4. Measurement Principles & Theory
Waveforms and Triggers
2012-10/4.3A
103
Barracuda & QABrowser Reference Manual
Is the integration of all signal which means the area below the curve above from point
1 to 7. During the measurement (exposure) the accumulated signal (dose, etc) is
displayed where applicable.
Signal rate (dose rate, tube current, etc.)
During the measurement the mean signal for the last second is displayed. When the
measurement (exposure) is over, point 8 above, the mean signal for the whole
measurement is displayed. This signal rate is calculated as all integrated signal (as
described above) divided by the irradiation time. If no irradiation time is possible to
calculate, the radiation time is used instead.
This means that for long measurements you may see a change in the rate value (dose
rate, etc) when the measurement is finished, if the signal level was changed during the
measurement.
Measurement Principle for the MPD4.6
The following are the key features of the MPD design:
· Small size
· Optimized filter packages for five different kV ranges
· Very sensitive and wide dynamic range
· Check filter for measurement geometry verification
· Single exposure estimation of total filtration and Quick-HVL
· Single exposure estimation of generator waveform type
The design of the detector package is very important to be able to measure kV and
dose correctly in the whole range of 20 to 155 kV.
The MPD design makes it possible to measure small field sizes, less than 3 mm width,
and low output levels down to approximately 1 µGy/s. Basically the detector packages
consist of four separate electrometer channels connected to detectors D1, D2, D3, and
D4 and a moveable filter package that can change to one of six positions, each a
combination of different filters for the detectors. One of these positions is used as a
"check-filter". It has the same filter thicknesses for both D1 and D2. When the detector
is perfectly positioned and both detectors have the same radiation the ratio between
the two signals should thus be exactly "1.000". This is very useful information, and
testing this makes sure that your measurement geometry is fine, giving reproducible
readings. The other 5 filter pairs have different thicknesses all optimized for different
ranges of the tube voltage; two (1 and 2) are used for the low mammography energy
range 20 to 45 kV, and three filters (3 - 5) are used for the radiography range 35 to
155 kV (35 - 75, 55 - 105, and 80 - 155 kV).
Using these four signals S1-S4 (from detectors D1 to D4) the MPD can accurately
calculate the corresponding tube voltage. The signal S3 is not affected by the
moveable filters and is designed to measure the dose. This detector is marked by a
square inside the rectangular detector area on the top panel. The reference depth for
the sensitive area of the dose detector is 8 mm under the MPD top panel surface. That
is indicated by the lower edge of the rectangular hole for the visual indication of the
filter position on the MPD front side.
The detector D4 is placed directly under D3 with additional filter in between. The ratio
between S3 and S4 is used to estimate the total filtration for the radiography range.
Using these signals together more accurate dose and tube voltage readings can be
obtained.
Since all signals is measured simultaneously and with a relative high speed, the MPD
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