Direct comparisons of the National Institute of Standards and Technolo
gy's (NIST) Ritz 20 kV to 100 kV standard free-air ionization chamber
and a portable variable-length free-air ionization chamber designed by
the University of Wisconsin - Madison Accredited Dosimetry Calibratio
n Laboratory (UW-ADCL) were made on NISTs low-energy tungsten x-ray ra
nge. As a result of this direct comparison, NIST has established a UW-
ADCL designed chamber, the Attix chamber, as the national standard cha
mber for the mammography energy x-ray range. The Ritz standard chamber
and the Attix standard chamber have been extensively compared using t
he new molybdenum and rhodium beam qualities. The results indicate tha
t exposure measurements in the mammography energy x-ray region with th
e two free-air chambers can be made with a discrepancy of less than 0.
35%. (C) 1997 American Association of Physicists in Medicine.