Mail Archives: geda-user/2017/07/04/16:56:09
"John Griessen (john AT ecosensory DOT com) [via geda-user AT delorie DOT com]"
<geda-user AT delorie DOT com> writes:
> On 07/03/2017 02:57 PM, Stephan Böttcher wrote:
>> No, sorry. What I have is for the RPi1. I hope to launch it on a
>> balloon into the stratosphere next week:
>>
>> http://www.ieap.uni-kiel.de/et/people/stephan/rpirena/
>
> "The scatterplot shows the pulse height of channel 2 vs channel 3, the
> histograms show cosmic muon spectra in those channels."
>
> So is the histogram chart x axis nanometers wavelength?
No, these are approximate mV pulse height of the shaper outputs. I did
not calibrate the energy scale. The pulse height is proportional to the
energy deposited in the silicon detectors by ionization.
The red dots are hits by muons, which are generated as secondary
particles from cosmic radiation in about 20km height above ground.
Muons have a half-live of a few microseconds, but travel close to the
speed of light, so in their reference frame the distance to ground is
compressed to a few meters. Muons pass through matter without cascading
into secondary particles, they just loose energy by ionization like any
particle with charge 1 at the speed of light (Minimum Ionizing
Particles, MIPs). That is how they reach ground. All other particles
cascade into showers of particles until the energy is distributed so
much that they get stopped, well above ground. Muons eventually decay
into an electrons and two neutrinos.
In these silicon detectors with thickness 300µm, muons loose on average
115 keV at vertical incidence. That energy is converted into electron
hole pairs inside the fully depleted silicon diode, 1 pair requires
3.6eV. The muons produce about 30000 electrons. The electronics
amplifies this charge pulse to a voltage pulse, which is sampled at
3MSPS and analysed by a digital filter.
The muon hits are identified by the fact that both detectors are hit at
the same time. Most of the other hits are also muons which miss the
other detector. Other hits may be due to x-rays or Compton scattering
from gamma rays from the environment.
The peaks in the spectra show the distribution of the energy deposits of
the muons. This is called the Landau distribution, here folded with the
path-length distribution due to angle of incidence, and background.
If you project just the red dots to either axis, a cleaner Landau
distribution results, with less background and less path length
variations.
> If so, what does the spike at 0 with zero notch at 10 or 20 mean?
The spike at zero is the electronic noise measured when the other
detector was hit.
The notch is the gap between the noise and the trigger threshold. The
pulse heights are sampled from the digital filter outputs when either
channel has a peak of more than 15mV.
> Is some of the energy really at effective "wavelengths" below 10 nm?
>
> If the scale is in nanometers, the slope up towards 10nm is more and
> more UV light. Would that be coming from ionized air inside the dark
> box? Ionized traces of outgassing circuit materials?
That slope can be the tail of the noise or indeed some x-ray
background. It looks like the tail of the noise to me.
> Is the gamma ray energy all in the high high frequency spike at or
> near zero?
Gamma rays are difficult to measure in silicon detectors. You need a
heavy detector with high-Z atoms inside to completely convert a gammy
photon into ionizing energy loss. The big box in that photo contained
at that time a CsI salt crystal, which can absorb gammay rays, and
converts the ionization enrgy into flashes of light. This is called a
scintillator. The light can be measured with the same kind of silicon
photodiodes. The calibration is not as easy, since the light yield
depends on temperature and doping of the scintillator, on the shaping
time of the electronicsm and the resulting balistic deficit, which is
the light that comes to late to be integrated by the shapers. The time
profile of the light emission depends on temperature as well.
In silicon, the gamma rays can be seen by Compton scattering. The gamma
kicks an electron, which then deposits part of the gammas energy in the
detector. The amount of energy depends on the elastic scattering angle
of the photon, up to a maximum when the photon is scattered 180°. That
maximum can sometimes be measured, when monoenergeic gamma rays from a
radioactive source are used. This is more difficult if the Comptom
electrons can escape the detector with some of the energy unseen.
Too much information?
Gruß,
--
Stephan
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