Energy loss of light ions (H+ and He+) in matter: high accuracy measurements and comparison ...

131 Pages·2017·4.33 MB·English

˜
UNIVERSIDADE DE SAO PAULO
´
INSTITUTO DE FISICA
Perda de energia de ´ıons leves (H+ and He+) na mate´ria:
medidas de alta acura´cia e comparac¸a˜o com o modelo de FEG
MSc. Marcos Vinicius Moro
Prof. Dr. Manfredo Harri Tabacniks (orientador)
Tese submetida em total cumprimento ao Insti-
tuto de F´ısica da Universidade de Sa˜o Paulo para
obtenc¸a˜odograudeDoutoremCieˆncias.
Bancajulgadora:
Prof. Dr. AlexandreA.P.Suaide(USP)
Profa. Dra. ClaudiaC.Montanari(UBAArgentina)
Prof. Dr. ManfredoH.Tabacniks(USP)
Prof. Dr. NilbertoH.Medina(USP)
Prof. Dr. PedroL.Grande(UFRGS)
Sa˜o Paulo, Brasil
– 2017 –

UNIVERSITY OF SAO PAULO
INSTITUTE OF PHYSICS
Energy loss of light ions (H+ and He+) in matter:
high accuracy measurements and comparison with the FEG model
MSc. Marcos Vinicius Moro
Prof. Dr. Manfredo Harri Tabacniks (advisor)
Thesis submitted in fulfillment to the Institute of
PhysicsoftheUniversityofSaoPaulotoobtainthe
degreeofPhDofScience.
ThesisCommittee:
Prof. Dr. AlexandreA.P.Suaide(USP)
Prof. Dr. ClaudiaC.Montanari(UBAArgentina)
Prof. Dr. ManfredoH.Tabacniks(USP)
Prof. Dr. NilbertoH.Medina(USP)
Prof. Dr. PedroL.Grande(UFRGS)
Sao Paulo, Brazil
– 2017 –

Tomymomanddadwhoalwaysbelievedinme.
TomyloveAlejandraSandoval.

Abstract
Thephenomenonofenergylossthatoccurswhenanioninteractswithmatter,alsocalledstopping
power,hasbeeninvestigatedformorethanacentury,andhasprovidedfindingsofinterest. However,
reliable procedures for obtaining accurate experimental measurements and a fully theoretical com-
prehension of the process are tasks still in high demand by the scientific community. Moreover,
stopping power data are prerequisites in several applications in modern science, such as engineering,
ion implantation and modification of materials, damage to electronics devices (e.g. space radiation),
medical physics (e.g. proton therapy), among others. In this thesis we i) develop a rigorous experi-
mental protocol to measure stopping power with high precision, and ii) investigate1 the collapse of
the free electron gas (FEG) model in energy loss of light ions (protons) at a low energy range in
transition and rare-earth metals. In the first part, we present an approach to obtain, with high accu-
racy, the stopping cross sections in the pure materials Al and Mo for protons in the energy range of
[0.9−3.6]MeVbymeansofthetransmissionmethod. Thetraceabilityofthesourcesofuncertainties
are fully evaluated and the final accuracy of the results is 0.63% (0.32% rand. and 0.54% syst.) for
Al,and1.5%(0.44%rand. and1.4%syst.) forMo,withbothresultsprimarilylimitedbythequality
and homogeneity of the stopping foils. For Al, this high accuracy represents an improvement com-
pared to the results obtained in previous studies and serves as a benchmark for our procedure. The
most important sources of uncertainties were random - the uncertainty in the peak positions and in
the Gaussian fits; and systematic - the non-uniformity thickness of the foils (a special procedure was
developed to correct this). Even though the final uncertainty for Mo is higher than for Al, our results
improve on the amount of data currently available for the energy range considered. Both data sets
are compared with the most commonly employed theoretical models and Monte Carlo codes in the
literature. In the second part, electronic stopping cross sections of nontrivial solids, that is, transition
andrareearthmetals(TaandGd)forslowprotonsareexperimentallyinvestigated,andthedatawere
compared with the results for Pt and Au, to understand how energy losses in these metals are cor-
related with electronic band structures, and to understand the failure of the FEG model predictions.
The higher stopping powers found for Ta and Gd cannot be explained by means of the FEG model;
however, these effects are successfully correlated with the high density of states (DOS) of both the
occupied and unoccupied electronic levels in these metals. For the case of Gd, the experimental data
are extended in the energy range until the Bragg’s peak is reached. The two parts of this thesis were
publishedinPhysicalReviewA93022704(2016),andinPhysicalReviewLetters18103401(2017),
respectively.
1AssuggestedbyProf.Dr.PeterBauerfromJohannesKeplerUniversity,Austria.
i

Resumo
Ofenoˆmenodeperdadeenergiaquandoum´ıoninteragecomamate´ria,tambe´mconhecidocomo
poder de freamento, vem sendo investigado por mais de um se´culo, gerando grandes descobertas.
Entretanto, conseguir obter medidas experimentais com alta precisa˜o, ou elaborar um completo en-
tendimentoteo´ricodosprocessosdeperdadeenergiasa˜otarefasextremamentedif´ıceiseaindamuito
requeridas pela comunidade cient´ıfica. Ale´m disso, dados de perda de energia sa˜o pre´-requisitos em
va´rias aplicac¸o˜es e ramos da cieˆncia moderna, tais como: engenharia, implantac¸a˜o e modificac¸a˜o de
materiais, danos em dispositivos eletroˆnicos (radiac¸a˜o espacial), f´ısica me´dica (pro´ton terapia), etc.
Esta tese tem dois focos: i) desenvolver um rigoroso protocolo experimental para medir stopping
power com alta precisa˜o e ii) investigar2 a quebra de validade do modelo de Ga´s de Ele´trons Livres
(FEG) para a perda de energia de pro´tons lentos em metais de transic¸a˜o e terra raras. Na primeira
parte apresentamos uma abordagem experimental para obter com alta precisa˜o o poder de freamento
em materiais puros (Al e Mo) para pro´tons no intervalo de energia de [0,9−3,6] MeV pelo me´todo
de transmissa˜o. A rastreabilidade das fontes de incerteza foi determinada e as incertezas finais en-
contradas foram: 0,63% (0,32% aleat. e 0,54% sist.) para Al e 1,5% (0,44% aleat. e 1,4% sist.)
paraMo,ambasdevidoaqualidadeehomogeneidadedasfolhasfreadoras. ParaAl,estaacura´ciarep-
resenta um avanc¸o comparado com publicac¸o˜es anteriores e, assim, serviu como uma refereˆncia de
nossoprocedimento. Asmaisimportantesfontesdeincertezaforam: aleato´riaincertezadasposic¸o˜es
dospicosedosajustesGaussianosesistema´ticana˜o-uniformidadedasfolhas-alvo(umprocedimento
foidesenvolvidoparacorrigirisso). EmboraaincertezafinaldoMoe´ umpoucomaiordoquedoAl,
nossosresultadosajudaramacomplementarabaixaquantidadededadosdispon´ıveisparaointervalo
de energia considerado. Ambos conjuntos de dados foram comparados com os mais comuns mod-
elos teo´ricos e co´digos de Monte Carlo na literatura. Para a segunda parte, poder de freamento em
metais na˜o ta˜o comuns tais como transic¸a˜o (Ta) e terras-raras (Gd) para pro´tons com baixas veloci-
dades foram experimentalmente investigados, e os dados comparados com resultados de Pt e Au, a
fimdeentendercomoostoppingpowerdestesmetaisesta´ correlacionadocomasestruturasdebandas
eletroˆnicas,eassimtentarexplicarafalhadomodelodeFEG.Osaltosvaloresdasperdasdeenergias
encontradas para Ta e Gd na˜o puderam ser explicadas pelo modelo de FEG, e portanto foram cor-
relacionadoscomadensidadedeestados(DOS)emambososn´ıveisocupadosena˜oocupadosdestes
metais. Para o caso do Gd, os dados experimentais foram estendidos em um intervalo de energia
ate´ alcanc¸arem o pico de Bragg. A primeira parte desta tese foi publicada na Physical Review A 93
022704(2016),easegundapartenaPhysicalReviewLetters18103401(2017).
2ComosugeridopeloProf.Dr.PeterBauerdaJohannesKeplerUniversity,A´ustria.
iii