instrumental effects on hed anisotropy measurements oskari saloniemi, srl workshop 14.8.2007
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Instrumental effects on HED anisotropy measurements
Oskari Saloniemi, SRL workshop 14.8.2007
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particle detector HEDBasic data products are:- proton spectra from 10-140 MeV with 1 minute time resolution.- directional distributions of protons (3-5 channels) and Helium (3-1 channels)- pulse height data from the heavier nuclei
- directional measurements provide THE best view of the particle flux intensity environment the detector was in. - Particle detectors count particles. It is the user’s responsibility to remove the instrument’s effects from the data to deduce the particle intensities- Could the directional measure- ments also give the best view of the instrument effects?
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Main instrument effects affecting directional measurements
• The (directional) geometric factor– function of energy (stopping layer) and direction
• broad energy channels or very steep spectrum induce spectral variations of geometric factor and geometrical midpoint of angular bins
• wide angular bins induce dynamic variations as a function of anisotropy
• On board software errors and rejection logics
• uppermost detection layer susceptible for double hits caused by high ”noise” from energetic electrons and protons.
• hardware failure
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ERNE/HED viewcone
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correction coefficient due to anisotropy
2nd of May 1998 Protons 17-22 MeV
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Binning error correctionparticles from sectors 1,7,13 and 19 are saved in the next sectors
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approach 1• Calculate the ”acceptance” of each directional bin from the data itself
-> sum up huge amounts of measurements from long enough periods and normalize the result to form a probability distribution.
• consistent anisotropy will be lost, but short term anisotropy should be unaffected.
• acceptance should contain all instrumental effects, since it is determined from the data itself.
• the first approach was to exclude all anisotropic measurements and to define acceptance to 90 different spectral indeces.-> decent results and one doctoral thesis
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approach 2
• Not all dynamic effects could be explained by the spectral differences.
• New acceptance matrices were formed as a function of total intensity in the anisotropy channels. All measurements were accepted.
• works well on most events.
• still used today.
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acceptance approach
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approach 3
• Acceptance matrices formed as a function of sample rate divider (SRD) value– SRD [1-128] is used by the hardware to restrict (divide) the
amount of lower priority (3) protons (and electrons) further in the data analysis.
– SRD can indicate situations where the instrument is susceptible to double hits, which eventually are suspected of causing the sometimes observed overload situation
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Solution
• define a total intensity correction coefficient for each ring.
• define 90 degree and 180 degree symmetry from the matrices and cancel them out with proper periodic functions.
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after the corrections
correction matrix factor of the previos twooriginal acceptance matrix
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after the corrections
correction matrix factor of the previos twooriginal acceptance matrix
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but OSS 2.50 (4.7.2001->)...
correction matrix factor of the previos twooriginal acceptance matrix
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but OSS 2.50 (4.7.2001->)...
correction matrix factor of the previos twooriginal acceptance matrix
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future work• instrumental response to high
background intensities through simulations.
• to study the instrument behaviour as a function of high background (SRD?) and spectra by factoring out a more realistic geometrical factor determined by the GEANT simulations from the acceptance matrices.
• proper acceptance determined for all periods of measurements
• What situations (spectral and intensity) most cause the phenomena seen in the instrument response?
• What is the nature of the response change for the underlying anisotropy? Can it be recovered using simple statistical correction factor?
open questions