1 | |
---|
2 | \chapter{Abrasion-ablation Model} |
---|
3 | |
---|
4 | \section{Introduction} |
---|
5 | \label{Intro} |
---|
6 | The abrasion model is a simplified macroscopic model for nuclear-nuclear |
---|
7 | interactions based largely on geometric arguments rather than detailed |
---|
8 | consideration of nucleon-nucleon collisions. As such the speed of the |
---|
9 | simulation is found to be faster than models such as G4BinaryCascade, |
---|
10 | but at the cost of accuracy. The version of the model implemented is |
---|
11 | interpreted from the so-called abrasion-ablation model described by Wilson |
---|
12 | {\normalsize\it{}et al} \cite{aaWilson},\cite{aaTownsend} together with an |
---|
13 | algorithm from Cucinotta to approximate the secondary nucleon energy |
---|
14 | spectrum \cite{aaCucinotta}. By default, instead of performing an ablation |
---|
15 | process to simulate the de-excitation of the nuclear pre-fragments, the Geant4 |
---|
16 | implementation of the abrasion model makes use of existing and more detailed |
---|
17 | nuclear de-excitation models within Geant4 (G4Evaporation, G4FermiBreakup, |
---|
18 | G4StatMF) to perform this function (see section \ref{deexcitation}). However, |
---|
19 | in some cases cross sections for the production of fragments with large |
---|
20 | $\Delta$A from the pre-abrasion nucleus are more accurately determined using a |
---|
21 | Geant4 implementation of the ablation model (see section \ref{ablation}). |
---|
22 | |
---|
23 | |
---|
24 | \noindent The abrasion interaction is the initial fast process in which the |
---|
25 | overlap region between the projectile and target nuclei is sheered-off (see |
---|
26 | figure \ref{fig:1}) The spectator nucleons in the projectile are assumed to |
---|
27 | undergo little change in momentum, and likewise for the spectators in the |
---|
28 | target nucleus. Some of the nucleons in the overlap region do suffer a change |
---|
29 | in momentum, and are assumed to be part of the original nucleus which then |
---|
30 | undergoes de-excitation. |
---|
31 | |
---|
32 | |
---|
33 | \noindent Less central impacts give rise to an overlap region in which the |
---|
34 | nucleons can suffer significant momentum change, and zones in the projectile |
---|
35 | and target outside of the overlap where the nucleons are considered as |
---|
36 | spectators to the initial energetic interaction. |
---|
37 | |
---|
38 | |
---|
39 | \noindent The initial description of the interaction must, however, take into |
---|
40 | consideration changes in the direction of the projectile and target nuclei due |
---|
41 | to Coulomb effects, which can then modify the distance of closest approach |
---|
42 | compared with the initial impact parameter. Such effects can be important for |
---|
43 | low-energy collisions. |
---|
44 | |
---|
45 | |
---|
46 | \section{Initial nuclear dynamics and impact parameter} |
---|
47 | \label{InitDyn} |
---|
48 | For low-energy collisions, we must consider the deflection of the nuclei as a |
---|
49 | result of the Coulomb force (see figure \ref{fig:2}). Since the dynamics are |
---|
50 | non-relativistic, the motion is governed by the conservation of energy |
---|
51 | equation: |
---|
52 | |
---|
53 | \begin{equation} |
---|
54 | E_{tot} = \frac{1}{2}\mu \dot r^2 + \frac{{l^2 }}{{2\mu r^2 }} + \frac{{Z_P Z_T e^2 }}{r} |
---|
55 | \label{eqn1} |
---|
56 | \end{equation} |
---|
57 | %Equation ??.1 |
---|
58 | |
---|
59 | \noindent where: |
---|
60 | |
---|
61 | \(E_{tot}\) $\>$ = total energy in the centre of mass frame; |
---|
62 | |
---|
63 | \(r\),\(\dot r\) $\>$ = distance between nuclei, and rate of change of distance; |
---|
64 | |
---|
65 | \(l\) $\>$ = angular momentum; |
---|
66 | |
---|
67 | \(\mu\) $\>$ = reduced mass of system {\normalsize\it{}i.e.} \(m_1m_2/(m_1+m_2)\); |
---|
68 | |
---|
69 | \(e\) $\>$ = electric charge (units dependent upon the units for \(E_{tot}\) and \(r\)); |
---|
70 | |
---|
71 | \(Z_P\), \(Z_T\) $\>$ = charge numbers for the projectile and target nuclei. |
---|
72 | |
---|
73 | \noindent The angular momentum is based on the impact parameter between the |
---|
74 | nuclei when their separation is large, {\normalsize\it{}i.e.} |
---|
75 | |
---|
76 | \begin{equation} |
---|
77 | E_{tot} = \frac{1}{2}\frac{{l^2 }}{{\mu b^2 }}\; \Rightarrow \;l{}^2 = 2E_{tot} \mu b^2 |
---|
78 | \label{eqn2} |
---|
79 | \end{equation} |
---|
80 | %Equation ??.2 |
---|
81 | |
---|
82 | \noindent At the point of closest approach, \(\dot r\)=0, therefore: |
---|
83 | |
---|
84 | |
---|
85 | \begin{equation} |
---|
86 | \begin{array}{l} |
---|
87 | E_{tot} = \frac{{E_{tot} b^2 }}{{r^2 }} + \frac{{Z_P Z_T e^2 }}{r} \\ |
---|
88 | r^2 = b^2 + \frac{{Z_P Z_T e^2 }}{{E_{tot} }}r \\ |
---|
89 | \end{array} |
---|
90 | \label{eqn3} |
---|
91 | \end{equation} |
---|
92 | %Equation ??.3 |
---|
93 | |
---|
94 | \noindent Rearranging this equation results in the expression: |
---|
95 | |
---|
96 | \begin{equation} |
---|
97 | b^2 = r(r - r_m ) |
---|
98 | \label{eqn4} |
---|
99 | \end{equation} |
---|
100 | %Equation ??.4 |
---|
101 | |
---|
102 | \noindent where: |
---|
103 | |
---|
104 | \begin{equation} |
---|
105 | r_m = \frac{{Z_P Z_T e^2 }}{{E_{tot} }} |
---|
106 | \label{eqn5} |
---|
107 | \end{equation} |
---|
108 | %Equation ??.5 |
---|
109 | |
---|
110 | \noindent In the implementation of the abrasion process in Geant4, the |
---|
111 | square of the far-field impact parameter, \(b\), is sampled uniformly subject |
---|
112 | to the distance of closest approach, \(r\), being no greater than |
---|
113 | \(r_P\) + \(r_T\) (the sum of the projectile and target nuclear radii). |
---|
114 | |
---|
115 | \section{Abrasion process} |
---|
116 | \label{abrasion} |
---|
117 | In the abrasion process, as implemented by Wilson {\normalsize\it{}et al} |
---|
118 | \cite{aaWilson} it is assumed that the nuclear density for the projectile is |
---|
119 | constant up to the radius of the projectile (\(r_P\)) and zero outside. This |
---|
120 | is also assumed to be the case for the target nucleus. The amount of nuclear |
---|
121 | material abraded from the projectile is given by the expression: |
---|
122 | |
---|
123 | \begin{equation} |
---|
124 | \Delta _{abr} = FA_P \left[ {1 - \exp \left( { - \frac{{C_T }}{\lambda }} \right)} \right] |
---|
125 | \label{eqn6} |
---|
126 | \end{equation} |
---|
127 | %Equation ??.6 |
---|
128 | |
---|
129 | \noindent where F is the fraction of the projectile in the interaction zone, |
---|
130 | \(\lambda\) is the nuclear mean-free-path, assumed to be: |
---|
131 | |
---|
132 | \begin{equation} |
---|
133 | \lambda = \frac{{16.6}}{{E^{0.26} }} |
---|
134 | \label{eqn7} |
---|
135 | \end{equation} |
---|
136 | %Equation ??.7 |
---|
137 | \noindent \(E\) is the energy of the projectile in MeV/nucleon and \(C_T\) is |
---|
138 | the chord-length at the position in the target nucleus for which the |
---|
139 | interaction probability is maximum. For cases where the radius of the target |
---|
140 | nucleus is greater than that of the projectile ({\normalsize\it{}i.e.} |
---|
141 | \(r_T > r_P\)): |
---|
142 | |
---|
143 | \begin{equation} |
---|
144 | C_T = \left\{ {\begin{array}{*{20}c} |
---|
145 | {2\sqrt {r_T^2 - x^2 } } & {:x > 0} \\ |
---|
146 | {2\sqrt {r_T^2 - r^2 } } & {:x \le 0} \\ |
---|
147 | \end{array}} \right. |
---|
148 | \end{equation} |
---|
149 | |
---|
150 | \noindent where: |
---|
151 | |
---|
152 | \begin{equation} |
---|
153 | x = \frac{{r_P^2 + r^2 - r_T^2 }}{{2r}} |
---|
154 | \label{eqn8} |
---|
155 | \end{equation} |
---|
156 | %Equation ??.8 |
---|
157 | |
---|
158 | \noindent In the event that \(r_P > r_T\) then \(C_T\) is: |
---|
159 | |
---|
160 | \begin{equation} |
---|
161 | C_T = \left\{ {\begin{array}{*{20}c} |
---|
162 | {2\sqrt {r_T^2 - x^2 } } & {:x > 0} \\ |
---|
163 | {2r_T } & {:x \le 0} \\ |
---|
164 | \end{array}} \right. |
---|
165 | \end{equation} |
---|
166 | |
---|
167 | \noindent where: |
---|
168 | |
---|
169 | \begin{equation} |
---|
170 | x = \frac{{r_T^2 + r^2 - r_P^2 }}{{2r}} |
---|
171 | \label {eqn9} |
---|
172 | \end{equation} |
---|
173 | %Equation ??.9 |
---|
174 | |
---|
175 | \noindent The projectile and target nuclear radii are given by the expression: |
---|
176 | |
---|
177 | \begin{equation} |
---|
178 | \begin{array}{l} |
---|
179 | r_P \approx 1.29\sqrt {r_{RMS,P}^2 - 0.84^2 } \\ |
---|
180 | r_T \approx 1.29\sqrt {r_{RMS,T}^2 - 0.84^2 } \\ |
---|
181 | \end{array} |
---|
182 | \label{eqn10} |
---|
183 | \end{equation} |
---|
184 | %Equation ??.10 |
---|
185 | \noindent The excitation energy of the nuclear fragment formed by the |
---|
186 | spectators in the projectile is assumed to be determined by the excess surface |
---|
187 | area, given by: |
---|
188 | |
---|
189 | \begin{equation} |
---|
190 | \Delta S = 4\pi r_P^2 \left[ {1 + P - \left( {1 - F} \right)^{{\raise0.7ex\hbox{$2$} \!\mathord{\left/ |
---|
191 | {\vphantom {2 3}}\right.\kern-\nulldelimiterspace} |
---|
192 | \!\lower0.7ex\hbox{$3$}}} } \right] |
---|
193 | \label{eqn11} |
---|
194 | \end{equation} |
---|
195 | %Equation ??.11 |
---|
196 | |
---|
197 | \noindent where the functions \(P\) and \(F\) are given in section |
---|
198 | \ref{PandF}. Wilson {\normalsize\it{}et al} equate this surface area to the |
---|
199 | excitation to: |
---|
200 | |
---|
201 | \begin{equation} |
---|
202 | E_S = 0.95\Delta S |
---|
203 | \label{eqn12} |
---|
204 | \end{equation} |
---|
205 | %Equation ??.12 |
---|
206 | |
---|
207 | \noindent if the collision is peripheral and there is no significant |
---|
208 | distortion of the nucleus, or |
---|
209 | |
---|
210 | \begin{equation} |
---|
211 | \begin{array}{l} |
---|
212 | E_S = 0.95\left\{ {1 + 5F + \Omega F^3 } \right\}\Delta S \\ |
---|
213 | \Omega = \left\{ {\begin{array}{*{20}c} |
---|
214 | 0 & {:A_P > {\rm 16}} \\ |
---|
215 | {1500} & {:A_P < 12} \\ |
---|
216 | {1500 - 320\left( {A_P - 12} \right)} & {:12 \le A_P \le 16} \\ |
---|
217 | \end{array}} \right. \\ |
---|
218 | \end{array} |
---|
219 | \label{eqn13} |
---|
220 | \end{equation} |
---|
221 | %Equation ??.13 |
---|
222 | \noindent if the impact separation is such that \(r << r_P\)+\(r_T\). |
---|
223 | \(E_S\) is in MeV provided \(\Delta S\) is in fm$^2$. |
---|
224 | |
---|
225 | \noindent For the abraded region, Wilson {\normalsize\it{}et al} assume that |
---|
226 | fragments with a nucleon number of five are unbounded, 90\% of fragments with |
---|
227 | a nucleon number of eight are unbound, and 50\% of fragments with a nucleon |
---|
228 | number of nine are unbound. This was not implemented within the Geant4 |
---|
229 | version of the abrasion model, and disintegration of the pre-fragment was only |
---|
230 | simulated by the subsequent de-excitation physics models in the |
---|
231 | G4DeexcitationHandler (evaporation, {\normalsize\it{}etc.} or |
---|
232 | G4WilsonAblationModel) since the yields of lighter fragments were already |
---|
233 | underestimated compared with experiment. |
---|
234 | |
---|
235 | \noindent In addition to energy as a result of the distortion of the fragment, |
---|
236 | some energy is assumed to be gained from transfer of kinetic energy across the |
---|
237 | boundaries of the nuclei. This is approximated to the average energy |
---|
238 | transferred to a nucleon per unit intersection pathlength (assumed to be 13 |
---|
239 | MeV/fm) and the longest chord-length, \(C_l\), and for half of the |
---|
240 | nucleon-nucleon collisions it is assumed that the excitation energy is: |
---|
241 | |
---|
242 | |
---|
243 | \begin{equation} |
---|
244 | E_X^* = \left\{ { |
---|
245 | \begin{array}{*{20}c} |
---|
246 | {13 \cdot \left[ {1 + \frac{{C_t - 1.5}}{3}} \right]C_l } & {:C_t > 1.5{\rm fm}} \\ |
---|
247 | {{\rm 13} \cdot {\rm C}_l } & {:C_t \le 1.5{\rm fm}} \\ |
---|
248 | \end{array}} \right. |
---|
249 | \label{eqn14} |
---|
250 | \end{equation} |
---|
251 | %Equation ??.14 |
---|
252 | |
---|
253 | \noindent where: |
---|
254 | |
---|
255 | \begin{equation} |
---|
256 | C_l = \left\{ { |
---|
257 | \begin{array}{*{20}c} |
---|
258 | {2\sqrt {r_P^2 + 2rr_T - r^2 - r_T^2 } } & {r > r_T } \\ |
---|
259 | {2r_P } & {r \le r_T } \\ |
---|
260 | \end{array}} \right. |
---|
261 | \end{equation} |
---|
262 | \begin{equation} |
---|
263 | C_t = 2\sqrt {r_P^2 - \frac{{\left( {r_P^2 + r^2 - r_T^2 } \right)^2 }}{{4r^2 }}} |
---|
264 | \label{eqn15} |
---|
265 | \end{equation} |
---|
266 | %Equation ??.15 |
---|
267 | |
---|
268 | \noindent For the remaining events, the projectile energy is assumed to be |
---|
269 | unchanged. Wilson {\normalsize\it{}et al} assume that the energy required to |
---|
270 | remove a nucleon is 10MeV, therefore the number of nucleons removed from the |
---|
271 | projectile by ablation is: |
---|
272 | |
---|
273 | \begin{equation} |
---|
274 | \Delta _{abl} = \frac{{E_S + E_X }}{{10}} + \Delta _{spc} |
---|
275 | \label{eqn16} |
---|
276 | \end{equation} |
---|
277 | %Equation ??.16 |
---|
278 | |
---|
279 | \noindent where \(\Delta _{spc}\) is the number of loosely-bound spectators |
---|
280 | in the interaction region, given by: |
---|
281 | |
---|
282 | \begin{equation} |
---|
283 | \Delta _{spc} = A_P F\exp \left( { - \frac{{C_T }}{\lambda }} \right) |
---|
284 | \label{eqn17} |
---|
285 | \end{equation} |
---|
286 | %Equation ??.17 |
---|
287 | \noindent Wilson {\normalsize\it{}et al} appear to assume that for half of the |
---|
288 | events the excitation energy is transferred into one of the nuclei (projectile |
---|
289 | or target), otherwise the energy is transferred in to the other (target or |
---|
290 | projectile respectively). |
---|
291 | |
---|
292 | \noindent The abrasion process is assumed to occur without preference for the |
---|
293 | nucleon type, {\normalsize\it{}i.e.} the probability of a proton being abraded |
---|
294 | from the projectile is proportional to the fraction of protons in the original |
---|
295 | projectile, therefore: |
---|
296 | |
---|
297 | \begin{equation} |
---|
298 | \Delta Z_{abr} = \Delta _{abr} \frac{{Z_P }}{{A_P }} |
---|
299 | \label{eqn18} |
---|
300 | \end{equation} |
---|
301 | %Equation ??.18 |
---|
302 | |
---|
303 | \noindent In order to calculate the charge distribution of the final fragment, |
---|
304 | Wilson {\normalsize\it{}et al} assume that the products of the interaction lie |
---|
305 | near to nuclear stability and therefore can be sampled according to the |
---|
306 | Rudstam equation (see section \ref{ablation}). The other obvious condition is |
---|
307 | that the total charge must remain unchanged. |
---|
308 | |
---|
309 | \section{Abraded nucleon spectrum} |
---|
310 | \label{spectrum} |
---|
311 | Cucinotta has examined different formulae to represent the secondary protons |
---|
312 | spectrum from heavy ion collisions \cite{aaCucinotta}. One of the models |
---|
313 | (which has been implemented to define the final state of the abrasion process) |
---|
314 | represents the momentum distribution of the secondaries as: |
---|
315 | |
---|
316 | \begin{equation} |
---|
317 | \psi (p) \propto \sum\limits_{i = 1}^3 {C_i \exp \left( { - \frac{{p^2 }}{{2p_i^2 }}} \right)} + d_0 \frac{{\gamma p}}{{\sinh \left( {\gamma p} \right)}} |
---|
318 | \label{eqn19} |
---|
319 | \end{equation} |
---|
320 | %Equation ??.19 |
---|
321 | |
---|
322 | \noindent where: |
---|
323 | |
---|
324 | \(\psi (p)\) \indent = number of secondary protons with momentum \(p\) per |
---|
325 | unit of momentum phase space [c$^3$/MeV$^3$]; |
---|
326 | |
---|
327 | \(p\) \indent = magnitude of the proton momentum in the rest frame of the |
---|
328 | nucleus from which the particle is projected [MeV/c]; |
---|
329 | |
---|
330 | \(C1\), \(C2\), \(C3\) \indent = 1.0, 0.03, and 0.0002; |
---|
331 | |
---|
332 | \(p1\), \(p2\), \(p3\) \indent = \(\sqrt \frac{2} {5} p_F\), |
---|
333 | \(\sqrt \frac{6} {5} p_F\), 500 [MeV/c] |
---|
334 | |
---|
335 | \(p_F\) \indent = Momentum of nucleons in the nuclei at the Fermi surface |
---|
336 | [MeV/c] |
---|
337 | |
---|
338 | \(d_0\) \indent = 0.1 |
---|
339 | |
---|
340 | \(\frac 1 \gamma\) \indent = 90 [MeV/c]; |
---|
341 | |
---|
342 | \noindent G4WilsonAbrasionModel approximates the momentum distribution for the |
---|
343 | neutrons to that of the protons, and as mentioned above, the nucleon type |
---|
344 | sampled is proportional to the fraction of protons or neutrons in the original |
---|
345 | nucleus. |
---|
346 | |
---|
347 | \noindent The angular distribution of the abraded nucleons is assumed to be |
---|
348 | isotropic in the frame of reference of the nucleus, and therefore those |
---|
349 | particles from the projectile are Lorentz-boosted according to the initial |
---|
350 | projectile momentum. |
---|
351 | |
---|
352 | \section{De-excitation of the projectile and target nuclear pre-fragments |
---|
353 | by standard Geant4 de-excitation physics} |
---|
354 | \label{deexcitation} |
---|
355 | Unless specified otherwise, G4WilsonAbrasionModel will instantiate the |
---|
356 | following de-excitation models to treat subsequent particle emission of the |
---|
357 | excited nuclear pre-fragments (from both the projectile and the target): |
---|
358 | |
---|
359 | \trivlist |
---|
360 | \item 1 G4Evaporation, which will perform nuclear evaporation of |
---|
361 | ($\alpha$-particles, $^3$He, $^3$H, $^2$H, protons and neutrons, in |
---|
362 | competition with photo-evaporation and nuclear fission (if the nucleus has |
---|
363 | sufficiently high A). |
---|
364 | |
---|
365 | \item 2 G4FermiBreakUp, for nuclei with \(A \le\) 12 and \(Z \le\) 6. |
---|
366 | |
---|
367 | \item 3 G4StatMF, for multi-fragmentation of the nucleus (minimum energy for |
---|
368 | this process set to 5 MeV). |
---|
369 | \endtrivlist |
---|
370 | |
---|
371 | \noindent As an alternative to using this de-excitation scheme, the user may |
---|
372 | provide to the G4WilsonAbrasionModel a pointer to her own de-excitation |
---|
373 | handler, or invoke instantiation of the ablation model (G4WilsonAblationModel). |
---|
374 | |
---|
375 | \section{De-excitation of the projectile and target nuclear pre-fragments by |
---|
376 | nuclear ablation} |
---|
377 | \label{ablation} |
---|
378 | A nuclear ablation model, based largely on the description provided by |
---|
379 | Wilson {\normalsize\it{}et al} \cite{aaWilson}, has been developed to provide |
---|
380 | a better approximation for the final nuclear fragment from an abrasion |
---|
381 | interaction. The algorithm implemented in G4WilsonAblationModel uses the same |
---|
382 | approach for selecting the final-state nucleus as NUCFRG2 and determining the |
---|
383 | particles evaporated from the pre-fragment in order to achieve that state. |
---|
384 | However, use is also made of classes in Geant4's evaporation physics to |
---|
385 | determine the energies of the nuclear fragments produced. |
---|
386 | |
---|
387 | \noindent The number of nucleons ablated from the nuclear pre-fragment |
---|
388 | (whether as nucleons or light nuclear fragments) is determined based on the |
---|
389 | average binding energy, assumed by Wilson {\normalsize\it{}et al} to be |
---|
390 | 10 MeV, {\normalsize\it{}i.e.}: |
---|
391 | |
---|
392 | \begin{equation} |
---|
393 | A_{abl} = \left\{ {\begin{array}{*{20}c} |
---|
394 | {Int\left( {\frac{{E_x }}{{10MeV}}} \right)} \hfill & {:A_{PF} > Int\left( {\frac{{E_x }}{{10MeV}}} \right)} \hfill \\ |
---|
395 | {A_{PF} } \hfill & {:otherwise} \hfill \\ |
---|
396 | \end{array}} \right. |
---|
397 | \label{eqn20} |
---|
398 | \end{equation} |
---|
399 | %Equation ??.20 |
---|
400 | |
---|
401 | \noindent Obviously, the nucleon number of the final fragment, \(A_F\), is |
---|
402 | then determined by the number of remaining nucleons. The proton number of the |
---|
403 | final nuclear fragment (\(Z_F\)) is sampled stochastically using the Rudstam |
---|
404 | equation: |
---|
405 | |
---|
406 | \begin{equation} |
---|
407 | \sigma (A_F ,Z_F ) \propto \exp \left( { - R\left| {Z_F - SA_F - TA_F^2 } \right|^{{\raise0.7ex\hbox{$3$} \!\mathord{\left/ |
---|
408 | {\vphantom {3 2}}\right.\kern-\nulldelimiterspace} |
---|
409 | \!\lower0.7ex\hbox{$2$}}} } \right) |
---|
410 | \label{eqn21} |
---|
411 | \end{equation} |
---|
412 | %Equation ??.21 |
---|
413 | |
---|
414 | \noindent Here \(R\)=\(11.8/AF^{0.45}\), \(S\)=\(0.486\), and |
---|
415 | \(T\)=\(3.8 \cdot 10^{-4}\). Once \(Z_F\) and \(A_F\) have been calculated, |
---|
416 | the species of the ablated (evaporated) particles are determined again using |
---|
417 | Wilson's algorithm. The number of $\alpha$-particles is determined first, on |
---|
418 | the basis that these have the greatest binding energy: |
---|
419 | |
---|
420 | \begin{equation} |
---|
421 | N_\alpha = \left\{ {\begin{array}{*{20}c} |
---|
422 | {Int\left( {\frac{{Z_{abl} }}{2}} \right)} & {:Int\left( {\frac{{Z_{abl} }}{2}} \right) < Int\left( {\frac{{A_{abl} }}{4}} \right)} \\ |
---|
423 | {Int\left( {\frac{{A_{abl} }}{4}} \right)} & {:Int\left( {\frac{{Z_{abl} }}{2}} \right) \ge Int\left( {\frac{{A_{abl} }}{4}} \right)} \\ |
---|
424 | \end{array}} \right. |
---|
425 | \label{eqn22} |
---|
426 | \end{equation} |
---|
427 | %Equation ??.22 |
---|
428 | |
---|
429 | \noindent Calculation of the other ablated nuclear/nucleon species is |
---|
430 | determined in a similar fashion in order of decreasing binding energy per |
---|
431 | nucleon of the ablated fragment, and subject to conservation of charge and |
---|
432 | nucleon number. |
---|
433 | |
---|
434 | \noindent Once the ablated particle species are determined, use is made of the |
---|
435 | Geant4 evaporation classes to sample the order in which the particles are |
---|
436 | ejected (from G4AlphaEvaporationProbability, G4He3EvaporationProbability, |
---|
437 | G4TritonEvaporationProbability, G4DeuteronEvaporationProbability, |
---|
438 | G4ProtonEvaporationProbability and G4NeutronEvaporationProbability) and the |
---|
439 | energies and momenta of the evaporated particle and the residual nucleus at |
---|
440 | each two-body decay (using G4AlphaEvaporationChannel, G4He3EvaporationChannel, |
---|
441 | G4TritonEvaporationChannel, G4DeuteronEvaporationChannel, |
---|
442 | G4ProtonEvaporationChannel and G4NeutronEvaporationChannel). If at any stage |
---|
443 | the probability for evaporation of any of the particles selected by the |
---|
444 | ablation process is zero, the evaporation is forced, but no significant |
---|
445 | momentum is imparted to the particle/nucleus. Note, however, that any |
---|
446 | particles ejected from the projectile will be Lorentz boosted depending upon |
---|
447 | the initial energy per nucleon of the projectile. |
---|
448 | |
---|
449 | \section{Definition of the functions P and F used in the abrasion model} |
---|
450 | \label{PandF} |
---|
451 | In the first instance, the form of the functions $P$ and $F$ used in the |
---|
452 | abrasion model are dependent upon the relative radii of the projectile and |
---|
453 | target and the distance of closest approach of the nuclear centres. Four |
---|
454 | radius condtions are treated. |
---|
455 | |
---|
456 | \noindent |
---|
457 | \underline{\bf $r_T > r_P$ and $r_T-r_P \le r \le r_T+r_P$} : |
---|
458 | |
---|
459 | \begin{equation} |
---|
460 | P = 0.125\sqrt {\mu \nu } \left( {\frac{1}{\mu } - 2} \right)\left( {\frac{{1 - \beta }}{\nu }} \right)^2 - 0.125\left[ {0.5\sqrt {\mu \nu } \left( {\frac{1}{\mu } - 2} \right) + 1} \right]\left( {\frac{{1 - \beta }}{\nu }} \right)^3 |
---|
461 | \end{equation} |
---|
462 | |
---|
463 | \begin{equation} |
---|
464 | F = 0.75\sqrt {\mu \nu } \left( {\frac{{1 - \beta }}{\nu }} \right)^2 - 0.125\left[ {3\sqrt {\mu \nu } - 1} \right]\left( {\frac{{1 - \beta }}{\nu }} \right)^3 |
---|
465 | \end{equation} \\ |
---|
466 | |
---|
467 | \noindent where: |
---|
468 | |
---|
469 | \begin{eqnarray} |
---|
470 | \nu = \frac{{r_P }}{{r_P + r_T }} \\ |
---|
471 | \beta = \frac{r}{{r_P + r_T }} \\ |
---|
472 | \mu = \frac{{r_T }}{{r_P }} |
---|
473 | \end{eqnarray} |
---|
474 | |
---|
475 | |
---|
476 | \noindent |
---|
477 | \underline{\bf $r_T > r_P$ and $r < r_T-r_P$} : |
---|
478 | |
---|
479 | \begin{equation} |
---|
480 | P = - 1 |
---|
481 | \end{equation} |
---|
482 | \begin{equation} |
---|
483 | F = 1 |
---|
484 | \end{equation} |
---|
485 | |
---|
486 | \noindent |
---|
487 | \underline{\bf $r_P > r_T$ and $r_P-r_T \le r \le r_P+r_T$} : |
---|
488 | |
---|
489 | \begin{eqnarray} |
---|
490 | P = 0.125\sqrt {\mu \nu } \left( {\frac{1}{\mu } - 2} \right)\left( {\frac{{1 - \beta }}{\nu }} \right)^2 |
---|
491 | \end{eqnarray} |
---|
492 | \begin{eqnarray} |
---|
493 | - 0.125\left\{ {0.5\sqrt {\frac{\nu }{\mu }} \left( {\frac{1}{\mu } - 2} \right) - \left[ {\frac{{\sqrt {1 - \mu ^2 } }}{\nu } - 1} \right]\sqrt {\frac{{2 - \mu }}{{\mu ^5 }}} } \right\}\left( {\frac{{1 - \beta }}{\nu }} \right)^3 \nonumber |
---|
494 | \end{eqnarray} |
---|
495 | |
---|
496 | \begin{eqnarray} |
---|
497 | F = 0.75\sqrt {\mu \nu } \left( {\frac{{1 - \beta }}{\nu }} \right)^2 |
---|
498 | \end{eqnarray} |
---|
499 | \begin{eqnarray} |
---|
500 | - 0.125\left[ {3\sqrt {\frac{\nu }{\mu }} - \frac{{\left[ {1 - \left( {1 - \mu ^2 } \right)^{{\raise0.7ex\hbox{$3$} \!\mathord{\left/ |
---|
501 | {\vphantom {3 2}}\right.\kern-\nulldelimiterspace} |
---|
502 | \!\lower0.7ex\hbox{$2$}}} } \right]\sqrt {1 - \left( {1 - \mu } \right)^2 } }}{{\mu ^3 }}} \right]\left( {\frac{{1 - \beta }}{\nu }} \right)^3 \nonumber |
---|
503 | \end{eqnarray} |
---|
504 | |
---|
505 | |
---|
506 | \noindent |
---|
507 | \underline{\bf $r_P > r_T$ and $r < r_T-r_P$} : |
---|
508 | |
---|
509 | \begin{equation} |
---|
510 | P = \left[ {\frac{{\sqrt {1 - \mu ^2 } }}{\nu } - 1} \right]\sqrt {1 - \left( {\frac{\beta }{\nu }} \right)^2 } |
---|
511 | \end{equation} |
---|
512 | \begin{equation} |
---|
513 | F = \left[ {1 - \left( {1 - \mu ^2 } \right)^{{\raise0.7ex\hbox{$3$} \!\mathord{\left/ |
---|
514 | {\vphantom {3 2}}\right.\kern-\nulldelimiterspace} |
---|
515 | \!\lower0.7ex\hbox{$2$}}} } \right]\sqrt {1 - \left( {\frac{\beta }{\nu }} \right)^2 } |
---|
516 | \end{equation} |
---|
517 | |
---|
518 | \begin{figure} |
---|
519 | \includegraphics[scale=0.8]{hadronic/theory_driven/AbrasionAblation/abrasion.eps} |
---|
520 | \caption{In the abrasion process, a fraction of the nucleons in the projectile |
---|
521 | and target nucleons interact to form a fireball region with a velocity between |
---|
522 | that of the projectile and the target. The remaining spectator nucleons in |
---|
523 | the projectile and target are not initially affected (although they do suffer |
---|
524 | change as a result of longer-term de-excitation).} |
---|
525 | \label{fig:1} % Give a unique label |
---|
526 | \end{figure} |
---|
527 | |
---|
528 | \begin{figure} |
---|
529 | \includegraphics[scale=0.55, angle=0]{hadronic/theory_driven/AbrasionAblation/deflection.eps} |
---|
530 | \caption{Illustration clarifying impact parameter in the far-field (\(b\)) and |
---|
531 | actual impact parameter (\(r\)).} |
---|
532 | \label{fig:2} % Give a unique label |
---|
533 | \end{figure} |
---|
534 | |
---|
535 | \section{Status of this document} |
---|
536 | 18.06.04 created by Peter Truscott \\ |
---|
537 | |
---|
538 | \begin{latexonly} |
---|
539 | |
---|
540 | \begin{thebibliography}{} |
---|
541 | |
---|
542 | \bibitem{aaWilson} |
---|
543 | J W Wilson, R K Tripathi, F A Cucinotta, J K Shinn, F F Badavi, S Y Chun, |
---|
544 | J W Norbury, C J Zeitlin, L Heilbronn, and J Miller, "NUCFRG2: An evaluation |
---|
545 | of the semiempirical nuclear fragmentation database," NASA Technical Paper |
---|
546 | 3533, 1995. |
---|
547 | |
---|
548 | \bibitem{aaTownsend} |
---|
549 | Lawrence W Townsend, John W Wilson, Ram K Tripathi, John W Norbury, |
---|
550 | Francis F Badavi, and Ferdou Khan, "HZEFRG1, An energy-dependent semiempirical |
---|
551 | nuclear fragmentation model," NASA Technical Paper 3310, 1993. |
---|
552 | |
---|
553 | \bibitem{aaCucinotta} |
---|
554 | Francis A Cucinotta, "Multiple-scattering model for inclusive proton |
---|
555 | production in heavy ion collisions," NASA Technical Paper 3470, 1994. |
---|
556 | |
---|
557 | \end{thebibliography} |
---|
558 | |
---|
559 | \end{latexonly} |
---|
560 | |
---|
561 | \begin{htmlonly} |
---|
562 | |
---|
563 | \section{Bibliography} |
---|
564 | |
---|
565 | \begin{enumerate} |
---|
566 | \item |
---|
567 | J W Wilson, R K Tripathi, F A Cucinotta, J K Shinn, F F Badavi, S Y Chun, |
---|
568 | J W Norbury, C J Zeitlin, L Heilbronn, and J Miller, "NUCFRG2: An evaluation |
---|
569 | of the semiempirical nuclear fragmentation database," NASA Technical Paper |
---|
570 | 3533, 1995. |
---|
571 | |
---|
572 | \item |
---|
573 | Lawrence W Townsend, John W Wilson, Ram K Tripathi, John W Norbury, |
---|
574 | Francis F Badavi, and Ferdou Khan, "HZEFRG1, An energy-dependent semiempirical |
---|
575 | nuclear fragmentation model," NASA Technical Paper 3310, 1993. |
---|
576 | |
---|
577 | \item |
---|
578 | Francis A Cucinotta, "Multiple-scattering model for inclusive proton |
---|
579 | production in heavy ion collisions," NASA Technical Paper 3470, 1994. |
---|
580 | |
---|
581 | \end{enumerate} |
---|
582 | |
---|
583 | \end{htmlonly} |
---|
584 | |
---|