14.8  The Source_custom McStas Component

A flexible pulsed or continuous source with customisable parameters. Multiple pulses can be simulated over time.

Identification

• Author: Pablo Gila-Herranz, based on ’Source_pulsed’ by K. Lieutenant

• Origin: Materials Physics Center (CFM-MPC), CSIC-UPV/EHU

• Date: May 2025

Description

Produces a custom pulse spectrum with a wavelength distribution as a sum of up to 3 Maxwellian distributions and one of undermoderated neutrons.

Usage:

By default, all Maxwellian distributions are assumed to come from anywhere in the moderator. A custom radius r_i can be defined such that the central circle is at temperature T1, and the surrounding ring is at temperature T2. The third Maxwellian distribution at temperature T3 always comes from anywhere in the moderator.

Multiple pulses can be simulated over time with the n_pulses parameter. The input flux is in units of [1/(cm^2 sr s Å)], and the output flux is per second, regardless of the number of pulses. This means that the intensity spreads over n_pulses, so that the units of neutrons per second are preserved. To simulate only the neutron counts of a single pulse, the input flux should be divided by the frequency. Bear in mind that the maximum emission time is given by t_pulse * tmax_multiplier, so short pulses with long tails may require a higher tmax_multiplier value (3 by default).

To simulate a continuous source, set a pulse length equal to the pulse period (e.g. t_pulse=1.0, freq=1.0). Note that this continuous beam is simulated over time, unlike other continuous sources in McStas which produce a Dirac delta at time 0.

Parameters xwidth and yheight must be provided for rectangular moderators, otherwise a circular shape is assumed.

Model description:

The normalised Maxwellian distribution for moderated neutrons is defined by [1] <div class="latex"> $M(\lambda)=\frac{2a^2}{T^2\lambda^5}\exp\left(-\frac{a}{T\lambda^{2}}\right)$ </div> where <div class="latex"> $a=\left(\frac{h^2}{2m_{N}k_{B}}\right)$ </div> and the joining function for the under-moderated neutrons is given by [1] <div class="latex"> $M(\lambda)_{um}=\frac{1}{\lambda(1+\exp(\lambda\chi-\kappa))}$ </div>

The normalised time structure of the long pulse is defined by [2] <div class="latex"> $N_{t<=t_p}=1-\exp\left(-\frac{t}{\tau/n}\right)$ </div> <div class="latex"> $N_{t>t_p}=\exp\left(-\frac{t-t_p}{\tau}\right)-\exp\left(-\frac{t}{\tau/n}\right)$ </div> where tp is the pulse period, tau is the pulse decay time constant, and n is the ratio of decay to ascend time constants.

Parameters for some sources:

HBS thermal source: t_pulse=0.016, freq=24.0, xwidth=0.04, yheight=0.04,

T1=325.0, I1=0.68e+12, tau1=0.000125,
I_um=2.47e+10, chi_um=2.5

HBS cold source: t_pulse=0.016, freq=24.0, radius=0.010,

T1=60.0, I1=1.75e+12, tau1=0.000170,
I_um=3.82e+10, chi_um=0.9

HBS bi-spectral:

t_pulse=0.016, freq=24.0, radius=0.022, r_i=0.010,
T1= 60.0, I1=1.75e+12, tau1=0.000170,
T2=305.0, I2=0.56e+12, tau2=0.000130,
I_um=3.82e+10, chi_um=2.5

PSI cold source: t_pulse=1, freq=1, xwidth=0.1, yheight=0.1,

T1=296.2, I1=8.5e+11, T2=40.68, I2=5.2e+11

References: [1] J. M. Carpenter et al, Nuclear Instruments and Methods in Physics Research Section A, 234, 542-551 (1985). [2] J. M. Carpenter and W. B. Yelon, Methods in Experimental Physics 23, p. 127, Chapter 2, Neutron Sources (1986).

Input parameters

Parameters in boldface are required; the others are optional.

Links

• Component source code found in file Source_custom.comp.

• This model is implemented in an interactive notebook to fit custom neutron sources.