GBS Elektronik MCA166-USB Behavior at different Temperatures - Manual (Page 3)

This causes the FWHM determination algorithm to work incorrect and the fluctuation of channels used for
centroid calculation is much larger than one channel. This also leads to an increased centroid error. To avoid this,
it is recommended to adjust the MCA resolution in a way that the FWHM is about 4 - 8 channels.
To reduce the range of possible centroid errors, it can be stated that peaks with an area <30 counts are hardly
recognizable and centroid errors < 5%FWHM seem hard to believe. So for practical purposes the following may
be assumed (if FWHM=3...12channels and the peak is large compared to background):
Table 2
Area Centroid error
peak area<30 not a peak
30<peak area <400
∆
E
FWHM
peak area
≅
peak area>400 FWHM*0.05
4. Temperature drift of the MCA 166
At first, it is evaluated how the drift changes with time. Knowing the thermal time constant allows to judge how
long it necessary to wait until the MCA runs stable.
It also tells that for measurement times short compared to the time constant, resolution losses due to gain drift can
be reduced.
0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0
T im e (m in )
0 .9 8 8
0 .9 9
0 .9 9 2
0 .9 9 4
0 .9 9 6
0 .9 9 8
1
C
e
n
tr
o
id
d
ri
ft
M CA 166 w arm ing up from about -7°C
Fig. 2. Reaction of the MCA166 on a sudden temperature change from -7°C to +20°C. The thermal time
constant (1/e) can be evaluated as 33 minutes.
planar high resolution HPGe detector type GL0310 which was
kept at constant temperature measuring a Co60 source was connected to the MCA.Spectra were taken in 5
minute intervals and the drift of the 1333 keV peak was evaluated. Shaping time was 2µs, Gain =5*0.6.
The drift reaction caused by switching on the device is similar to that on a thermal change of 4-7°C. So, for
perfect stability it is a good idea to leave the MCA at least 3 hours running to come to a thermal equilibrium. For
short measurements (few minutes) drift will not affect resolution, as drift is very slow. However, energy
calibration may have to be readjusted a few times.
In the next experiment it was measured how gain changes with temperature and if there are other dependencies.
For this experiment a GL1015R with a Ra226 source was used. The MCA was serial number 140 (one of the first
series). The count rate was about 6500 cps and 13% dead time (2µs shaping time). The MCA was in a climatic
chamber and all the time connected to the charger. After temperature adjustments the next measurement was
started earliest after 4h to allow the MCA to find its thermal equilibrium. This temperature test was also meant to
check the reliability of the MCA electronics. Below -10°C, the battery voltage was too low to allow correct MCA
operation without charger. The tests were stopped at -40°C due to lack of time and because this is far out of
specifications (normal minimum temperature -5°C).
In first order and within the specified temperature range, the gain change with temperature can be considered
linear. Only at very low temperatures there seems to be a nonlinear effect. Much more interesting is that the drift
depends strongly on the gain setting. The drift is sometimes positive, sometimes negative and sometimes nearly
negligible.