A multichannel analyzer counts, stores, analyzes, and displays data measured from a sample source. It is necessary to calibrate the MCA to account for the effects related to differing source-detector geometries. Calibration of a MCA comparing the measured peak values of a known source to a standardized value for that source in order to determine the operating gain of the particular MCA configuration.
Calibration typically is a manual operation, wherein a sample of known composition, because of its nature, produces in the MCA a histogram with well-defined, isolated peaks and a small statistical uncertainty. The expected typical values of the peaks are well established physical data for the known sample. The operator then compares the locations and heights of the peaks collected from the known sample to the expected values for that known sample. This is accomplished by having the operator identify a peak at an appropriate channel and compare the measured value at that channel to the expected value. Next, the operator must set the measured values to correspond to the expected values for the known sample. This is accomplished by having the operator select the peak at the appropriate channel and enter the expected value for that channel. The calibration system then establishes the relationship between the measured value and the expected value and computes the necessary adjustment to correct for any inconsistency. This process is repeated for several peaks to establish the proper correlation between measured values and actual values. The larger the number of peaks for which the measured value and the expected value are available, the greater the accuracy of the correlation and the greater the time necessary to complete the calibration.
Next, the relationship between the measured data and the parameters corresponding to the matched element is determined. The calibration relationship is applied to subsequently measured data from samples of unknown composition to correct the measurements for errors introduced in the data collection process and convert the analysis results from peak position in channels to peak position in energy.
Further, calibration for the MCA must be set every time changes are made to the MCA, such as changing the parameters or the detector. Each new detector produces a signal of a different energy, resulting in a different channel to energy relationship. Moving the MCA alters the effects introduced by the surroundings of the MCA.
As a result, calibration of the MCA is a time consuming process which must often be repeated. The frequency of repetition increases the chance that errors will be made in the calibration process because of the amount of precise physical data which must be manually entered into the process.
Automatic calibration of the MCA is accomplished by having the MCA compare the data collected from a sample of known composition with an internal library of data containing the expected values. The calibration is performed by determining the closest possible match of the measured data to an element of the internal library.
Accordingly, one object of the present invention is to substantially reduce the time necessary to calibrate the MCA by automatic calibration.
An additional object of the present invention is to reduce the amount of precise information which must be entered by the operator to reduce the likelihood of operator error.
A further object of the present invention is to allow calibration results to be archived for future use.