In order to describe a cell, one can use intersections, unions or exclusions of surfaces (and also exclusion of other cells). Thus, a cell will be defined with

• a cell number, identifying the cell

• either a material number with its density or a ‘0’ if the cell is void,

• a list of surfaces (or cells) with operators (intersection: a blank; union: a colon ‘:’; exclusion ‘#’).

First, let us see how cells are built from surfaces; for the sake of simpli­city we will deal with a two-dimensional empty cell (see figure 5.3).

10-123-4

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Figure 5.3. A simple geometry (a) and a somewhat more difficult one (b). Circled num­bered are the cell numbers, plain numbers are surface numbers.

In this example, the ‘-’ sign before surface 1 means that one considers the region below surface 1 (i. e. y — d1 < 0, if surface 1 is given by y — dj = 0). Similarly, the region on the left of surface 4 is considered (x — d4 < 0) whereas the ‘positive’ surfaces 2 and 3 mean that the region being considered is to the right and above them respectively. Cell 1 is thus the intersection of these four ‘signed’ surfaces. The ‘exterior’, i. e. cell 2, can be defined as

2 0 1:-2:-3:4

that is, as the union of the outside region defined by the surfaces; the exclusion operator can also be used:

2 0 #1

which means that cell 2 is the whole space except cell 1.

Consider now the more difficult case of figure 5.3(b). First, forget cell 3 and surface 6. Because cell 1 has a concave corner the cell has to be defined also with unions, namely:

10-123 (-4:-5) (Fig. 5.3(b) without surface 6)

Now, taking into account surface 6 and cell 3,

10-123 (-4:-5) 6 (Fig. 5.3(b))

3 0-6

where ‘-6’ means the inside of circular surface 6.

The outside of cell 1 could be defined as

2 0 1:-2:-3:(4 5)

or, with the exclusion operator,

2 0 #1 #3

Don’t forget to exclude cell 3 also! Indeed, #1 means ‘the whole space except cell 1′ and cell 3, although it is inside cell 1, does not belong to that cell.

To illustrate a real input geometry with a short example, let us consider the geometry of figure 5.4.

First simple geometry c

c Cell cards c

10-1 $ the inner sphere

20-23-41 $ the cylinder without the sphere

3 0 #2 #1 $ exterior c

c Surface cards

Figure 5.4. Simple geometry: a sphere with R = 5 cm inside a cylinder centred on the Z axis with R = 20 cm and height = 40 cm.

c

1 SO 5 $ centred sphere with R=5 cm

2 CZ 20 $ infinite cylinder with R=20cm

3 PZ -20 $ bottom plane intersecting the cyl.

4 PZ 20 $ top plane intersecting the cyl.

c end of the file

Suppose that this geometry is written in a file named ‘mygeom’. In order to see this geometry, simply type on the command line:

mcnp ip i=mygeom

A prompt ‘plot>’ allows you to view a section perpendicular to the z axis at the value z0 by the command

pz z0

Other views can be seen with ‘px’ and ‘py’. This shows a 2D view of the geometry. If there are errors in the geometry the ‘bad’ surfaces are drawn with a dotted red line.