F8 – Discriminator Plot
Gem-5 User’s Manual 79
Current operating discriminator settings are stored in settings file DISCRIM.DF (on
page 299).
The low-level discriminator (usually called LLD, shown as Discriminator on the
screen) circuit on the Detector Board suppresses pulses of amplitudes below its level.
The LLD setting affects the detector efficiency and its response to different energies.
Consequently it also affects the background count rate. The LLD should be operating
above the system noise level, which typically is about 10 mV but depends on the HV
setting, temperature and PMT. The system noise level may be estimated by running
the F8 – Discrimination Plot. Below a certain value of the LLD (usually about 10
mV) the count rate rises quickly. This is the point below which system noise starts to
contribute to the background counts. There should be some margin left between the
LLD setting and the actual system noise level to account for higher noise at increased
operating temperatures. In most cases, there is no need to change from the default
setting.
The upper level discriminator (usually called ULD) suppresses the pulses of
amplitudes above its level. Note that maximum ULD voltage setting ULD 5V
disables the ULD rejection as the maximum amplitude of pulses is below 5V. Setting
ULD below its maximum (5.0 V), typically below 4.75V will reject high amplitude
pulses. The ULD setting is applied while monitor is in operation.
Setting ULD below Pulse Amplitude V during the Amplifier Pulse Test will give
count errors as test pulses are rejected.
The default LLD and ULD values usually work well; therefore it is not essential to
perform a discriminator plot during every routine calibration. However, the
Discriminator Plot can be a useful diagnostic tool if noise is suspected for a particular
detector.
The screen title on the right top corner shows the detector location and detector
number of the currently selected detector. For example: Feet 1, Left Bottom 2, Left
Middle 3, Left Top 4, Head 5, Right Top 6, Right Middle 7 or Right Bottom 8.The
number in small font at the top right of the screen gives the sequential zone number
assigned to the detector that is used on various information screens and printouts.
Use the inset diagram to confirm which zone is currently selected. Use PgDn, PgUp,
Home, or End keys to select the next, previous, first or last detector, respectively.
Discriminator Plot screens have a dark green background for Gamma values.
The Discriminator Plot is performed on the selected detector only, or on all detectors
if this option is selected. The plot points are saved in DISC3_##.CSV files in the
D:\MonitorProgram\DATALOG directory, where ### is the detector number e.g.
DISC3_01.CSV. These CSV files can be opened by any spreadsheet program such
as Microsoft Excel.
Some useful hints for viewing CSV files using Excel are provided in the appendix,
Viewing Datalog Files with Excel, on page 303.
To Next Page
Loading...
Need help?
General
