Dark counts
The intention of this chapter is to show that CEMs do not generate dark pulses under normal conditions. Practise nevertheless shows that dark pulses may also arise in substantial amounts, but their source is not easily found.
The measurement of the dark count rate is always a measurement of the system CEM - preamplifier/discriminator - HV power supply - counter, including all cables and electrical connections and sometimes, when the discriminator is on a very low level, of the whole environment. In this case a movement of a person close to the preamplifier can generate counts in the electronic circuit.
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There are many possible sources for dark counts. Your system only works flawlessly if the dark count rate of a CEM is under 2 counts/minute. |
Before starting the burn-in process, we first measure the dark count rate of all CEMs by switching on the high voltage to 2300 V for the standard type CEMs and 3000 V for the EDR type CEMs respectively. A significant dark count rate would show that something is wrong with the detector itself or one of the other above mentioned components.
Dark Count Rate as a function of CEM opening
Figures 4 and 5 show the dark count spectrum of a standard CEM type KBL5RS and KBL25RS to demonstrate the dark count rate for a clean and well working system.
The measurements of the dark count rate for two CEMs with different input funnel sizes (KBL25RS with 25 mm diameter and KBL5RS with 5 mm diameter) show that the "normal" dark pulse rate depends on the size of the CEM. The reason for this is that these dark pulses are created by natural cosmic radiation (muons) and larger CEM surfaces detect more muons than small ones. Lead shielding can of course reduce this natural background.
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Fig. 4 Dark count rate of a KBL5RS |
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Fig. 5 Dark count rate of a KBL25RS |
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Dark Counts caused by wrong settings of the dead time and discriminator threshold
A possible cause of dark counts can be a wrong setting of, above all, the discriminator threshold of the measuring system. An example is shown in the graphics of the count rate against the applied voltage (Fig. 2 and Fig. 3), which were measured with positive high voltage and the measuring system MEASARplus. The discriminator threshold in Fig. 3 was on - 3 mV, so that noise from the electronic system was counted as well. Please note the different absolute count rate between Fig. 2 and Fig. 3.
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User Report |
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Courtesy of Paul Scheier |
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Univ. Innsbruck |
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Detection of extremly low count rates |
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Superfluid He droplets are formed by expansion of He at a temperature of 12K and a pressure of 23bar through a 5 micrometer nozzle into vacuum. The average droplet contains several 10000 atoms and has a temperature of 0.37K. Carbon dioxide is introduced into a differentially pumped chamber at a pressure of 2.5 mPa and picked up by the supersonic jet of He droplets. In the following differentially pumped chamber electron attachment to the doped He droplets is performed with a free electron beam with an energy of 13eV. The resulting anions are mass analyzed with a two sector field mass spectrometer.
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The detector, a KBL510 is operated in a pulse counting mode and following a Cu-Be dynode kept at ground potential. The rear end of the CEM is at 3.6kV and the pulses are capacitively coupled to a home built preamplifier. Each point of the mass spectrum is measured for 30s and the spacing between two points is 0.01 Thomson. The raw data are shown in red and after applying a filtering procedure (23 points Savitzky-Golay) the blue curve is obtained. The noise level is well below 0.05cps and is mainly due to cosmic radiation. |
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Two sector field mass spectrometer for detection of superfluid He anions |
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