Sans spheres intensity fix

mcstas-comps/examples/ILL/ILL_H15_SAM/ILL_H15_SAM.instr

@@ -35,8 +35,8 @@
 *	mcrun ILL_H15_SAM.instr --gravity -n1e8 -dSmallQ guide1=0 guide2=0 guide3=0 lsd=6.8
 *
 * (Corresponding test-suite entries:)
-* %Example: ILL_H15_SAM.instr --gravity lambda=5.2 Detector: psd_I=1272.79
-* %Example: ILL_H15_SAM.instr --gravity guide1=0 guide2=0 guide3=0 lsd=6.8 Detector: psd_I=1980.7
+* %Example: ILL_H15_SAM.instr --gravity lambda=5.2 Detector: psd_I=15486.6
+* %Example: ILL_H15_SAM.instr --gravity guide1=0 guide2=0 guide3=0 lsd=6.8 Detector: psd_I=25468.3
 *
 * %P
 * lambda:      	[AA]	Wavelength mean value

mcstas-comps/examples/Mantid/templateSANS_Mantid/templateSANS_Mantid.instr

@@ -26,8 +26,8 @@
 * 2) Generate an IDF using mcdisplay templateSANS_Mantid --format=Mantid -n0
 * 3) mcrun templateSANS_Mantid -n1e6 --format=NeXus
 *
-* %Example: lambda=6 Detector: PSDrad_I=7.55196e+06
-* %Example: lambda=6 --format=NeXus Detector: PSDrad_I=7.55196e+06
+* %Example: lambda=6 Detector: PSDrad_I=96709700.0
+* %Example: lambda=6 --format=NeXus Detector: PSDrad_I=96709700.0
 *
 * %Parameters
 * lambda: [AA]          Mean wavelength of neutrons

mcstas-comps/examples/Templates/templateSANS/templateSANS.instr

@@ -19,7 +19,7 @@
 * %Description
 * Very simple test instrument for the Sans_spheres component
 *
-* %Example: lambda=6 Detector: detector_I=0.0445623
+* %Example: lambda=6 Detector: detector_I=0.556137
 *
 * %Parameters
 * lambda: [AA]          Mean wavelength of neutrons

mcstas-comps/examples/Tests_samples/Test_Sans_spheres/README.md

@@ -0,0 +1,36 @@
+# The `Test_Sans_spheres` Instrument
+
+*McStas: A small instrument to compare the Sans_spheres and SANS_spheres2 components*
+
+## Identification
+
+- **Site:** Test_samples
+- **Author:** Daniel Lomholt Christensen
+- **Origin:** Niels Bohr Institute @ UCPH
+- **Date:** 18/08/2026
+
+## Description
+
+```text
+This instrument is a small SANS instrument that simulates the Sans_spheres
+component, and the SANS_spheres2 component, in order to ensure that they
+give the same value in this specific case.
+```
+
+## Input parameters
+
+Parameters in **boldface** are required; the others are optional.
+
+| Name | Unit | Description | Default |
+|------|------|-------------|---------|
+| E_i | meV | Energy of the simulated neutrons. | 5 |
+| use_SANS_spheres2 | 1 | Flag to select whether to use the Sans_spheres component or the SANS_spheres2 component. | 0 |
+| improved_res | 1 | Flag to indicate whether the instrument should run in the insanely good resolution mode. | 0 |
+| flux_mult | n/s | Factor that is multiplied onto the source component in order to adjust the flux. | 0.5512 |
+
+## Links
+
+- [Source code](Test_Sans_spheres.instr) for `Test_Sans_spheres.instr`.
+- [Additional information](Test_Sans_spheres.md)
+
+---

mcstas-comps/examples/Tests_samples/Test_Sans_spheres/Test_Sans_spheres.instr

@@ -0,0 +1,183 @@
+/*******************************************************************************
+*         McStas instrument definition URL=http://www.mcstas.org
+*
+* Instrument: Test_Sans_spheres
+*
+* %Identification
+* Written by: Daniel Lomholt Christensen
+* Date: 18/08/2026
+* Origin: Niels Bohr Institute @ UCPH
+* %INSTRUMENT_SITE: Test_samples
+*
+* A small instrument to compare the Sans_spheres and SANS_spheres2 components
+*
+* %Description
+* This instrument is a small SANS instrument that simulates the Sans_spheres 
+* component, and the SANS_spheres2 component, in order to ensure that they 
+* give the same value in this specific case.
+*
+* %Example: use_SANS_spheres2=0 Detector: psd_det_I=9.73795e+06
+* %Example: use_SANS_spheres2=1 Detector: psd_det_I=9.73795e+06
+*
+*
+* %Parameters
+* E_i: [meV] Energy of the simulated neutrons.
+* use_SANS_spheres2: [1] Flag to select whether to use the Sans_spheres component or the SANS_spheres2 component.
+* improved_res: [1] Flag to indicate whether the instrument should run in the insanely good resolution mode.
+* flux_mult: [n/s] Factor that is multiplied onto the source component in order to adjust the flux. 
+*
+* %Link
+*
+* %End
+*******************************************************************************/
+DEFINE INSTRUMENT Test_Sans_spheres(E_i=5, int use_SANS_spheres2=0, int improved_res = 0, flux_mult=0.5512)
+
+DECLARE
+%{
+  double focus_w;
+  double focus_h;
+  double samp_w;
+  double samp_h;
+  double dE;
+  double src_rad;
+  double src_focus_w;
+  double src_focus_h;
+%}
+
+INITIALIZE
+%{
+  focus_w = 1;
+  focus_h = 1;
+  samp_w = 0.01;
+  samp_h = 0.01;
+  dE = 0.475;
+  src_rad = 0.05;
+  src_focus_w = samp_w;
+  src_focus_h = samp_h;
+  if (improved_res){
+    samp_w = 0.000001;
+    samp_h = 0.000001;
+    dE = 0.00000000000001;
+    src_rad = 0.0000000001;
+    src_focus_w = samp_w - samp_w * 0.01;
+    src_focus_h = samp_h - samp_h * 0.01;
+  }
+%}
+
+TRACE
+
+COMPONENT Origin = Progress_bar()
+  AT (0,0,0) ABSOLUTE
+
+COMPONENT source = Source_simple(
+    radius = src_rad,
+    dist = 10.495,
+    focus_xw = src_focus_w,
+    focus_yh = src_focus_h,
+    E0 = E_i,
+    dE = dE,
+    flux = 1e14*flux_mult
+) AT (0, 0, 0) RELATIVE Origin
+ROTATED (0, 0, 0) RELATIVE Origin
+
+
+COMPONENT post_src_det = PSD_monitor(
+    nx = 200,
+    ny = 200,
+    xwidth = src_rad*2,
+    yheight = src_rad*2,
+    restore_neutron = 1
+) AT (0, 0, 0.001) RELATIVE source
+
+
+
+COMPONENT post_source_slit = Slit(
+    radius = 0.005
+) AT (0, 0, 0.025) RELATIVE source
+
+
+COMPONENT between_slits_det = PSD_monitor(
+    nx = 200,
+    ny = 200,
+    xwidth = 0.005,
+    yheight = 0.005,
+    restore_neutron = 1
+) AT (0, 0, 0.001) RELATIVE PREVIOUS
+
+
+COMPONENT pre_samp_arm = Arm(
+    ) AT (0, 0, 10.495) RELATIVE source
+
+
+COMPONENT pre_samp_slit = Slit(
+    radius = 0.005
+) AT (0, 0, 0) RELATIVE pre_samp_arm
+
+
+
+COMPONENT pre_samp_det = PSD_monitor(
+    nx = 200,
+    ny = 200,
+    filename="pre_det",
+    xwidth = samp_h,
+    yheight = samp_h,
+    restore_neutron = 1
+) AT (0, 0, 0.0005) RELATIVE pre_samp_arm
+
+
+
+
+COMPONENT arm_sample = Arm(
+
+) AT (0, 0, 0.005) RELATIVE pre_samp_arm
+ROTATED (0, 0, 0) RELATIVE pre_samp_arm
+
+COMPONENT sans_spheres = Sans_spheres(
+  R = 100, Phi = 1e-2, Delta_rho = 5, sigma_abs = 0,
+  xwidth=samp_w, yheight=samp_h, zdepth=0.001,
+  focus_xw=focus_w,
+  focus_yh=focus_h,
+  target_index=4
+) WHEN(use_SANS_spheres2 == 0) 
+AT (0,0,0) RELATIVE arm_sample
+
+COMPONENT sans_spheres2 = SANS_spheres2(
+  xwidth = samp_w, yheight = samp_h, zthick = 0.001,
+  dsdw_inc = 0, singlesp = 1,
+  R = 100, phi = 1e-2, drho = 5e11,
+  Qmind = 0.001, Qmaxd = 100.25,
+  sc_aim =1, sans_aim = 1
+) WHEN (use_SANS_spheres2 == 1)
+AT (0,0,0) RELATIVE PREVIOUS
+
+
+
+COMPONENT total_scat = PSD_monitor_4PI(
+    nx = 360,
+    ny = 360,
+    radius=0.1,
+    restore_neutron=1,
+    filename="total_scat"
+) //WHEN (total_scattering==1)
+AT (0, 0, 0) RELATIVE arm_sample
+
+
+COMPONENT arm_det = Arm(
+) AT (0, 0, 0) RELATIVE arm_sample
+ROTATED (0, 0, 0) RELATIVE arm_sample
+
+COMPONENT psd_det = PSD_monitor(
+    nx = 1000,
+    ny = 1000,
+    filename="post_det",
+    xwidth = 1,
+    yheight = 1,
+    restore_neutron = 1
+) AT (0, 0, 5) RELATIVE arm_det
+
+
+FINALLY
+%{
+%}
+END
+

mcstas-comps/examples/Tests_samples/Test_Sans_spheres/Test_Sans_spheres.md

@@ -0,0 +1,62 @@
+# The Test_Sans_spheres Instrument
+
+*McStas Simple instrument used for ensuring that Sans_spheres and SANS_spheres2 remain consistent*
+
+---
+
+## Identification
+
+- **Instrument:** `Test_Sans_spheres`
+- **Author:** Daniel Lomholt Christensen
+- **Date:** 18/08/2026
+- **Origin:** Niels Bohr Institute @ UCPH
+- **Instrument Site:** `Test_samples`
+
+### Summary
+
+A small instrument used to compare the `Sans_spheres` and `SANS_spheres2` components.
+
+
+## Description
+
+```text
+This instrument is a simple SANS setup designed to simulate both the
+`Sans_spheres` component and the `SANS_spheres2` component, allowing direct
+verification that they produce identical results for this specific test case.
+```
+
+## Input Parameters
+
+| Name                | Unit | Description                                                                      | Default |
+| ------------------- | ---- | -------------------------------------------------------------------------------- | ------- |
+| E\_i            | meV  | Energy of the simulated neutrons.                                                | -       |
+| use\_SANS\_spheres2 | 1    | Selects the sample component. `0` uses `Sans_spheres`, `1` uses `SANS_spheres2`. | 0       |
+| improved\_res       | 1    | Enables the instrument's high-resolution mode.                                   | 0       |
+| flux\_mult          | n/s  | Multiplicative factor applied to the source flux.                                | 1       |
+
+---
+## Links
+
+- [Source code](Test_Sans_spheres.instr) for `Test_SANS.instr`.
+
+---
+
+
+## Data treated results
+
+The results from this instrument have been integrated azimuthally using Pyfai and compared to an analytical 
+comparison. The below plots show the instrument in the normal (so called "low" resolution mode),
+and in the improved\_resolution mode.
+
+First, the normal resolution mode:
+![Normal resolution mode](low_res.png)
+
+
+And secondly the high resolution mode:
+
+![High resolution mode](high_res.png)
+
+
+Note, that in the high resolution mode, the neutrons will not move their complete distance through the sample, and therefore
+when calculating the analytical values, the flight length that is attenuated was approximated to $0.48\cdot l_{\rm full}$ where 
+$l_{\rm full}$ is the zdepth of the sample

mcstas-comps/samples/SANS_spheres2.comp

@@ -343,6 +343,9 @@ TRACE
       intersect = box_intersect (&t0, &t1, x, y, z, vx, vy, vz, xwidth, yheight, zthick);
       if (!intersect)
         ABSORB;
+      if (singlesp == 1) {
+        p *= exp (-v * t1 * Stot); // Attenuate beam if no multiple scattering is simulated
+      }
       PROP_DT (t1);
 
     } else {