  We measured signals of low amplitudes originating from cosmic rays, using two
rectangular-block scintillation detectors at various positions. The signals
were analyzed by a slightly modified signal analyzer from project 'MuonLab',
designed to measure the lifetime and velocity of muons from cosmic radiation in
high school education. In our experiment we focused on the possibilities of the
apparatus to measure the time difference 'deltatime (DT)' between signals in
two scintillation detectors and the signal amplitude 'pulse height (PH)' of the
signal in one detector. We performed measurements in two arrangements: first
vertical, detectors parallel and above each other, second horizontal, detectors
parallel and next to each other. We observed that in both the vertical and
horizontal arrangements, low-amplitude signals from cosmic rays in the two
detectors showed unexpected coincidences. Under normal experimental conditions
these effects were not observed because the amplification in the
photomultiplier (PMT) as well as the threshold voltage were intentionally
chosen to avoid noise and excessively high count rates. The rate at which the
coincidences occur is very small, more than a thousand times smaller than the
count rates of the individual detectors. However, the measurement of the
deltatime spectrum appears to be a sensitive way to detect the presence of
these rare and unexpected effects. The time differences between the signals at
the two analyzer inputs were measured with a resolution of 0.5 ns and the pulse
height measurement had a resolution of approximately 8 mV. It is estimated that
the low-amplitude signals involved in the coincidences must respond to energy
absorptions of the order of 100 keV in the scintillator material.
