Gbs-elektronik MCA166-USB Behavior at different Temperatures Manuel d'utilisateur télécharger pdf

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 Co

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

Spectrum

∑

(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

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Page 1 - Application Note

Application NoteBehavior of the MCA 166 at different Temperatures andGain settings and limits of centroid accuracy10.1.2001Jörg BrutscherGBS Elektroni

Page 2

channels above half maximum are used, which represent 76% of the total peak area. This yields in a statisticalcentroid error depending on peak area N

Page 3

This causes the FWHM determination algorithm to work incorrect and the fluctuation of channels used forcentroid calculation is much larger than one ch

Page 4 - Temperature Drift 1/°C

Fig. 3. Gain change depending on temperature. The drift depends very much on the gain setting.This behavior was evaluated a bit closer. So the drift b

Page 5

factor 2 period. At the gain of 15 and the gain of 30 this oscillation is disturbed. This can be explained byadditional attenuators which are switched

Page 6

0306090120Time (min)0.9940.9960.9981Drift (relative)with stabilizationwithout stabilizationFig. 7. Peak drift of the MCA166 with detector GL0310 and a

Page 7

NkeVcpskeVopts=∗∗ ∗ ⋅=−22140021861610300005123.The stabilization cycle time is here 21s and the expected peak broadening is neglible (0.6%).The

Page 8

8. Conclusions-The temperature drift of the MCA166 is a complex oscillating function of the gain setting. Positive as well asnegative values for the d

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