diff --git a/thesis/chapters/chap4_alcap_phys.tex b/thesis/chapters/chap4_alcap_phys.tex index a5bd278..0501d5b 100644 --- a/thesis/chapters/chap4_alcap_phys.tex +++ b/thesis/chapters/chap4_alcap_phys.tex @@ -347,18 +347,18 @@ $n_{avg} = (0.3 \pm 0.02)A^{1/3}$~\cite{Singer.1974}. The neutron emission can be explained by several mechanisms: \begin{enumerate} \item Direct emission follows reaction~\eqref{eq:mucap_proton}: these neutrons - have fairly high energy, from a few \si{\mega\electronvolt}~to as high as 40--50 - \si{\mega\electronvolt}. + have fairly high energy, from a few \si{\si{\MeV}}~to as high as 40--50 + \si{\si{\MeV}}. \item Indirect emission through an intermediate compound nucleus: the energy transferred to the neutron in the process~\eqref{eq:mucap_proton} is 5.2 - \si{\mega\electronvolt} if the initial proton is at rest, in nuclear + \si{\si{\MeV}} if the initial proton is at rest, in nuclear environment, protons have a finite momentum distribution, therefore the mean excitation energy of the daughter nucleus is around 15 to 20 - \si{\mega\electronvolt}~\cite{Mukhopadhyay.1977}. This is above the nucleon + \si{\si{\MeV}}~\cite{Mukhopadhyay.1977}. This is above the nucleon emission threshold in all complex nuclei, thus the daughter nucleus can de-excite by emitting one or more neutrons. In some actinide nuclei, that excitation energy might trigger fission reactions. The energy of indirect - neutrons are mainly in the lower range $E_n \le 10$ \si{\mega\electronvolt} + neutrons are mainly in the lower range $E_n \le 10$ \si{\si{\MeV}} with characteristically exponential shape of evaporation process. On top of that are prominent lines might appear where giant resonances occur. \end{enumerate} @@ -382,7 +382,7 @@ data. There are two reasons for that: neutron emission. The rate is about 15\% for light nuclei and reduces to a few percent for medium and heavy nuclei. \item The charged particles are short ranged: the emitted protons, - deuterons and alphas are typically low energy (2--20~\mega\electronvolt). + deuterons and alphas are typically low energy ( \SIrange{2}{20}{\MeV}). But a relatively thick target is normally needed in order to achieve a reasonable muon stopping rate and charged particle statistics. Therefore, emulsion technique is particularly powerful. @@ -411,9 +411,9 @@ statistics and in fair agreement with Morigana and Fry Protons with higher energy are technically easier to measure, but because of the much lower rate, they can only be studied at meson facilities. Krane and colleagues~\cite{KraneSharma.etal.1979} measured proton emission from -aluminium, copper and lead in the energy range above 40 \mega\electronvolt~and +aluminium, copper and lead in the energy range above \SI{40}{\MeV} and found a consistent exponential shape in all targets. The integrated yields -above 40 \mega\electronvolt~are in the \sn{}{-4}--\sn{}{-3} range (see +above \SI{40}{\MeV} are in the \sn{}{-4}--\sn{}{-3} range (see Table~\ref{tab:krane_proton_rate}), a minor contribution to total proton emission rate. \begin{table}[htb] @@ -462,16 +462,16 @@ The aforementioned difficulties in charged particle measurements could be solved using an active target, just like nuclear emulsion. Sobottka and Wills~\cite{SobottkaWills.1968} took this approach when using a Si(Li) detector to stop muons. They obtained a spectrum of charged particles up to 26 -\mega\electronvolt~in Figure~\ref{fig:sobottka_spec}. The peak below 1.4 -\mega\electronvolt~is due to the recoiling $^{27}$Al. The higher energy events +\si{\MeV}~in Figure~\ref{fig:sobottka_spec}. The peak below 1.4 +\si{\MeV}~is due to the recoiling $^{27}$Al. The higher energy events including protons, deuterons and alphas constitute $(15\pm 2)\%$ of capture events, which is consistent with a rate of $(12.9\pm1.4)\%$ from gelatine observed by Morigana and Fry. This part has an exponential -decay shape with a decay constant of 4.6 \mega\electronvolt. Measday +decay shape with a decay constant of 4.6 \si{\MeV}. Measday noted~\cite{Measday.2001} the fractions of events in -the 26--32 \mega\electronvolt~range being 0.3\%, and above 32 -\mega\electronvolt~range being 0.15\%. This figure is in agreement with the -integrated yield above 40 \mega\electronvolt~from Krane et al. +the 26--32 \si{\MeV}~range being 0.3\%, and above 32 +\si{\MeV}~range being 0.15\%. This figure is in agreement with the +integrated yield above 40 \si{\MeV}~from Krane et al. In principle, the active target technique could be applied to other material such as germanium, sodium iodine, caesium iodine, and other scintillation @@ -480,7 +480,7 @@ identification like in nuclear emulsion, the best one can achieve after all corrections is a sum of all charged particles. It should be noted here deuterons can contribute significantly, Budyashov et al.~\cite{BudyashovZinov.etal.1971} found deuteron components to be -$(34\pm2)\%$ of the charged particle yield above 18 \mega\electronvolt~in +$(34\pm2)\%$ of the charged particle yield above 18 \si{\MeV}~in silicon, and $(17\pm4)\%$ in copper. \begin{figure}[htb] \centering @@ -547,7 +547,7 @@ protons were taken. Wyttenbach et al.\ saw that the cross section of each reaction decreases exponentially with increasing Coulomb barrier. The decay constant for all -$(\mu^-,\nu pxn)$ is about 1.5 per \mega\electronvolt~of Coulomb barrier. They +$(\mu^-,\nu pxn)$ is about 1.5 per \si{\MeV}~of Coulomb barrier. They also commented a ratio for different de-excitation channels: \begin{equation} (\mu^-,\nu p):(\mu^-,\nu pn):(\mu^-,\nu p2n):(\mu^-,\nu p3n) = 1:6:4:4, @@ -572,7 +572,7 @@ nucleus is formed, and then it releases energy by statistical emission of various particles. Three models for momentum distribution of protons in the nucleus were used: (I) the Chew-Goldberger distribution $\rho(p) \sim A/(B^2 + p^2)^2$; (II) Fermi gas at zero temperature; and (III) -Fermi gas at a finite temperature ($kT = 9$ \mega\electronvolt). +Fermi gas at a finite temperature ($kT = 9$ \si{\MeV}). A very good agreement with the experimental result for the alpha emission was obtained with distribution (III), both in the absolute percentage and the energy @@ -598,7 +598,7 @@ the nucleon, the average excitation energy will increase, but the proton emission rate does not significantly improve and still could not explain the large discrepancy. He concluded that the evaporation mechanism can account for only a small fraction of emitted protons. Moreover, the high energy protons -of 25--50 \mega\electronvolt~cannot be explained by the evaporation mechanism. +of 25--50 \si{\MeV}~cannot be explained by the evaporation mechanism. He and Lifshitz~\cite{LifshitzSinger.1978, LifshitzSinger.1980} proposed two major corrections to Ishii's model: \begin{enumerate} @@ -611,14 +611,14 @@ major corrections to Ishii's model: is possibility for particles to escape from the nucleus. \end{enumerate} With these improvements, the calculated proton spectrum agreed reasonably with -data from Morigana and Fry in the energy range $E_p \le 30$ \mega\electronvolt. +data from Morigana and Fry in the energy range $E_p \le 30$ \si{\MeV}. Lifshitz and Singer noted the pre-equilibrium emission is more important for heavy nuclei. Its contribution in light nuclei is about a few percent, increasing to several tens of percent for $100