GBS Elektronik MCA166-USB Behavior at different Temperatures - Manual

Application Note
Behavior of the MCA 166 at different Temperatures and
Gain settings and limits of centroid accuracy
10.1.2001
Jörg Brutscher
GBS Elektronik GmbH
1. Problem
2. Basic limits
3. Centroid error
4. Temperature drift of the MCA 166
5. Temperature drift of detectors
6. Stabilisation
7. Linearity and accuracy of gain setting
8. Conclusions
1. Problem
For very accurate measurements, all possible influences on measurement performance have to be known and
understood. This helps to estimate errors and to improve measurement conditions. In this document, the
temperature drift and the linearity of the gain and its dependence on settings have been examined.
2. Basic limits
Gamma spectroscopy can be a very accurate measurement method in terms of measuring photon energy. The
limits are mostly given by detector resolution. For example, the FWHM of the Co60 1333 keV peak with
measured with a HPGe (High purity Germanium) detector is typical 1.85 keV. A useful assumption is that the
centroid of a peak can be calculated with an error of 10% of its FWHM. This is a measurement error in this case
of 1.4E-4=140 ppm.
With other detectors the error is larger.
Table 1: typical photon energy measurement errors for different detectors.
detector condition relative error
HPGe 1.85 keV FWHM at 1333 keV 1.4E-4
CZT 12 keV FWHM at 662 keV 1.8E-3
NaI 7.5% FWHM at 662 keV 7.5E-3
Gamma spectroscopy with semiconductor or scintillation detectors is a relative measurement method which relies
on calibration by standard photon sources. So main requirements are linearity and stability. The most interesting
point is the peak drift caused by temperature change. Temperature drift may be caused by the detector crystal, the
preamplifier and the main amplifier within the MCA. For accurate measurements it is desirable to keep
conditions such that the drift does not exceed the relative errors mentioned in table 1.
Absolute values of gain are not critical because the MCA has to be calibrated anyway. The absolute accuracy of
the MCA166 gain setting is a few percent.
2. Centroid error
Centroid
i Spectrum
Spectrum
i
i l
h
i
i l
h
= =
=
∑
∑
*
(1)
The peak centroid is the sum of the background corrected channel
contents multiplied by the channel number and divided by the sum of
the background corrected channel contents. Only the channels above
the half maximum are used (MCA measurement software algorithm).
From this definition it can be derived that there is a systematical and a
statistical component of the centroid error.
Statistical error
The measurement of a peak in a spectrum can be seen as N repeated measurements of the corresponding photon
energy, where N is the peak area. The standard deviation of a single measurement is 42.5% of the FWHM of the
gaussian distribution. The standard deviation of the average is normally the standard deviation of a single
measurement divided by the square root of the number of single measurements. In the definition only the