27 - Deadtime Measurement

Software Manual

Section 27

Previous Section
Home
Next Section

Deadtime Measurement

Introduction

A deadtime measurement consists of many repeated isotopic measurements, where the count rate is constantly changed between runs. A typical deadtime measurement on Si runs over night and consists of more than a hundred individual Si isotopic measurements with the 28Si count rate varying between 200,000 and 1,000,000 c/s. Here we discuss the changes that have to be made to a normal isotopic measurement to make it useful as a deadtime measurement.

For the most part, the setup is similar to that of other isotopic measurements.

Iso Setup 1

Follow the standard routine for setting up an isotopic measurement in IsoSetup 1. Typical parameters are 28Si for 1 s, 29 Si for 5 s and 30 Si for 5 s. Of course, the instrument has to be adjusted for high mass resolution to avoid interferences from 28SiH etc.

Iso Setup 2

In IsoSetup 2 we have to select a special option for deadtime measurements (see below). Otherwise, everything here is normal. Typical settings for a Si deadtime measurement are 5 blocks with 4 cycles each, track all peaks every 4 cycles, and monitor delta 29/28 and delta 30/28.

Measure Deadtime Based on

This option in IsoSetup 2 has to be selected for deadtime measurements. Select the main (most abundant) isotope, whose count rate will be continuously adjusted during the deadtime measurement.

Iso Setup 3

Choose file names etc. as usual in IsoSetup 3. Make sure that data files in the "New Format" are being created. These files are needed for the subsequent deadtime calculations.

Large Number of Runs...

In IsoSetup 3, this red text is the only indication that this is actually a deadtime measurement. If this red text is not present, go back to IsoSetup 2 and select the option under "Measure Deadtime Based on". A deadtime measurement only makes sense with a really large number of selected runs. Don't be shy. If you select 1000 runs and let it run over night, you can always change this number to something lower the next morning.

Print Options

Although any options here are ok, keep in mind that you may have a measurement of more than 100 runs. Is it really necessary to print one page with detailed results for each run? The print option shown here "General Setup (1st Run) & Short Run Results" is the best choice and it results in only 3 lines of text per run.

Start Measurement

Click on this button in IsoSetup 3 to start the deadtime measurement. Note that the measurements themselves are identical to normal isotopic measurements. First, however, come a few deadtime-specific steps.

Choose the range of count rates

Follow the instructions in this window, so that the computer can change the secondary ion count rates by modifying the L3 settings. Then choose the range of count rates desired. The program will adjust the count rate to the highest chosen value before the first run and use the next count rates for successive runs. Then it will start with the first value again and keep cycling through the selected values.

Calibrate (Center) Mass Peaks

Even if you have selected peak centering for every peak, you only have to calibrate the main isotope in this preliminary centering routine. There will be another centering later when the actual measurement starts. This is only the centering of the main isotope in preparation for the next step. This is the last place to make sure that the count rate of the main isotope is higher than the maximum chosen in the previous step. Try to adjust so that the count rate is about 20% higher than required.

Checking Range of Count Rates

In this step the computer will try to adjust the count rate of the main isotope to the different values that have been requested by varying L3. The bit settings on the horizontal axis are meaningless units, but a lower (negative) bit value results in a lower voltage on L3. The red horizontal lines indicate the requested count rates (in this case: 200k, 400k, 600k, 800k and 1 million c/s). The dots are the measurement points and the red points are the chosen settings. The chosen settings (red dots) are not always the very closest ones (to the red lines), but they are usually ok within one bit. At this step, this is only to check the whole range of count rates. The acutal L3 settings for the measurements will be determined before each run.

Usually it is not a problem to get to lower count rates by changing L3, but occasionally the count rates do not get high enough. Therefore, this routine times out if it cannot get a high enough count rate within 20 steps. Hence the line at the bottom of the window. If the routine does time out, you have a chance to repeat the last three steps.

Set Count Rate

The next window that pops up looks similar to the previous one, but has only one red horizontal line. This is the actual adjustment of the count rate for the first run. This window will appear before each run. No user input is required for this window.

Calibrate Mass Peaks

This time we perform the 'normal' mass calibration that comes at the beginning of any isotopic measurement. All the peaks that have been selected for centering should now be in the list of masses to be calibrated. This is the last user input that is required before the measurement starts.

Output File

The data file produced by these deadtime measurements are identical to those from other isotopic measurements - with one exception: None of the results shown (count rates and ratios) have been corrected for deadtime. Only this way it is possible to 'see' the deadtime effect. In other words, the deadtime correction value has been set to 0.0 ns for this measurement. This does not affect other measurements that are being performed later. As soon as the deadtime measurement is over, the deadtime correction value will be set back to the one that was defined in Initialize.

Calculate the Deadtime

After the deadtime measurement has completed a large number of runs, the data files have to be transferred to the SGI workstation. A program there can then be used to calculate the EM deadtime.

FJ Stadermann
Back to Top
Next Section


Deadtime Measurement



	Introduction A deadtime measurement consists of many repeated isotopic measurements, where the count rate is constantly changed between runs. A typical deadtime measurement on Si runs over night and consists of more than a hundred individual Si isotopic measurements with the 28Si count rate varying between 200,000 and 1,000,000 c/s. Here we discuss the changes that have to be made to a normal isotopic measurement to make it useful as a deadtime measurement. For the most part, the setup is similar to that of other isotopic measurements.

	Iso Setup 1 Follow the standard routine for setting up an isotopic measurement in IsoSetup 1. Typical parameters are 28Si for 1 s, 29 Si for 5 s and 30 Si for 5 s. Of course, the instrument has to be adjusted for high mass resolution to avoid interferences from 28SiH etc.

	Iso Setup 2 In IsoSetup 2 we have to select a special option for deadtime measurements (see below). Otherwise, everything here is normal. Typical settings for a Si deadtime measurement are 5 blocks with 4 cycles each, track all peaks every 4 cycles, and monitor delta 29/28 and delta 30/28.



	Measure Deadtime Based on This option in IsoSetup 2 has to be selected for deadtime measurements. Select the main (most abundant) isotope, whose count rate will be continuously adjusted during the deadtime measurement.

	Iso Setup 3 Choose file names etc. as usual in IsoSetup 3. Make sure that data files in the "New Format" are being created. These files are needed for the subsequent deadtime calculations.



	Large Number of Runs... In IsoSetup 3, this red text is the only indication that this is actually a deadtime measurement. If this red text is not present, go back to IsoSetup 2 and select the option under "Measure Deadtime Based on". A deadtime measurement only makes sense with a really large number of selected runs. Don't be shy. If you select 1000 runs and let it run over night, you can always change this number to something lower the next morning.



	Print Options Although any options here are ok, keep in mind that you may have a measurement of more than 100 runs. Is it really necessary to print one page with detailed results for each run? The print option shown here "General Setup (1st Run) & Short Run Results" is the best choice and it results in only 3 lines of text per run.

	Start Measurement Click on this button in IsoSetup 3 to start the deadtime measurement. Note that the measurements themselves are identical to normal isotopic measurements. First, however, come a few deadtime-specific steps.



	Choose the range of count rates Follow the instructions in this window, so that the computer can change the secondary ion count rates by modifying the L3 settings. Then choose the range of count rates desired. The program will adjust the count rate to the highest chosen value before the first run and use the next count rates for successive runs. Then it will start with the first value again and keep cycling through the selected values.



	Calibrate (Center) Mass Peaks Even if you have selected peak centering for every peak, you only have to calibrate the main isotope in this preliminary centering routine. There will be another centering later when the actual measurement starts. This is only the centering of the main isotope in preparation for the next step. This is the last place to make sure that the count rate of the main isotope is higher than the maximum chosen in the previous step. Try to adjust so that the count rate is about 20% higher than required.



	Checking Range of Count Rates In this step the computer will try to adjust the count rate of the main isotope to the different values that have been requested by varying L3. The bit settings on the horizontal axis are meaningless units, but a lower (negative) bit value results in a lower voltage on L3. The red horizontal lines indicate the requested count rates (in this case: 200k, 400k, 600k, 800k and 1 million c/s). The dots are the measurement points and the red points are the chosen settings. The chosen settings (red dots) are not always the very closest ones (to the red lines), but they are usually ok within one bit. At this step, this is only to check the whole range of count rates. The acutal L3 settings for the measurements will be determined before each run. Usually it is not a problem to get to lower count rates by changing L3, but occasionally the count rates do not get high enough. Therefore, this routine times out if it cannot get a high enough count rate within 20 steps. Hence the line at the bottom of the window. If the routine does time out, you have a chance to repeat the last three steps.



	Set Count Rate The next window that pops up looks similar to the previous one, but has only one red horizontal line. This is the actual adjustment of the count rate for the first run. This window will appear before each run. No user input is required for this window.



	Calibrate Mass Peaks This time we perform the 'normal' mass calibration that comes at the beginning of any isotopic measurement. All the peaks that have been selected for centering should now be in the list of masses to be calibrated. This is the last user input that is required before the measurement starts.



	Output File The data file produced by these deadtime measurements are identical to those from other isotopic measurements - with one exception: None of the results shown (count rates and ratios) have been corrected for deadtime. Only this way it is possible to 'see' the deadtime effect. In other words, the deadtime correction value has been set to 0.0 ns for this measurement. This does not affect other measurements that are being performed later. As soon as the deadtime measurement is over, the deadtime correction value will be set back to the one that was defined in Initialize.

	Calculate the Deadtime After the deadtime measurement has completed a large number of runs, the data files have to be transferred to the SGI workstation. A program there can then be used to calculate the EM deadtime.