Modelovanje gasnih detektora čestica visokih energija primenom tehnike elektronskih rojeva
Applications of electron swarm theory in modeling of gaseous particle detectors
Author
Bošnjaković, Danko V.Mentor
Petrović, Zoran Lj.
Committee members
Đorđević, AntonijeCvetić, Jovan
Marinković, Predrag
Dujko, Saša
Metadata
Show full item recordAbstract
Zahvaljujući svojim dobrim performansama i niskoj ceni po jedinici
zapremine, gasni detektori su najčešće korišćeni detektori u eksperimentalnoj fizici
visokih energija. Pored fizike visokih energija, ovi detektori nalaze primene u mnogim
drugim oblastima, poput dozimetrije i zaštite od zračenja, medicine, fizike kosmičkog
zračenja i geofizike. Postoje brojni modeli ovih uređaja. Bez obzira da li su analitički ili
numerički, stohastički ili deterministrički, svi ovi modeli koriste transportne i/ili sudarne
podatke za elektrone u gasovima. Nažalost, uobičajeno je da se unutar detektorske
zajednice, ovi podaci kao i tehnike za njihov proračun primenjuju nekritički. Ova
praksa, kao što je diskutovano u ovoj disertaciji, veoma često vodi ka pojednostavljenim
modelima detektora i neadekvatnoj metodologiji za pristup i analizu. Imajući ove
činjenice u vidu, osnovni cilj ove disertacije je da na osnovama transportne teorije
rojeva elektrona, ukaže na čitav spektar važnih aspekata u proračunu ...i implemetanciji
transportnih podataka u modelovanju kao i da na konkretnim modelima detektora
pokaže kako podaci i njihova implementacija utiču na izračunate signale i performanse
detektora. Osim toga, u ovom radu su detaljno analizirane netipične pojave u transportu
elektrona indukovane eksplicitnim efektima nekonzervativnih sudara i njihove
potencijalne implikacije u modelima.
Specifičnosti transporta elektrona u ukrštenom električnom i magnetskom polju,
ilustrovane su na primeru detektora tipa Time Projection Chamber (TPC) koji se koriste
za trodimenzionalnu rekonstrukciju putanje čestica. Kod ovih detektora magnetsko polje
ima ulogu u redukciji bočne difuzije od koje direktno zavisi prostorna rezolucija dok
npr. linearnost rekonstrukcije zavisi od osetljivosti brzine drifta na temperaturu i
nehomogenost magnetskog polja. U ovom kontekstu potencijalne optimizacije radnih
uslova, a takođe imajući u vidu i neželjene varijacije parametara tokom rada, sistematski
je razmotren uticaj električnog i magnetskog polja, temperature i pritiska gasa kao i
udela nečistoća u gasnoj smeši, na transportne osobine elektrona u TPC detektoru.
Posebna pažnja posvećena je detektoru sa pločastim elektrodama visoke
otpornosti (Resistive Plate Chamber, RPC) koji se koriste za timing i triggering u
brojnim eksperimentima fizike visokih energija kao i u drugim oblastima. Rešavanjem
Boltzmannove jednačine i primenom Monte Karlo tehnike, koju smo sa jedne strane
koristili da proverimo rezultate dobijene Boltzmannovom jednačinom, a sa druge strane
za dobijanje prostorno razloženih transportnih parametara elektrona, identifikovani su iobjašnjeni transportni fenomeni elektrona poput negativne diferencijalne provodnosti i
grejanja zahvatom elektrona koji su uočeni u gasnim smešama RPC detektora koji se
primenjuju na eksperimentima u CERN-u.
U ovoj disertaciji, razmotreni su teorijski principi klasičnog fluidnog modela i
modela zasnovanog na difuzionoj jednačini i hidrodinamičkoj aproksimaciji na osnovu
kojih je razvijen numerički 1.5-dimenizionalni fluidni model RPC detektora. Ovim
modelom, ispitivan je razvoj elektronske lavine i strimera pod dejstvom efekata
prostornog naelektrisanja i fotojonizacije u gasu. Pokazano je da nepravilna
implementacija transportnih podataka, zanemarivanjem eksplicitnih efektata
nekonzervativnih sudara, može dovesti do greške od nekoliko stotina procenata u
proračunu indukovanog signala. Razmatrana je i osetljivost izračunatog signala
detektora na promene setova preseka za rasejanje elektrona u individualnim gasovima
gasnih smeša.
Konačno, u ovoj disertaciji razvijen je i mikroskopski Monte Karlo model RPC
detektora koji se zasniva na praćenju pojedinačnih trajektorija elektrona i njihovih
sudara sa molekulima gasa. Pomoću ovog modela, ipitivana je stohastika elektronskog
lavinskog procesa. Takođe, koristeći različite setove preseka za rasejanje elektrona u
individualnim gasovima gasne smeše, izračunate su vremenska rezolucija i efikasnost
RPC detektora koje se dobro slažu sa eksperimentalnim vrednostima.
Owing to their good performance characteristics and low price per unit
volume, gaseous particle detectors remained the most commonly used detectors in high
energy physics experiments. In addition to high energy physics, these detectors have
also found applications in other fields such as radiation protection and dosimetry,
medicine, cosmic ray physics and geophysics. A number of methods to model particle
detectors have been developed. Being analytical or numerical, stochastic or
deterministic, detailed knowledge of electron swarm transport properties as well as
reliable cross sections for electron scattering are required as an input in modeling. The
highly applied nature of the field, has inevitably driven the modeling of these systems
more towards empiricism, and unfortunately often away from its roots in the
fundamental transport theory of electron swarms. Thus, the main goal of this work is to
bridge the gap between the fundamental transport theory of electron swarms and
application...s in the field of particle detectors. This goal is achieved by considering many
elements of the theory which are important for accurate calculation and correct
implementation of electron transport data in modeling. In addition, we provide the
examples of specific detector models in which the incorrect implementation of data
affects the calculated signal and detector performance characteristics. In this work, we
discuss atypical manifestation of electron transport phenomena induced by the explicit
effects of non-conservative collisions and potential implications arising from their
inclusion in the models.
The peculiarities of electron transport in crossed electric and magnetic fields are
illustrated using the example of Time Projection Chamber (TPC), a detector employed
for three-dimensional reconstruction of particle trajectories. In this detector, the
magnetic field suppresses the transverse diffusion, which directly affects the spatial
resolution. On the other hand, the reconstruction linearity depends on the sensitivity of
drift velocity on the gas temperature and non-uniformity of the magnetic field. With this
as one of the motivating factors, and also having in mind the unwanted variation of
detector parameters with time, we systematically study the influence of the electric and
magnetic fields, gas pressure and temperature, as well as the impact of impurities in the
gaseous mixture, on the electron transport properties in TPC.
Special attention is given to the Resistive Plate Chambers (RPCs) which are
used for timing and triggering purposes in many high energy physics experiments and elsewhere. The Boltzmann equation is used for the determination of electron swarm
transport properties under conditions when transport is greatly affected by nonconservative
collisions. A Monte Carlo simulation technique has been used with the aim
of verifying the results based on the Boltzmann equation as well as for the evaluation of
spatially resolved electron transport data. This segment of data is used to explain the
existence of certain kinetic phenomena, including negative differential conductivity and
attachment heating, which are important for the detector behavior.
Within the framework of the classical fluid model and using the diffusion
equation in association with the hydrodynamic approximation, we have developed a
numerical 1.5-dimension fluid model of an RPC. This model is used to study the
electron avalanche and streamer development under the influence of space charge
effects and photoionization. We have shown that improper use of the data, especially
the lack of correct representation of the explicit effects of non-conservative collisions,
can lead to errors of a several hundred percents for the calculated signals. In addition,
we discuss the sensitivity of the output detector signals with respect to the sets of cross
sections for electron scattering.
Finally, in this thesis we present our microscopic Monte Carlo model of RPC
based on the tracking of individual electron trajectories and their collisions with the gas
molecules. Using this model, we study the electron avalanche fluctuations and the
related processes. The detection efficiency and timing resolution are calculated using
different sets of cross sections for electron scattering. We have found that our results
agree very well with the measured data.