The DECO team at the Wisconsin IceCube Particle Astrophysics Center (WIPAC) has published a second paper entitled Measurement of camera image sensor depletion thickness with cosmic rays. This paper was accepted by the Journal of Instrumentation. The full text of the paper can be found here. The abstract is below:
Camera image sensors can be used to detect ionizing radiation in addition to optical photons. In particular, cosmic-ray muons are detected as long, straight tracks passing through multiple pixels. The distribution of track lengths can be related to the thickness of the active (depleted) region of the camera image sensor through the known angular distribution of muons at sea level. We use a sample of cosmic-ray muon tracks recorded by the Distributed Electronic Cosmic-ray Observatory to measure the thickness of the depletion region of the camera image sensor in a commercial smart phone, the HTC Wildfire S. The track length distribution prefers a cosmic-ray muon angular distribution over an isotropic distribution. Allowing either distribution, we measure the depletion thickness to be between 13.9~μm and 27.7~μm. The same method can be applied to additional models of image sensor. Once measured, the thickness can be used to convert track length to incident polar angle on a per-event basis. Combined with a determination of the incident azimuthal angle directly from the track orientation in the sensor plane, this enables direction reconstruction of individual cosmic-ray events.
The DECO team at the Wisconsin IceCube Particle Astrophysics Center (WIPAC) has published a paper entitled Detecting particles with cell phones: the Distributed Electronic Cosmic-ray Observatory and is presenting at International Cosmic Ray Conference (ICRC) at The Hague, Netherlands on August 6, 2015 (webpage). The full paper can be found here. The abstract is below:
In 2014 the number of active cell phones worldwide for the first time surpassed the number of humans. Cell phone camera quality and onboard processing power (both CPU and GPU) continue to improve rapidly. In addition to their primary purpose of detecting photons, camera image sensors on cell phones and other ubiquitous devices such as tablets, laptops and digital cameras can detect ionizing radiation produced by cosmic rays and radioactive decays. While cosmic rays have long been understood and characterized as a nuisance in astronomical cameras, they can also be identified as a signal in idle camera image sensors. We present the Distributed Electronic Cosmic-ray Observatory (DECO), a platform for outreach and education as well as for citizen science. Consisting of an app and associated database and web site, DECO harnesses the power of distributed camera image sensors for cosmic-ray detection.
As we have discussed in our talks and other blog posts, noise is a large factor in creating events, and we don’t want noise, whether it be thermal, the most common, or magnetic. One of the great things about being able to actually see event images on a computer is that an event created by noise is easy to identify based off the pixels seen in the image. For example, events created by noise rarely have a small group of pixels at a higher brightness or intensity in comparison to the other pixels in the image. Events created by cosmic rays have a grouping of pixels that is significantly brighter than the surrounding pixels. We can see these clusters of pixels by altering the color range of the image. Because the Charged-couple devices (CCDs) in the cameras only use red, green(2), and blue, all we have to do is distort the color range so that those colors are more pronounced. If the color range of an image is reduced to all green, it is easier to spot the bright spot(s) in the image. If you don’t reduce the color range, the image is black and the brightened pixels are almost impossible to see. Now it’s just a process of filtering out events created by noise.
Over the past few months, many queries have been conducted and many numbers recorded in regards to pixel intensity. A simple way to eliminate events created by noise is to come up with a set minimum pixel intensity per image. While this method may not eliminate all events created by noise–and very well could eliminate lower energy cosmic ray particles–it will significantly reduce the volume of events created which makes analysis much easier. There is also a higher certainty that the events registered are cosmic rays because the filter will only show high-energy events. Determining the minimum pixel intensity is the current focus of the project on a broader scale.
In terms of the tests run in Wyoming to determine if there was a relationship between altitude and flux, please visit this section of our wiki page.
Here is our set-up for running the aforementioned Cobalt-60 tests with the DECO App:
Testing the DECO app with Cobalt-60 (emmiting gamma rays) and a HTC Wildfire.
We will run with this set-up (source is 10cm away from the phone) until we get 1,000 events.
One of our projects, DECO, has made a lot of progress since our last post in April. For those of you who don’t know what DECO (Distributed Electronic Cosmic-ray Observatory) is, the quick and dirty summary of the project involves the use of Android phones to detect cosmic rays–byproducts of supernovae, black holes, and other high-energy astronomical phenomena–using a CCD in the phones’ camera.
Over the summer, Veronica Hoyo, an undergrad at Haverford College, tested the application used to detect and collect data while placing the phones at various angles. Her results varied based on the angle. The scientific poster can be found here and the raw data here. Because of these results, we have been testing to see if there is a direct correlation between tilt angle and frequency.
We have found that there is a connection between cos^2θ and frequency. There is some constant the effects the frequency of events. To solve for this constant, work in the next coming weeks will involve using a Cobalt-60 gamma-ray emitter to create “fake” events to speed up testing.
Justin Vandenbroucke, a post-doctoral at Stanford University, has also done tests as of late underground (-16meters), in Italy (+800ft), and on an airplane with a cruising altitude of 36,000ft. Below are his results concerning event rate:
1) underground: 2.07 +/- 0.39
2) 800 feet altitude: 7.34 +/ 1.60
3) ~36,000 feet altitude (combining the two flights): 51.4 +/- 10.3
This is great because there should be a higher concentration of cosmic rays closer to space.
Until next time…
-The DECO Team