Fluke Biomedical TNT 12000 - Manual (Page 104)

TNT 12000
Users Manual
B-2
Window Material.................................................... 0.25 mm thick, graphite-coated polycarbonate (lexan). A 1.5 by 1.6 cm
piece of 0.0025 cm thick aluminum is attached to the back of the
window with the diagnostic sticker to improve the energy response for
diagnostic measurements.
Window Density .................................................... 32 mg/cm
2
Active Window Area ............................................. 12.32 cm
2
, centered within the chamber body
Connector .............................................................. Side-mounted triaxial two-lug BNC connector
Collector Plate....................................................... Centrally mounted collector is a 0.25 mm thick graphite-coated acrylic
plate, 3.18 cm in diameter. A 1.27 x 0.89 cm guard region is electrically
isolated from the collector area.
Electrical Specifications
Sheet Resistivity ................................................... All areas sprayed with graphite conductive coating measure less than
6 k/square but greater than 3 k_/square.
Leakage Current.................................................... With 300 volt bias in effect and 0.1 volt potential imposed between the
guard and collector, at 20 ° C and 50% relative humidity, the leakage
current is < 10 fA within 60 seconds of applying the voltage. The
leakage under the above bias conditions is < 100 fA within 60 seconds
of applying the voltage over both the full operating temperature range
of 8 – 38
° C and for 20 – 80 % relative humidity, but only for conditions
in which the absolute humidity is _ 20 g/m
3
(the equivalent R.H. is
75 % at 25 ° C, 60 % at 30 ° C, and 50 % at 35 ° C).
Radiological Specifications
Energy Range........................................................ 30 to 150 kVp for diagnostic measurements; 20 to 50 kVp for
mammographic measurements.
Nominal Sensitivity
L100 .................................................................... 2.00 x 10 8 R/C ( 1.75 x 10 6 Gy/C ) at 22 ° C and 1013 hPa
MV30................................................................... 2.21 x 10 8 R/C ( 1.94 x 10 6 Gy/C ) at 22 ° C and 1013 hPa
DV70 ................................................................... 2.01 x 10 8 R/C ( 1.76 x 10 6 Gy/C ) at 20 ° C and 1013 hPa
DH70................................................................... 1.89 x 10 8 R/C ( 1.66 x 10 6 Gy/C ) at 20 ° C and 1013 hPa
MH30 .................................................................. 2.16 x 10 8 R/C ( 1.89 x 10 6 Gy/C ) at 20 ° C and 1013 hPa
L100, DV70, and DN70 nominal sensitivities apply to the diagnostic
side of the chamber, while MV30 and MH30 apply to the
mammographic side.
Multiply values by 0.00876 to convert from R to
Gy.
Sensitivity Range .................................................. Sensitivity range is ± 8 % of nominal
Reference Point..................................................... The reference point is located 6.5 cm directly below the center of the
either active window region. This corresponds to the center of the
active volume.
Incident Beam Direction....................................... For diagnostic measurements, the reference direction of incident
radiation is perpendicular to the entrance window with the
“DIAGNOSTIC FOCUS” sticker attached, and with this window facing
the x-ray source. For mammographic measurements, the reference
direction of incident radiation is perpendicular to the entrance window
with the “MAMMOGRAPHY FOCUS” sticker attached, and with this
window facing the x-ray source.
Angular Dependence ............................................ The ion chamber response to radiation incidence variations up to ± 8 °
of normal is within ± 1 % of the response to incident radiation striking
perpendicular to the entrance window surface.
Collection Efficiency............................................. The collection efficiency for both continuous intensity and 120 Hz half
wave rectified intensity is theoretically calculated using Boag’s
equations. An ionization potential of 300 V is used in both cases. An
effective plate separation distance of 0.73 cm was determined
experimentally and used to generate the curves shown in Figure B-2.