The Beamloss Monitor system was calibrated using X-rays from a Philips PW2773/00 tube equipped with a copper anode. The X-rays leave the tube through a 300 µ m thick, beryllium window. The beam is defined by a 7 cm long brass collimator with a 2 mm diameter bore. X-rays were chosen because their intensity can easily be adjusted. A full (internal) report of the calibration experiment is available here for HERMES members. The remainder of this page shows only the more important results. Sometimes in the text small graphs are shown. Click on these to get a readable plot. Note: some gains have been changed since the calibration experiment, to make better use of the dynamic range of the amplifiers.
The first quantity to be determined is the value of the high voltage to be
applied to the chamber to achieve full saturation of the ionization current.
Saturation in these chambers is achieved between 50 and 100 V. The value
chosen to operate the chambers is 250 V.
The uniformity of the response is measured by moving the collimated beam of
X-rays in the vertical (= direction of the axis of the cylindrical gasvolume)
and horizontal directions. In each of these cases the absorbed energy is
calculated using the known wavelength of the radiation and absorption
coëfficient of the Argon gas. The measured ionization current is
subsequently compared to these calculations. Horizontal scan:
vertical scan:
The linearity of the response was tested by varying the intensity of the X-ray
beam. The following plot
shows the linearity in the range accessible by the X-ray set-up. A very good
linearity (better than 0.1%) was observed. The absolute calibration for the
dose measurement is obtained by calculating the energy lost in the gas at
a given current. A few measurements with dosimeters (TLD's) were added
to confirm the calibration. The final result is
dD/dt = 4.27 106 I,
where the doserate dD/dt is in Gy/s, and the current I in A.
The signal levels at different positions in the read-out chain can be calculated with this calculator.