Monte Carlo Models

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This is a summary page of existing Monte Carlo models of the NPDGamma experiment and auxiliary experiments. The idea behind this page is to create a list of who has models of what aspects of the experiment.

Contents

Kyle's Monte Carlo Models

All of these models are MCNPX models. Each model is built inside the full model framework with only the obvious changes of the target. When I make significant changes to the geometry of the full model, I incorporate it into the other models.

I'm using version 2.7 of MCNPX, which is the newest version. I've also modified the source code to calculate geometrical factors and to implement the neutron source with divergence.

Full Model

The full model is a work in progress. The concrete composition is a question mark and an approximation is being used. The SNS group has several standard concretes and I'm hoping to get a copy of a materials library that they are compiling. In reality, the cave is surrounded in varying thicknesses of steel shielding from a fraction of an inch to 2 inches in some places. The model just uses 1 inch of iron everywhere. The source is a 12 cm tall 10 cm wide rectangle approximately 1.88 meters from the center of the detector array. The source also has a small Gaussian divergence in each direction, 11 mrad in x and 15mrad in y. There are two kinds of lithium plastic in the model, the gray 1/8 inch thick plastic and the ~1mm thick white plastic. The white lithium is only used inside the hydrogen target. At this time, I'm unsure of the full composition of the white lithium and all I know is that it is 70% LiF enriched to 90% Li-6 and there is 2.2% hydrogen by weight. All of the aluminum is 6061 and is modified with a thermal neutron treatment at 293.6 Kelvin, except for the target vessel which is at 20 Kelvin.

The following images are colored according material, but unfortunately MCNPX's visual editor ran out of colors so distinguishing some materials is difficult. The lightest blue is concrete, the medium dark blue is air, and the darkest blue is cesium iodide. Lead is colored orange. Aluminum is a pale green which looks almost the same as the nylon in the spin flipper. The hydrogen is yellow. In most cases, the lithium is too thin to see on the scale of these images.

Model is updated as of June 4, 2012 to include thicker walls and exterior blocks.

The full model shown in the x-y plane. The full model shown in the x-z plane.

Aluminum Target

The aluminum target was comprised of 35 individual discs. Each disc has a thickness of 3.175 mm and a radius of 5.715 cm. Each disc is separated by 3.175 mm. The stand was not included in the model. The composition of the aluminum was a complicated mixture of mostly aluminum with small amounts of other elements that comprise 6061 aluminum.

The 35 discs in the aluminum target shown in the x-z plane. The aluminum target model shown in the x-z plane.

The plot below is a plot of the energy deposition in each detector in units of MeV per source particle. This calculation completely isolated the aluminum discs and contains no contribution from aluminum or other materials elsewhere in the model. The calculation used 200 million source neutrons.

Aluminum target energy deposition calculation.

Boron Profile Monitor

Elise and Kyle made a model of the boron profile monitor to model its response to a cesium source. The CsI crystal in the BPM is a cylinder 3.81 cm in radius and 7.62 cm long. The aluminum cylinder is 0.8mm thick and the front face is 0.5mm thick. The cesium source is modeled as a uniform cylinder 8mm long and 3mm in radius. For a 0.662 MeV gamma source placed directly on the face of the BPM, the energy deposition is approximately 0.164 MeV/gamma. This means that 24.8% of the photon energy emitted from the source is deposited in the crystal. This small model has been incorporated into the full geometry so that the BPM response to neutrons can be modeled. In the full model, the aluminum cylinder is surrounded in 1/8 inches of gray lithium and the front face has 2 layers of the same lithium. There is also a 1.27cm radius disc of boron carbide.

The boron profile monitor model shown in the x-z plane. The boron profile monitor model zoomed in showing the cesium source.

Boron Slab

The boron slab is rotated 45 degrees about the vertical axis and is 0.1524 cm thick. It is made of enriched boron carbide, with 15.7% B-10, 62.6% B-11, and 21.7% C-12.

The boron target model shown in the x-z plane.

The plot below is a plot of the energy deposition in each detector in units of MeV per source particle. This calculation completely isolated the boron slab and contains no contribution from aluminum or other materials elsewhere in the model. The calculation used 10 million source neutrons.

Boron slab energy deposition calculation.

Cesium Source

It uses the same geometry as the other models but uses an isotropic point gamma source at 0.662MeV. It has the capability to move the source to an arbitrary position in space.

The plot below is a plot of the energy deposition in each detector in units of MeV per source particle. The calculation used 10 million source photons.

Cesium source energy deposition calculation.

Chlorine Target

The chlorine target is a liquid carbon tetrachloride inside an aluminum case. The radius of the cavity holding the liquid is 5.715 cm and the depth of the cavity is 5.588 mm The outer radius of the case is 6.1468 cm. The upstream face of the aluminum case is thinner than the downstream face at 0.762 mm compared to 2.667 mm. The cavity was assumed to be completely filled with carbon tetrachloride. The location of the chlorine target for the first run with was 4.9 cm downstream from the spin rotator inside ring 3.

The chlorine target model shown in the x-y plane. The chlorine target model shown in the x-z plane.

The plot below is a plot of the energy deposition in each detector in units of MeV per source particle. This calculation completely isolated the carbon tetrachloride and contains no contribution from aluminum or other materials elsewhere in the model. The calculation used 200 million source neutrons.

Chlorine target energy deposition calculation.

Hydrogen Target

The hydrogen target model uses para-hydrogen at 20K with the aluminum target vessel at 20K as well. The target vessel in the model is hermetically sealed in lithium fluoride. There are two thin layers of copper outside the lithium on the target vessel. The hydrogen target was built from CAD drawings and the position of all components of the hydrogen target was determined relative to the front face of the hydrogen target vacuum box. The position of the front face of the vacuum box was measured to be 2 inches from the lithium on the downstream face of the detector array aluminum frame. The ortho-para ratio can be changed in the MCNPX model, see Offline Analysis for a writeup. See Flux Mesh Plots for neutron flux meshes for different para concentrations.

The hydrogen target model shown in the x-y plane. The hydrogen target model shown in the x-z plane.

The plot below is a plot of the energy deposition in each detector in units of MeV per source particle. This calculation completely isolated the hydrogen volume and contains no contribution from aluminum or other materials elsewhere in the model. The hydrogen used in this calculation was 100% para hydrogen at 20 Kelvin. The calculation used 200 million source neutrons.

Hydrogen target energy deposition calculation.

Aluminum in Spin Flipper

I used an aluminum target 5.715cm in radius and 1.5875cm thick. This corresponds to 5 discs of the aluminum target that we used last year. Plotted below is the energy deposition per source neutron in each detector to give an idea of signal strengths for this target. The collimator is 3.5inches in diameter. The calculation used 10^7 source neutrons.

image:Alsf_signal.png

Chlorine in Spin Flipper

The chlorine target is in a teflon cylinder with outer radius 8.43cm and inner radius 6.35cm. The thickness of the front and rear windows of the target is 3.048mm. The volume containing CCl4 is a 6.35cm radius cylinder with a height of 5.588mm. There is 3.175mm of lithium behind the target. Plotted below is the energy deposition per source neutron in each detector to give an idea of signal strengths for this target. The collimator is 3.5inches in diameter. The calculation used 10^7 source neutrons.

image:Clsf_signal.png

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