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Dear Yoshiki,

As a result the weight shows you amount of information.

I recommend you to search "splitting" and "russian roulette" at MCNP.

Think about 2 regions, and photon transport.

1st region IMP:p=1

2nd region IMP=p=2

When photon passes to 2nd region from 1st region, it splits and MCNP generates 2 saparate photons. But their weight become 0.5. From now on each photon carries half information of the first one.

Reverse, if photon try to pass from 2nd region to 1st region, it have 1/2 chance to make it (russian roulette).

So if your tally is energy deposition, and if one of splitted photons transfers all of its energy at 2nd region, deposited energy will be half of the first photon energy.

I agree with you, physical meaning of weight is not very clear. But maybe this example helps you to understand. A better explanation will help me also.

Best regards,

Çağrı

At the basic level, the WGT entry on the source card is a tally multiplier. You can use the default (WGT=1.0), and then multiply your tally cards by your source strength (either particles/second or particles) to match the physical situation of your simulation. Another way to go is to set up the source with a WGT equal to the source in your physical geometry. For example, if you were interested in a dose rate at a location inside a Co-60 irradiator with a 1.0 Ci Co-60 source, then you would provide a WGT like the following:

WGT = 7.40e10

[source strength = (3.7e10 dis/s) * (~2 gammas/dis) = 7.4e10 gammas/s]

Your dose rate tally would already be scaled to the source. You could also use an integral number of gammas (say 100 second irradiation, WGT = 7.40e12) and get the total integrated dose. I tend to use 1 for the weight in my calculations and scale the tallies by the source strength, but you may have a different preference.

With regard to how MCNP uses weight during its calculations, the math behind the scenes with regard to Russian roulette, splitting, implicit capture, etc. is designed to provide a "fair game" - or adjusting the weight to make sure that a complete analog Monte Carlo simulation for the same problem would give you the same estimate of the radiation field quantities. The weight games are played to reduce the variance in the MC estimate and converge the estimate more efficiently. In any case, the F4 tally (track length estimate of cell fluence [or fluence rate]) uses this equation to provide its estimate:

F4 = (Weight [particles or particle/s] * Track Length [cm]) / Volume [cm**3]

F4 = particles / cm**2 (or particles / cm**2-s)

The other tallies use the weight in a similar manner.

Particle weight in essence has no physical equivalent, and is used purely for Monte Carlo transport. Particle weight adjusts the tally contribution. Weight window variance reduction is a common technique for speeding up the convergence of the result. This is usually performed by calculating importance function by means of deterministic transport code, and this sets the lower weight of the particle, and heigher weight is usually set at 5-times the lower weight.

Some other use of weights is for source description. For instance you have some source probability distribution, where probabilities vary by several orders of magnitude. If one were to use sampling based on probability, you would in principle undersample the problem. However, you could sample the source uniformly, and adjust the weight by the emission probability.

Monte Carlo codes mainly use weights to further propagate particles if by adjusting particle weight by total/absorption xs.

So there are many uses for particle weights.