Comparison of Triple Coincidence Rate of Cosmic Ray Activity for CROP Detectors to Geiger-Mueller Rate

 

 

 

 

Cosmic Ray Observatory Project

January 2003

AJ Killeen, Mike Lundgren and Ben Swiniarski

 

 

 

 

 

                Cosmic rays are extremely high energy particles, usually protons, that crash into Earth’s atmosphere. The energetic collisions produce a shower of particles. By the time this shower reaches the surface of the earth, the particles are mostly muons, electrons and positrons. These are the fragmentary remnants of the original collision. 

                To see cosmic ray showers you need a detector.  The Cosmic Ray Observatory Project (CROP) detectors are plastic panels doped with a scintillating material called a fluor that causes the plastic to light up whenever a charged particle passes through it.  A photomultiplier tube (PMT) mounted on top of the plastic panels detects the light. The CROP panels measure 60 cm by 60 cm by 2 cm (thick).

                Geiger-Mueller tubes also can detect the shower of cosmic ray particles. They were often used in early cosmic ray experiments. However in addition to beta rays which are electrons and positrons, Geiger-Mueller tubes also detect alpha particles, which are helium nuclei, and gamma rays, which are photons. In this experiment, we were looking for a correlation between cosmic ray activity measured by the CROP detectors and alpha, beta, and gamma ray activity measured by a Geiger-Mueller tube.

                In this experiment the CROP detectors are stacked vertically in a telescope (Diagram 1).

Diagram 1

 

The three detectors are each separated by a distance of 60 cm. A charged particle passing through the panel causes a flash of light. The flash of light, caused by the high energy particle, is then turned into an electrical pulse by the PMT.  Those signals are then sent to a discriminator.

                The discriminator counts only the electric pulses that are above a certain “threshold voltage”.  “Threshold voltage” is determined by a procedure which counts only those pulses that produce a signal above a given voltage. The procedure helps to reduce “background noise”. Before starting the experiment, we tested the panels until we arrived at a voltage which counted only those particles that were most likely caused by cosmic rays.  

                From the discriminator, the pulses are sent to the logic unit. The logic unit allows a count to be  recorded only if the signal from each of the three detectors happen within a time of 100 nanoseconds.  If a triple coincidence occurs, a signal is sent to a visual scalar.  The scalar is just a counting device that increments each time it receives a signal.

 

Diagram 2

                We began measuring triple coincidences in September 2002 and continued until December 2002. Each measurement counted the number of triple coincidences in 30 minutes. The triple coincidences histogram shows two peaks, one at about 50 counts and another at 120 counts (Diagram 2). The bin size is 10, roughly equal to the square root of the most frequent count rate. The larger count rate peak results from measurements early in the experiment. The lower count rate peak was the result of measurements later in the experiment. When the effect was noticed, the count rate for each detector  was individually checked against the initial count rates recorded while setting the threshold for the detectors. All three detectors showed lower count rates, but we do not know why.

                The triple coincidence distribution is scattered, with measurements above 120 in irregular patterns. While this may be actual cosmic ray activity, it seems more likely that our detectors are not as consistent as we would like.

                At the same time that we were measuring cosmic ray activity with our detectors, we were recording radio activity with a Geiger-Mueller tube. The Geiger counter was invented in 1928 by H. Geiger and E.W. Mueller.  The Geiger counter detects radioactivity in the environment.  It sees alpha, beta, and gamma rays.  Alpha rays are the nuclei of helium atoms. They have a net positive charge.  They have a weak penetrating power because of their larger size.  They can be blocked by a few pieces of paper.  Beta rays are electrons or positrons. Electrons have a negative charge. Positrons have a positive charge. Beta rays can penetrate three millimeters of aluminum.  Gamma rays are high energy photons.  These have the greatest penetrating power.  They can pass through several centimeters of lead.             


                The Geiger-Mueller tube is a cylinder filled with an inert gas. Down the axis of the tube runs a wire held at a large electrical voltage.  At the end of the tube, a thin mica window allows in lower energy rays that cannot penetrate the side of the counter to be detected and counted. When an energetic particle passes through the gas in the tube, a current passes down the wire and a count is registered. The number of counts per unit time measures the intensity of the radiation.  The energy of the particles detected by the Geiger-Mueller tube is measured in electron volts(eV).  An electron volt is a unit of energy equal to the work required to move one electron through a potential difference of one volt.  One electron volt is equal to 1.6 x 10-19 Joules.  One Joule is equal to 6.2 x 1018 eV.

                The second graph, a Geiger rate histogram shows that the peak of the counts was at about

 

Diagram 3

480 counts (Diagram 3). From the peak, the count rate decreases in roughly a bell curve pattern on both sides from the peak. The bin size is determined approximately by the square root of the most frequently occuring count rate. In this case, the most frequent count is 480, and the bin size is 20. The average count is 460. The histogram shows that the Geiger rate distribution is less spread than the triple coincidence rate. A possible reason is that cosmic ray activity is inherently more random than radio activity. The other possible reason is that our detectors are not as reliable and efficient as the Geiger counters.                                                               

                To see if there is any relation between the triple coincidence rate and the Geiger counter rate, the former was plotted on the vertical axis, and the latter on the horizontal axis (Diagram 4).

Diagram 4

There are two distinct bands of points, one below 70 triple coincidence counts and one above 70. The upper band represents counts that were made early on in the experiment, when the equipment was more sensitive. As the experiment went on, something happened, and the count rate suddenly dropped from an average of 110 counts to an average of about 40 counts per 30 minutes. In either event, the generally horizontal line formed by the two bands suggests cosmic ray activity is largely independent of radioactivity.                

 

 

 

Appendix

Specifications for the Geiger counter:

Sensor:  LND 712 halogen-quenched Geiger-Mueller tube with a mica window, 1.5 to  2.0        mg/cm2 thick.  It is rated at 1000 counts per minute using a Cesium-137 laboratory standard.

Power:  One 9-volt alkaline battery gives battery life of 2000 hours under normal conditions.

Accuracy:  Noninstrumental aligned: about plus or minus 20 percent of full scale.  Instrumental aligned: about plus or minus 10 percent of full scale.

Energy Sensitivity:  Alpha: Down to 2.5 MeV; typical efficiency at 3.6 MeV is greater than 80   percent.  Beta: 50 KeV, typical 35 percent detection efficiency.  Gamma and X-rays: down to 10   KeV typical through the mica window,  40 KeV through the case.

Audio Output:  Audio switch allows audible indication of each count.

Temperature Range: 0 to 50 degrees Celsius.