CRWriteup

Radio Observation of UHE Cosmic Rays • June 2015

Radio Observation of Ultra HighEnergy Cosmic Rays

John S. Conway, Scott A. McIntosh

University of California, Davis

Abstract

Cosmic ray air showers are created when high energy subatomic particles collide with other particles inEarth’s upper atmosphere. As the air showers propagate through the Earth’s magnetic field, radio frequencywaves are emitted. In this experiment we devised several arrays of RF antennas in order to observe EASsignals. We determined that the raw signal is too noisy and added several methods of noise reduction,ultimately concluding that the event rate of ultra-high energy EAS are very low.

I. Introduction

Earth’s upper atmosphere is constantlybombarded by ultra high energy sub-atomic particles traveling through space

known as cosmic rays. Particles at the lowerend of the energy spectrum are believed tooriginate in distant deep space objects likequasars and the remnants of supernovae, ter-minating in massively energetic collisions withgas atoms in our atmosphere, while the originof the most energetic particles still remains amystery[1].

During these collisions, the particles inter-act to create new particles which in turn collidewith other nearby atmospheric gas atoms, in acascading process. This chain reaction resultsin what is known as an Extensive Air Shower(EAS), which is essentially a cascade of chargedparticles.

Large area particle detector arrays, such asthe Pierre Auger Observatory on the Argen-tinian Pampas have observed showers up tothe highest energies[2]. The Auger Observa-tory uses a novel approach to observing EASby detecting radio wave emissions with sim-ple antennas that can easily be constructed atrelatively small cost.

The greatest challenge in this experimentarises in the fact that the more energetic theshower is, the less frequently it is likely to oc-

cur. At the upper end of the energy spectrum,cosmic ray particles arrive at a rate of once persquare kilometer per century[3].

The purpose of this experiment was to usedeploy an array of radio antennae in order topick up the RF signal created by the chargedparticles in an EAS as they travel throughEarth’s magnetic field.

II. Methods

Two arrays of three antennas were constructed,one consisting of spiral antenna and one con-sisting of "bowtie" style dipoles.

Initially we tried a Log Periodic Dipole ar-ray. One such antenna was constructed andwas determined to be too cumbersome dueto the size of the apparatus, and not effectiveenough at picking up signal.

The next attempt was with "inverted-V"type dipoles, but there was simply not enoughresolution on the acquired signal, so they weremodified into the "bowtie" style dipoles.

The dipoles were constructed of copper tub-ing that had been crimped and screwed into abowtie shape and mounted on wooden stands.The antennas were designed to have their fre-quency response centered around 70MHz, andthe ends of the antenna elements were con-nected to 1:1 baluns for impedance matchingpurposes, and fed to a BNC terminal. Figure 1

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Radio Observation of UHE Cosmic Rays • June 2015

below shows a photograph of one antenna:

figure 1: bowtie style dipole antenna

The three antennas were spaced approxi-mately 10 meters apart on the roof of the UCDavis physics building, and connected with50Ω coaxial cable to a GHz-sampling oscillo-scope and subsequently a digitizer set up ina laboratory on the third floor. The distribu-tion of antennas in space is shown in figure 2below:

figure 2: antenna distribution

Data was collected for several weeks withno promising results. It was determined thatthere was simply too much noise in the signal,which was believed to be coming from localDoppler radar stations and other terrestrialsources.

At this stage we determined that the radi-ation pattern of the spiral antenna would bemuch better suited to our needs, as it obtainssignal most effectively from every directionother than the plane of the spiral, effectively"nulling the horizon."

The spirals were constructed using12 gauge copper wire wound into anArchimedean spiral on wooden spars con-nected to a wooden plate and resting on topof a wooden base. The spirals were designedto have their frequency response centered atapproximately 70MHz, and the ends were con-nected to a 4:1 Guanella balun for impedancematching purposes, and fed to a BNC terminal.The inner and outer radii of the spiral weredetermined using the following equation:

f =c

2πR(1)

Where c is the speed of light, f is the fre-quency, and R is the spiral radius.

Figure 3 below shows a photograph of oneantenna:

figure 3: Archimedean spiral antenna

The antennas were again spaced approxi-mately 10 meters apart and terminated to thesame electronics readout setup as before.

Data was again collected for several weeks,and again determined to be too noisy. By exam-ining the signal with a spectrum analyzer, wewere able to determine that a bulk of the noisewas in the range of approximately 88MHz to108MHz, and thus FM radio signal.

Three π-section band-pass filters were con-structed to reduce low and high frequencynoise as much as possible. The normalizedfrequency response of the three filters is givenin figure 4 below:

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Radio Observation of UHE Cosmic Rays • June 2015

figure 4: normalized frequency response ofπ-section filters

Several more weeks of data was collectedand again determined to be too noisy. Since thenoise was intermittent and not consistent forall three antennas, we believed that it had beencoming from inside the physics building itself.In order to reduce this problem, ground planeswere constructed using large square sectionsof copper mesh placed under the bases of eachantenna and connected with copper wire to theground casing of the coaxial wire.

We again took data for another severalweeks and were still seeing a lot of noise withrelatively few signals that looked like possiblecosmic ray events. We decided to switch backto the original dipole antennas with the addi-tion of the ground planes and π-section filtersto examine the effect that these noise reductiondevices would have on the signal picked up bythem.

III. Results

A Python script was written to sort and vi-sualize the acquired data. The waveform ac-quired by each antenna was plotted individu-ally, along with the signal envelope from allthree antennas. The envelope shows how muchtotal signal was acquired over time by each an-tenna, which allows us to see the simultaneityof the event.

A cosmic ray event should consist of a fairlyshort pulse, that appears approximately identi-cal on all three channels in both time and am-

plitude, with the rising edge of the envelopesapproximately overlapping.

Shown below is an example of a waveformacquisition:

figure 5: possible cosmic ray waveform

As shown, the alignment of the rising edgeof the envelope is approximately overlappingon all three channels. The signal looks approxi-mately the same on all three channels, howeverthe signals are not aligned in time. This leadsus to believe that the signal is terrestrial inorigin.

Below is another possible cosmic ray wave-form:

figure 6: possible cosmic ray waveform

As shown, the signals are approximatelyaligned in time, and are very short lived pulses.Their envelopes are similar, however the signalon channel 2 is slightly louder than the othertwo channels. This signal could potentiallybe a cosmic ray event, but it is not altogetherconvincing.

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Radio Observation of UHE Cosmic Rays • June 2015

By far the most convincing acquisition weobtained is shown in figure 7 below:

figure 7: probable cosmic ray waveform

The signal is a very short lived burst, ap-proximately the same on all three channels, andthe envelopes are roughly overlapping. This iswhat we believe is what we believe a cosmicrays signal should look like, and the only suchsignal we saw in ten weeks of data collection.

IV. Discussion

By experimenting with arrays of several types,we have attempted to find a suitable style of an-tenna for observing radio signals from cosmicray air showers.

We have shown that we need to employnoise reduction strategies such as groundplanes and band-pass filters. Even with thesemeasures in place, there is still a great deal ofnoise. A suggested improvement to the experi-ment would be to move it to an area with lessRF noise, as the UC Davis physics building hasa lot of extraneous radio noise.

Since we only saw one incident of a veryconvincing cosmic ray signal in ten weeks ofdata acquisition, we have confirmed that theevent rate of these events is very low, which isin agreement with what we expected.

References

[1] H. Falcke. Radio Detection of Ultra HighEnergy Cosmic Rays. ArXiv Astro physicse-prints, arXiv:0804.0548v1, 2008.

[2] P. Abreau et. al. Antennas for the De-tection of Radio Emission Pulses fromCosmic-Ray induced Air Showers at thePierre Auger Observatory. ArXiv Astrophysics e-prints, arXiv:1209.3840v1, 2012.

[3] P. Gorham. Radio Detection of Ultra-high Energy Cosmic Rays and Neutri-nos. SLAC Summer Institute on ParticlePhysics (SSI04), Aug. 2-13, 2004

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