Search

AN ice sublimation or freeze-drying process will have maximum rate of vaporization per unit exposed area of frozen product at a specified temperature if the following conditions are satisfied : (i) The vapour pressure of ice sublimed on the condenser is negligible compared with ice at the product temperature, (ii) The condensing and product surfaces have a uniform and minimum separation. (iii) The permanent gas (usually air) pressure in the product-condenser interspace is a minimum, (iv) Mechanical obstruction (in the form of radiation shields, sterile caps, etc.) to vapour transfer from product to condenser is a minimum, (v) The product is of uniform and minimum thickness. The first four conditions follow from kinetic considerations and ensure that reversed vapour transfer, length of vapour path, and obstruction to vapour transfer are a minimum. The last condition as a consequence of the increase of drying-time with product thickness.

An easily demountable radiation shield has been developed for use with a Philips PW 1078 texture attachment. With the shield in position, the unfocused scattered x radiation is reduced from approximately 50 mR h−1 to a value of less than 01 mR h−1.

The gaps in the hinged shield used in many commercial x-ray diffractometers may be covered with a second hinged shield. The design of the second shield is described.

• A sterile, portable radiation shield has been developed to protect the surgeon who must remain in the operative field during radiation exposure. The shield is simple to construct and to use. There is no undue prolongation of operating time. The shield effectively reduces exposure to radiation. ( Arch Surg 115:347-348, 1980)

InP crystals were grown by the liquid encapsulated Czochralski (LEC) technique, in which the starting materials in a pBN crucible were previously encapsulated with B2O3 using an outer vacuum vessel. The electrical properties throughout the LEC-grown InP crystals were improved by using the pre-encapsulated starting materials. This is due to the reduced incorporation into the crystals of impurities such as S and C, which are evaporated from carbon heater and radiation shield materials.

The effect of a number of thick radiation shields of infinite thermal conductivity and of a type realizable in practice is analyzed by matrix algebra, and a method for calculating the temperatures of the various shields is also given. For identical thin shields it is found that the ratio of the power required to maintain a radiating surface of emissivity e0 at a given temperature with N shields to the power required to maintain the same temperature without shields is given by [1+e0(f+f2+f3+···fN)(1ek+1el−1)]−1, where ek, el are the emissivities of the two sides of a shield and f is the fraction of the radiation leaving the inside of the nth shield which is incident upon the n−1th shield.

The construction and performance of a simple helium Dewar is described. A large apparatus space is achieved within the small outside dimensions by replacing the normal liquid nitrogen jacket by a conduction-cooled metal radiation shield.