Brem converter optimization

The optimization of the photon yield from Al bremsstrahlung converter to get the maximum [math](\gamma,f)[/math] reaction rate in the case of U-238 target was simulated using Geant4 for the incident electrons with the energy E = 10.5 MeV. The electron beam had no divergence. The only physics used in the simulation was G4EmStandardPhysics electromagnetic physics.

The total [math](\gamma,f)[/math] cross section for U-238 target was taken fron nndc/BNL web site and provided below:

Since the electron energy is 10.5 MeV the photons will have maximum energy of 10.5 MeV. The information on the photofission cross section was obtained for the photon energies in the range of [5-10.5] MeV using polynomial fit and presented below for the magnified region of the U-238 [math](\gamma,f)[/math] cross section vs photon energy plot:

The photon yield as a function of the photon energy simulated for different bremsstrahlung converter lengths is provided below. In each case of the converter length the number of the incident electrons was equal to [math]10^6[/math]. No collimation cuts were applied.

After the normalization of the total yield to the number of incident electrons and photon energy, and weighting the yield by the U-238 [math](\gamma,f)[/math] cross section, the following photon yield was obtained (no collimation cuts were applied) :

As can be seen for the given U-238 target the highest reaction [math](\gamma,f)[/math] rate would be for the case of 10 mm Al radiator, assuming that all the photons composing the above distribution reach the target.

Now the collimation effect was taken into account. The collimator was placed 63 cm down the bremsstrahlung converter and had a diameter of 0.25". Bremsstrahlung converter had a shape of a disk with variable thickness and the diameter of 5.5 cm. Total number of the incident electrons was [math]3*10^7[/math]. The number of photons produced in the radiator and reached the collimator (Ng_tot) was counted. The value of a signal (Sig) is the number of photons in the range of energies [5,10.5] MeV. The value of a background (BKG) is the number of photons in the range of energies [0,5) MeV.

Weighted part of the bremsstrahlung spectra that will cause [math](\gamma,f)[/math] reaction of U-238 element:

l, mm	Ng_tot	Sig, [5,10.5] MeV	BKG, [0,5) MeV	BKG*, [1,5) MeV	Sig/BKG	Sig/BKG*	A([math]\mu[/math]barn/MeV/electron), U-238 [math](\gamma,f)[/math]
1	6319	293	6026	1339	0.0486	0.219	4.6
3	7538	380	7158	1778	0.0531	0.214	6.4
4	7450	409	7041	1801	0.0581	0.227	6.6
5	7726	414	7312	1865	0.0566	0.222	7.0
8	7510	412	7098	1989	0.0580	0.207	6.6
10	7179	406	6773	1934	0.0599	0.210	6.5
12	6954	416	6538	1889	0.0636	0.220	6.7
16	6333	400	5933	1846	0.0674	0.217	6.5
20	5756	412	5344	1777	0.0771	0.232	6.6
24	5143	377	4766	1673	0.0791	0.225	6.2

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The distance between Al bremsstrahlung converter and the collimator is 7 ft, the diameter of the collimator 0.25" and its length is 10 cm. Photons and electrons were observed behind the collimator. Surrounding is vacuum. The collimator absorbs everything that hits it. The total number of incident electrons was [math]10^{9}[/math].