Letter: Ardran/Crooks penetrameter cassette - necessity for re-calibration.
We have frequently been asked if the penetrameter cassette described by us (1968) requires frequent re-calibration. When this cassette was first designed it was our opinion that calcium tungstate intensifying screens did not alter in speed with time or use unless obviously worn, abraided, stained or dirty; we still hold this view.
Checking diagnostic x-ray beam quality.
Abstract A radiographic method is described for comparing the effective kilovoltage and inherent nitration of diagnostic X-ray machines. The apparatus consists of a modified Xray cassette covered by a sheet of lead perforated with two rows of holes; one row is associated with a strip of slow intensifying screen and provides a reference exposure, the other row has a strip of fast intensifying screen and a copper step wedge. The thickness of copper needed to give a film density which matches that of the reference exposure is called the copper reference number and provides an index of the beam quality. The device is calibrated against a constant potential generator for which the kV values are accurately known. For determining the effective kV of machine the beam is attenuated and hardened at source by means of a filter of 1·5 mm of Cu, corresponding approximately to the filtration (of the primary beam) provided by an abdomen plus Bucky grid, plus table top. The method is relatively independent of processing,...
Heritage Languages: In the 'Wild' and in the Classroom
Heritage speakers are people raised in a home where one language is spoken who subsequently switch to another dominant language. The version of the home language that they have not completely acquired – heritage language – has only recently been given the attention it deserves from linguists and language instructors. Despite the appearance of great variation among heritage speakers, they fall along a continuum based upon the speakers’ distance from the baseline language. Such a continuum-based model enables researchers and instructors to classify heritage speakers more accurately and readily. This article discusses the results of research on lower-proficiency speakers, identifying recurrent features of heritage languages in phonology, morphology, and syntax. Preliminary results indicate that different heritage languages share a number of structural similarities; this finding is important for the understanding of general processes involved in language acquisition. The article also presents implications of the main findings for language education and identifies areas needing further study.