FlowStar2 LB 514
14. Disturbances
121
Type
Scintillator used
Quantity
(Cell volume)
Existing ambient radiation
BG of cell
Discriminator setting
Cell
contamination
BG total
(mainly in solid cells)
Device BG
Cross-talk
Random coincidence
Noise
Figure 35: Factors affecting the background
If the BG values are significantly higher than the values listed in the table,
check first whether this is caused by the measuring cell or the instrument:
If the measuring cell is the cause, replace it by a well-flushed or new
empty admixture cell (Z-1000 or Z-500), where the in- and outlets are
well-protected from incident light by aluminum foil (even better: use
steel capillaries). In this case, only the “instrument BG” has to be meas-
ured. For service personnel we recommend measuring the individual
rate of PM A(left) and PM B(right) i
n the “SERVICE” menu. The individ-
ual rate is normally between 50 and 450 cps. If the measurement cham-
ber has been open for a long time, these values may be higher.
Cau-
tion:
Both PM’s always have different single rates!
Up to 1000 cps will be compensated without any problem by the coin-
cidence circuit. Higher single rates in both counting channels are
caused by insufficient light protection or errors in the instrument.
If the single rate is higher in one counting channel only, this may be
caused by the high voltage or the PM.
The “instrument BG” is OK if the values specified are measured with
the empty, clean admixture cell (Z-500 or Z-1000), even though the BG
increases after installation of the new measuring cell.
