# I can’t find my error when using USRBIN

Dear FLUKA experts:

I can’t understand where I am making a mistake.

The task is to solve a simple problem:
A source of 14 MeV neutrons. A detector in the form of a cubic plastic A-150 with a side length of 1 cm located at a distance of r = 40 cm from the source. How long will it take for the detector to accumulate an absorbed dose of D = 5 Gy if the neutron output from the source is N = 5e11 n/s?

Direct calculation:

1. Φ = N / (4 * pi * r^2) = 5e11/ (4 * pi * 1600) = 2.48e7 n/(s * cm^2) - neutron flux at a distance of 40 cm from the target
2. D = Φ * k * t
k - kerma. For 14 MeV neutrons and material - plastic A-150, it is approximately 2.5E-13 Gy/(cm^2*pr)

t = D/(Φ*k) = 5/(2.48e7 * 2.5e-13) = 8.06e5 s = 224 hr

It will take 224 hours to accumulate 5 Gy.

FLUKA calculation gives a result of about 3 hours:

The result of the calculation by the USRBIN command with What(2)=DOSE - 3.5781e-08 GeV/g/pr for the detector in the form of a cubic with a side, equal to 1 cm

1. 3.5781e-08 * 1.6021764e-07 = 5.73e-15 Gy/pr (USERBIN, Note 5)
2. 5.73e-15 Gy/pr * 5e11 pr/s = 2.87e-3 Gy/s
3. 5 Gy / 2.87e-3 Gy/s = 1742 s = 0.5 hr

224 hr vs 0.5 hr. I don’t understand where the error is.

Best regards
Konstantin

USRBIN.flair (2.5 KB)
USRBIN.inp (1.5 KB)

I rather find (P. Pihet et al, 1992, Phys. Med. Biol. 37 1957) a value of 6E-11 Gy * cm^2, giving ⪅ 1 hr.

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FLUKA win, again! Thank u!