LB 134
Design and Mode of Operation of the LB134
11
Units
Each nuclide and each measurement type (see above) has one
unit for the measured value and one unit for the integral or dose
value. The units consist of strings of up to 6 characters; they
can be entered from the keyboard or like any other parameter
via the USB interface. These units appear on the display behind
the respective measurement results and also with the stored or
transmitted data. Examples: Bq, pCi, Bq/cm², µSv/h, mrem/h,
etc.
Integral calculation time
During the measurement, the current integral value or the dose
and the integration time elapsed until then is determined every
second by addition. If you have selected the unit counts per
second (cps), you get counts through adding up (counts). If you
have selected the unit µSv/h, as for the dose rate, then the time
base is not second but hour. Therefore, before the addition, the
value must be divided by 3600, since every second is added
up. When entering the integral calculation time or the time base,
this divisor is selected correctly. You can choose between sec-
onds, minutes or hours.
Measuring range limits
There is an upper measuring range value for each channel.
OVR appears on the display if this value is exceeded. In this
case, the integral calculation (dose) and the integral time will be
stopped. The upper limits are determined and set by the manu-
facturer. When working with ZnS scintillator for contamination
measurements, please keep in mind a special feature because
of the method used. Since alphas cause quite a bit of afterglow
in the scintillator, the beta channel is blocked for about 1.3 ms
when an alpha event occurs. Therefore, the message "Beta in-
valid" appears in the beta channel if the alpha rate is greater
than 750 cps. The upper limit for the alpha channel is neverthe-
less 5,000 cps, for the beta channel 50,000 cps.
Fail threshold
Each detector has its own fail threshold and fail time because
the backgrounds are very different and we do not want to get
any false alarms, but on the other hand, we would like to get
failure messages as quickly as possible. In contamination
probes, failure monitoring always refers to the beta channel. A
counter/timer measurement monitoring whether the probe count
rate has fallen below the fail threshold is constantly running in
the background during a normal measurement. This test is car-
ried out at the end of the fail time and, where appropriate, the
error message "Fail" is output. Then, this measurement will start
new. Please proceed as follows to determine fail threshold and
fail time. Basically, you want a failure to be indicated as early as
possible and you choose a small fail time. However, this largely
depends on the background rate of the detector. If this back-
ground rate is very small, you have to adjust the fail time. When
determining the two parameters, it is assumed that the back-
ground rate of the detector is Gaussian distributed. To get no
Summary of Contents for LB 134
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Page 6: ...LB 134 Content IV...
Page 8: ...LB 134 Working with the User s Manual VI...
Page 10: ...LB 134 Safety Instructions VIII...
Page 34: ...LB 134 2 System Description 24 Figure 2 9 LB 134 with wall bracket...
