def getScatterAnglePDG(x, dt):
# return thetax, thetay, yx, yy
# given a velocity and timestep, compute a
# deflection angle and deviation due to multiple scattering
# Update the position/velocity and return deflection
#
# x is a 6-element vector (x,y,z,px,py,pz)
# returns deflection angles thetax, thetay and
# displacements yx, yy in two orthogonal directions to p
p = x[3:]
magp = np.linalg.norm(p) # must be in MeV
E = np.sqrt(magp**2 + Params.m**2)
v = p/E
beta = np.linalg.norm(v)
dx = (beta*2.9979e-1) * dt # in m
## in the following we generate a random deflection angle theta
## and transverse displacement y for the given momentum. This is taken
## from the PDG review chapter on the Passage of Particles through Matter
mat = Detector.getMaterial(x[0],x[1],x[2])
X0 = Params.materials[mat][3]
if X0<=0:
return np.zeros(6)
# rms of projected theta distribution.
theta0 = 13.6/(beta*magp) * abs(Params.Q) * np.sqrt(dx/X0) * (1 + 0.038*np.log(dx/X0))
# correlation coefficient between theta_plane and y_plane
rho = 0.87
getRandom = np.random.normal
z1 = getRandom()
z2 = getRandom()
yx = z1*dx*theta0 * np.sqrt((1-rho**2)/3) + z2*rho*dx*theta0/np.sqrt(3)
thetax = z2*theta0
z1 = getRandom()
z2 = getRandom()
yy = z1*dx*theta0 * np.sqrt((1-rho**2)/3) + z2*rho*dx*theta0/np.sqrt(3)
thetay = z2*theta0
return thetax, thetay, yx, yy
def multipleScatterKuhn(x, dt):
# use the method from Kuhn paper
if Detector.getMaterial(x[0],x[1],x[2])=='air':
return np.zeros(6)
p = x[3:]
theta = getScatterAngleKuhn(x, dt)
if theta==-1:
return multipleScatterPDG(x,dt)
vx = getNormVector(p)
# deflection in momentum
defl = np.linalg.norm(p) * (theta*vx)
return np.append(np.zeros(3), defl)
def multipleScatterPDG(x, dt):
# get the angles/displacements from above function and return the
# net change in x=(x,y,z,px,py,pz)
if Detector.getMaterial(x[0],x[1],x[2])=='air':
return np.zeros(6)
p = x[3:]
vx = getNormVector(p)
vy = np.cross(vx, p/np.linalg.norm(p))
thetax, thetay, yx, yy = getScatterAnglePDG(x, dt)
# transverse displacement
disp = yx*vx + yy*vy
# deflection in momentum
defl = np.linalg.norm(p) * (thetax*vx + thetay*vy)
return np.append(disp, defl)
def getKuhnScatteringParams(x, dt):
mat = Detector.getMaterial(x[0],x[1],x[2])
Z,A,rho,X0 = Params.materials[mat]
z = abs(Params.Q)
p = x[3:]
magp = np.linalg.norm(p)
v = p/np.sqrt(magp**2 + Params.m**2)
beta = np.linalg.norm(v)
ds = beta * 2.9979e1 * dt
Xc = np.sqrt(0.1569 * z**2 * Z*(Z+1) * rho * ds / (magp**2 * beta**2 * A))
b = np.log(6700*z**2*Z**(1./3)*(Z+1)*rho*ds/A / (beta**2+1.77e-4*z**2*Z**2))
if b<3:
if not Params.MSCWarning:
print "Warning: something (probably Q) is too small! Using PDG MSC algorithm."
Params.MSCWarning = True
return -1,-1
## we want to solve the equation B-log(B) = b. Using Newton-Raphson
B = b
prevB = 2*B
f = lambda x: x-np.log(x)-b
fp = lambda x: 1-1./x
while abs((B-prevB)/prevB)>0.001:
prevB = B
B = B - f(B)/fp(B)
# use B+1 for correction at intermediate angles
return Xc, B+1
def doEnergyLoss(x, dt):
## returns new x after losing proper amount of energy according to Bethe-Bloch
p = x[3:]
magp = np.linalg.norm(p)
E = np.sqrt(magp**2+Params.m**2)
gamma = E/Params.m
beta = magp/E;
me = 0.511; #electron mass in MeV
Wmax = 2*me*beta**2*gamma**2/(1+2*gamma*me/Params.m + (me/Params.m)**2)
K = 0.307075 # in MeV cm^2/mol
mat = Detector.getMaterial(x[0],x[1],x[2])
Z,A,rho,X0 = Params.materials[mat]
I,a,k,x0,x1,Cbar,delta0 = Params.dEdx_params[mat]
I = I/1e6 ## convert from eV to MeV
xp = np.log10(magp/Params.m)
if xp>=x1:
delta = 2*np.log(10)*xp - Cbar
elif xp>=x0:
delta = 2*np.log(10)*xp - Cbar + a*(x1-xp)**k
else:
delta = delta0*10**(2*(xp-x0))
# mean energy loss in MeV/cm
dEdx = K*rho*Params.Q**2*Z/A/beta**2*(0.5*np.log(2*me*beta**2*gamma**2*Wmax/I**2) - beta**2 - delta/2)
dE = dEdx * beta*2.9979e1 * dt
if dE>(E-Params.m):
return np.zeros(6)
newmagp = np.sqrt((E-dE)**2-Params.m**2)
x[3:] = p*newmagp/magp
return x
