# Muons¶

A cosmic ray model can be used to generate cosmics in gemc.

The model is based on the parameterization [1]:

$\dfrac{a^{cos (b\theta)}}{cp^2}$

The default parameters

$\begin{split}a = 55.6 \\ b = 1.04 \\ c = 64.0\end{split}$

represent a fit to experimental data [2].

To use the default parameters the COSMICRAYS option can be used. For example, to generate cosmic with momentum ranging from 1 to 10 GeV:

<option name="COSMICRAYS"   value="default, 1, 10"/>


The above will generate an equal amount of positive and negative muons.

User can set the parameters as well. For example:

<option name="COSMICRAYS"   value="55, 2, 66, 3, 4"/>


will set (a,b,c) to (55, 2, 66), and momentum range [3-4] GeV.

# Neutrons¶

Neutrons are selected by the 4th field in the COSMICRAYS data card, corresponding to the type of cosmic ray particle: muon || neutron. Default (null string): muon is selected.

Generation of neutron cosmic rays:

The vertical distribution of cosmic neutrons is described by $$j(E) dE \sim E^{-\gamma}dE$$ with $$\gamma = 2.95 \pm 10$$ (from a fit of data by several experiments by Ashton et al. (Cosmic rays at ground level (1973), updated by Nature 256, 387 (1975), see picture).

The zenith angle dependence is given by $$I(\theta) = I(0)*cos^n(\theta)$$ with $$\theta = 3.5 \pm 1.2$$ (from Heidbreder et al., J. Geophys. Pres. 76, 2905 (1971))

# Target Area¶

By default the cosmic rays hit an area 50cm around (0,0,0).

The option COSMICAREA can be used to target a specific area and radius of interest. For example:

<option name="COSMICAREA"   value="0*cm, 0*cm, 1*m, 5*m"/>


will target an area at (0, 0, 1m) with 5m of radius.

# Source Area¶

Be default the particles are generated on a sphere with radius specified by the COSMICAREA option. The COSMICAREA can have an additional argument to generate the cosmics on a cylinder instead. For example:

<option name="COSMICAREA"   value="0*cm, 0*cm, 1*m, 5*m, cyl"/>


will generate cosmics on a cylinder with radius 5m and height 2.5m (half the radius).

Footnotes

 [1] A. Dar, Phys.Rev.Lett, 51,3,p.227 (1983)
 [2] Harut Avakian, Jefferson Lab.