam current. For a polymer film firmly mounted on a 200 mesh grid and irradiated with 100 kV electrons, the rise is 1-3 K nA [7, 16]. Thus a focused spot 2 /xm in diameter with the beam current density used for high resolution, 1 A cm , has a total current of 3 X 10 A and a temperature rise of 30-90°C. 

Polymer microscopy normally uses a beam current ten or a hundred times less than that used in the example above, so the temperature rise is small. Temperature measurement to confirm this is difficult, but TEM observation of low melting point (40-50°C) crystals is convincing [99]. On the other hand, poor thermal contact with the support or a restricted thermal path, as when an 

Uniform radiation fields can be measured absolutely in such absorbed energy units by calorimetry, but it would be very difficult to measure the energy absorbed in the tiny illuminated region of a TEM specimen. The case of the SEM is even more complicated, as the dose will vary strongly with depth. To avoid these problems the dose in the electron microscope is normally defined as an incident dose or electron flux in coulombs per square meter (C/m ) or electrons per square nanometer (e/nm ), where le/nm2 = 0.16C/m2. 

The conversion factor between incident electron dose in Qlrr (or e/nm ) and absorbed dose in rads or grays depends on the rate of energy loss of the incident lOOkeV electrons. This has been 

There have been many calculations of the equilibrium temperature rise of a specimen in the beam of a TEM, usually for the ideal case of perfect thermal contact with the surroundings 

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