Physics 405 lab Manual

Appendix C: The Personal Computer Analyzer and OxfordWin-MCA software

The multichannel analyzer (MCA) used for this experiment is a PC plug-in board coupled with software provided by Oxford Instruments. A brief description of the use of the MCA follows. For a more detailed description of the MCA program and its consult the manual or the on-line help files.

To start up the program, start the computer and double-click on the icon labeled OxfordWin-MCA. The main display screen should come up. There may or may not be a spectrum present when the program starts: if there is, you will want to clear it before acquiring any new data. To get a quick overview of the functionality of the buttons across the top of the MCA window, start up the HELP/Contents menu and choose Point and Click Definitions. You should also browse through the functions in the Menu Bar before attempting to use the program.

Program Startup
Spectrum Display and Memory Control
Basic Data Acquisition
Energy Calibration
Choosing a Region of Interest (ROI)
Saving Data to Disk
Multichannel Scaling Mode



1. Program startup

 When the program starts up, it should already be configured to run in Pulse Height Analysis mode (PHA), with the preset functionality enabled in Real Time. Check that this is the case via the following. Click on the 3rd from left of the Screen Display buttons, and the Hardware Controls should appear at the bottom. Go to the ADC2 option to check that the MCA is in PHA mode. Go to the Presets option to check that the presets are enabled and set for Real Time acquisition. Next go to the EDIT menu bar, click on System Parameters, to turn on Beep when Ready if you wish to hear a beep when data acquisition is completed.

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2. Spectrum Display and Memory Control

There are four channels of memory in the program. This feature can be used to store more than one spectrum at a time for qualitative comparison. Only one memory channel is active at any given time, the others are static. Use the multi-colored button in the upper left corner to find information about which channel is presently active. Counts will be acquired only in the active memory. The four buttons just to the right of the multi-colored one allow you to control how many channels of memory will be displayed at one time and whether they will be stacked or overlaid.

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3. Basic Data Acquisition

a) Clear the contents of a spectrum using the Erase button.

b) Begin acquiring data with the Acquire button.

c) Stop acquiring data with the Stop button.

d) Change the full-scale counts by either entering a maximum by hand or using the arrow keys.

e) Zoom or expand the horizontal scale using the MIN/MAX buttons and the horizontal cursor bar that appears when the horizontal scale is not on MAX.

f) The preset data acquisition time can be changed by going to the Preset card of the Hardware Controls box when it is displayed with the spectrum (the 3rd from left of the Screen Display buttons).

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4. Energy Calibration

The horizontal scale can be adjusted to be in proper energy units if the detector is calibrated. There are two suggested methods for performing an energy calibration. One is to acquire a spectrum which has two known peaks (such as 22Na) and use the Two Point Calibration method. The other is to acquire multiple peaks in the same spectrum and use the Region of Interest (ROI) centroids. The latter will be more accurate but requires more data.

a) Two Point Energy Calibration.

After acquiring a spectrum with two peaks whose energies are known, go to the Acq Parms pull down menu, to Manual Calibration, and click on Two Point Energy. A dialog box will open. Line the cursor up with the first peak and left-click on the mouse to enter the channel number of the peak into the dialog box. You must then enter the energy of the first peak. Do the same for the second peak. Information about particular sources is contained in the on-line library. Click on the Library Selection Button and install the Gamma library, then bring up the source of interest in the Nuclide Drop Down list to the right of the library box. You can then click on the Nuclides Radiation Table to find the energies of all known peaks for that particular source.

In this method, you will not get any information returned about the energy calibration but the horizontal scale will automatically be adjusted. You can test the results of the calibration on some other spectrum by bringing up the Spectral Line Markers to see how well the markers agree with the peaks in your spectrum.

b) Energy by ROI Centroids

This method allows you to use more than two peaks in the calibration, and allows for a possible quadratic dependence of the detector pulse height on energy. It also will give you the resulting fitted parameters for later use in offline analysis if desired. To use this method you must choose a Region of Interest around each peak (see ROI, below). Once you have chosen ROIís about each peak, go to Acq Parms, manual calibration, and bring up the Energy by ROI dialog box. You must enter an associated energy for each peak. Use the information in the Nuclides Radiation Table or in Table VII.1 in the lab manual.

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5. Choosing a Region of Interest (ROI)

The ROI feature can be used to determine the integral number of counts in a peak or in a given region of a spectrum, or to determine the centroid of a peak for energy calibrations.

To set a foreground ROI (type Unknown) be sure that the ROI  type selector is showing Unknown. While holding down CTRL place the cursor at the position where you want to start the ROI. As the cursor is dragged, a colored area corresponding to the type of ROI you are setting appears; this is the area in which the ROI is created.

After completion of dragging an ROI area, a full-screen marker appears, denoting the center of the ROI The ROI you have just set is the current ROI, which will appear in blue. More information can be found in section 9.3 of the OxfordWin-MCA manual, or in the help menu under Common Examples of Operation.

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6. Saving data to disk

This program does not appear to be able to subtract one spectrum from another. Thus, in order to perform a proper background subtraction, you must save your data to disk and use the program ANALYSIS. At the present time it is recommended that you save your data in ASCII format (*.asc) for it to be readable by the ANALYSIS program. However, you may also wish to save your data in ANS format (*.ans) in order to load it back into the MCA program at a later time. It appears that the MCA program will NOT read ascii files.

To save the data: choose Save as Spectrum in the File menu. You will be asked for a format and a filename. It is recommended that you save two copies, one each in ANS or ASCII formats. In order to use the ASCII format with the ANALYSIS program, you will have to edit the file (which can be done with the Windows Notepad application) and strip off the header information written with the data. To read in an existing spectrum in the MCA program, choose Load Spectrum in the File menu.

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7. Multi-channel Scaling mode

Rather than look at the pulse height spectrum of gamma rays from a radioactive source, it is sometimes of interest to simply count the number of decays as a function of time. This mode will be used in experiment 7 where the half-life of a radioactive source is measured. The PCA3 and OxfordWin-MCA support a mode called multi-channel scaling, where the x-axis represents a certain time bin and the y-axis represent a total number of counts which occured during that time bin.

To select the MCS mode, use the ADC2 tab of the hardware controls and select MCS as the operating mode. You cannot do this with data in memory, and a number of settings including group numbers, count inputs, and other settings may be invalidated by this selection.  Only when MCS is selected will the Presets panel show the MCS controls as displayed above.  Starting an acquisition in MCS mode will generate a trace across the screen where data is acquired in each channel for the period of time specified in dwell time.

The MCAs supported by Quantum MCA (the PCA-3 and PCA-Multiport) count events from an external signal input.  The Dwell Time (the amount of time for which data will be acquired in each channel) must be set using the Presets tab while in the Hardware Controls screen mode.  The PCA3 and PCA-Multiport only support a predetermined set of values for the dwell time.  The values supported can be found in the respective MCA hardware manuals. In the software you can enter any value.  When the dwell time entry control looses focus the value you have entered will be reset to the nearest value supported by the hardware.  If an MCA device supports multiple passes, you may enter the number of passes you desire in the MCS passes control. Consult your hardware manual to determine the input types and dwell times available and the maximum number of passes that can be performed.

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