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NeuCBOT v-2.0

Author: Shawn Westerdale

email: shawest@princeton.edu

Date: 5 Jun 2020

github: https://github.com/shawest/neucbot

NeuCBOT (Neutron Calculator Based On TALYS)

TABLE OF CONTENTS

1. About NeuCBOT
2. General
3. Dependencies
4. Directory Structure
5. Setup
6. Accessing the code
7. Accessing (alpha,n) reaction databases
8. Usage
9. The Basics
10. Material Composition Files
11. Isotope List Files
12. Alpha List Files
13. Output
14. Citations

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1. About NeuCBOT

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i. General

NeuCBOT is a tool for calculating (alpha,n) yields and neutron energy spectra for arbitrary materials under alpha exposure for arbitrary lists of alpha energies or in the presence of alpha-emitting contaminants.

The underlying physics, calculations, and validation of this code is presented in S. Westerdale, P.D. Meyers, Radiogenic Neutron Yield Calculations for Low Background Experiments, arXiv:1702.02465, 6 Feb 2017 and S. Westerdale, A Study of Nuclear Recoil Backgrounds in Dark Matter Detectors, Ph.D. thesis, Princeton University (2016)

The core of the calculations performed with this tool is the nuclear reaction database generated by TALYS-1.95 [1]. NeuCBOT v-1.0 used a database generated by TALYS-1.6; this database can be accessed as an alternative, if specified as a run option.

By default, we proivde data for every naturally occurring isotope for alpha energies ranging from 0 to 10 MeV in 0.01 MeV intervals.

If the user wishes to expand their copy of the database to include alpha energies or isotopes not included in the default database, the user may install TALYS on their local machine and run NeuCBOT with the -t option.

NeuCBOT uses a SRIM-generated [2] stopping power database to determine the stopping power of the user-specified material, and uses this stopping power to integrate over the alpha energies as they slow down.

If the user wants to perform these calculations for a set of alpha-emitting contaminants, they can specify the isotopes and their concentrations. NeuCBOT looks up decay data for these isotopes from the ENSDF [3] database and saves them in a local database for future use.

ii. Dependencies

This software is written in python.

NeuCBOT was written specifically for Python 2.6.6, and, to the extent that some scripts included in this package rely on bash, was written for bash 4.1.2.

This code itself doesn't do anything particularly fancy, and so it will likely work on systems running different versions of Python and bash. However, it is possible that changing syntax with different versions of Python and bash may require small tweaks to be made to the NeuCBOT code.

iii. Directory Structure

Databases are stored in the ./Data directory. Within this directory there are the following subdirectories:

• ./Data/StopingPowers/ : SRIM-generated stopping powers for each element
• ./Data/Decays/ : Downloaded ENSDF files with alpha decay data for given isotopes
• ./Data/abundances.dat : Natural abundances of every isotope, from [4]
• ./Data/Isotopes/ : TALYS-generated (alpha,n) reaction data library

Within the ./Data/Isotopes/ directory, there is a subdirectory for each element, and a subdirectory within this subdirectory for each isotope of that element.

Within an isotope's directory, there is a direcotry called NSpectra/ which contains outgoing neutron energy spectra for alpha particles of a given energy undergoing the (alpha,n) reaction on this isotope.

The TalysOut/ direcotry contains the output from TALYS, from each simulated alpha particle at a given energy upon the isotope. Files in this directory include cross sections for the various alpha-induced nuclear reactions that may occur, including the (alpha,n) reaction, as well as cross sections for producing various excited nuclei and gammas at various energies. These files may be of particular interest to people wishing to explore gammas that may be correlated with (alpha,n) neutrons.

For questions or comments, feel free to send me an email.

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2. Setup

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i. Accessing the code

The code for NeuCBOT is stored in https://github.com/shawest/neucbot.

To access the code, you can either download it directly by clicking the "Clone or download" button and choosing "Download ZIP", or checking out the repository with the command

git clone https://github.com/shawest/neucbot.git

This will create a directory called neucbot in your current directory.

ii. Accessing (alpha,n) reaction databases

There are two options for obtaining (alpha,n) reaction databases.

Databases for all naturally occurring isotopes ranging from 0 to 10 MeV alpha energies have been precompiled. These databases have been grouped together by element are are storred as gzipped tarballs on github at

https://github.com/neucbot-datasets

Databases can be downloaded from this github repository and unzipped with the command

tar -xvzf X.tar.gz

where X is the chemical symbol of the element. These unzipped files should be stored in ./Data/Isotopes/ in order to be read by NeuCBOT.

README files in each element's database gives details of how the cross sections were calculated, including the version of TALYS. Repositories with element names correspond to v-1.0, made with TALYS-1.6; those labeled _v2 were made with TALYS-1.95.

For convenience, NeuCBOT comes with a bash script in the ./Scripts directory called download_element.sh. This script takes the chemical symbol of an element as an argument, and downloads the database for that element, setting it up properly. This script can be run from ./neucbot with the command

./Scripts/download_element.sh X

where X is the chemical symbol of the element.

Data from previous versions can be downloaded using the alternative script labeled with the corresponding version. E.g.,

./Scripts/download_element_v2.sh X

If NeuCBOT is run with the -d option, it will automatically run download_element.sh for each element missing from your local database, using the calculations corresponding to the latest versions. Versions of the database can optionally be specified after the -d argument (e.g. "-d v1" or "-d v2").

Alternatively, you can generate your own database,if you have TALYS installed on your computer. To do so, run NeuCBOT with the -t option. Doing so will save input files in a directory called TalysInput, in the isotope's database directory.

This option allows you to calculate (alpha,n) yields and spectra for isotopes and alpha energies not included in this database, though it should be noted that this will typically slow down the code.

The TALYS software can be downloaded from the homepage at http://www.talys.eu/

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3. Usage

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i. The Basics

NeuCBOT can be run from the command line of any unix-based operating system with the command

./neucbot.py [list of options]

where the list of options is a series of parameters given to NeuCBOT. Each option starts with a hyphen and is followed by any arugments needed by that option.

A list of arguments is given below, follwed by parameters required by that option (written in square brackets) and a description of what that option does in parentheses.

• -h [no arguments] (print help message)
• -l [alpha list file name] (file with a list of alpha energies to be used)
• -c [decay chain file name] (file with a list of alpha-emitting contaminants)
• -m [material composition file name] (file with a description of the material composition)
• -s [alpha step size in MeV] (the step size to be used when integrating over the alpha energy, minimum of 0.01)
• -t [no arguments] (tells NeuCBOT to run TALYS for reactions not in libraries)
• -d [no arguments] (if an element is missing from the (alpha,n) database, automatically run the download_element.sh script to download the element's database)
• -o [output file name] (name of text file to store output to)
• --print-alphas [no arguments] (prints a list of alpha energies being used)
• --print-alphas-only [no arguments] (same as --print-alphas, but aborts after printing)
• --force-recalculation [no arguments] (if TALYS is installed in your machine, run it for each alpha energy and each isotope, overwriting pre-existing entries in the database if necessary)

In order to run NeuCBOT, the user must provide a material description (-m material_file_name) and either an alpha energy list (-l alpha_list_name) OR a list of contaminants in your decay chain of interest (-c contaminants_list_name).

All other options are optional.

If the -o option is not specified, all output will be displayed to the terminal.

If the -s option is not specified, a default step size of 0.01 MeV will be assumed.

NeuCBOT comes with acrylic as an example material, stored in ./Materials/Acrylic.dat.

We also provide the ²³²Th, ²³⁵U, and upper and lower ²³⁸U decay chains (split above ²²⁶Ra), as well as the ²¹⁰Pb decay chain as example decay chains. These chains are stored in ./Chains/.

Examples of alpha lists can be seen in ./AlphaLists/ where we include alpha energies emitted by several isotopes found in the decay chains.

Example usage of neucbot with both of these options is given below

./neucbot.py -m Materials/Acrylic.dat -c Chains/Th232Chain.dat

or

./neucbot.py -m Materials/Acrylic.dat -l AlphaLists/Rn220Alphas.dat

It should be noted that the order of these options does not matter.

If any isotopes listed in the material composition description are not present in the (alpha,n) reaction library, NeuCBOT will throw an error.

ii. Material Composition Files

This section describes the anatomy of a material composition file. One such file must be given to NeuCBOT as an argument to the -m option.

These text files consist of three columns.

The first column is the chemical symbol of an element in the material. Capitalization does not matter.

The second column is the mass number of this isotope. If 0 is specified, it will be assumed that all naturally occurring isotopes of this element are present at their natural abundances, as reported in [4].

The third column is the percent mass of the specified element or isotope.

For example

c 12 45

c 13 55

would describe a material that is made of 45% ¹²C and 55% ¹³C, by mass.

More realistically,

c 0 59.984

o 0 31.962

h 0 8.054

is the composition of acrylic, assuming carbon, oxygen, and hydrogen isotopes are all present according to their natural abundances.

After reading a material composition file, NeuCBOT normalizes the composition so that all isotopes add up to 100%. The user may therefore specify mass fractions as percentages or decimals, as they prefer.

All lines in this file that start with a # are skipped by NeuCBOT, allowing the user to leave comments in these files.

iii. Isotope List Files

Decay chains are specified in text files with two columns.

The first column specifies the name of the isotope, which is the chemical symbol followed by the mass number, and the second column is the percent of decays of the chain in which the specified isotope occurs, relative to the top of the chain.

It is important to note that the second column must be given in percent probability of the isotope appearing.

As an example, the ²³²Th decay chain is specified as

Th232 100

Th228 100

Ra224 100

Rn220 100

Po216 100

Bi212 35.94

Po212 64.06

Similar to the material composition description, lines that start with a # are skipped by NeuCBOT.

iv. Alpha List Files

Alpha list files are structured very similiarly to isotope list files.

Each text file has two columns. The first column specifies an alpha energy in MeV, and the second column specifies the percent porbability of an alpha of that energy being produced.

For example, ²¹⁶Po alphas can be described with a file that reads as

6.7783 99.9981

5.985 0.0019

Like in the two above cases, lines that start with a # are skipped by NeuCBOT.

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4. Output

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The output of NeuCBOT is hopefully mostly staightforward to understand.

As NeuCBOT runs, it prints the alpha energy every 10 keV, so its progress can be tracked. It will also output any relevant warnings it may encounter while running. The most likely warnings will be missing isotopic data in the (alpha,n) database. If the alpha energies simulated are all under 10 MeV and only naturally occuring isotopes are being simulated, the simplest solution is to run NeuCBOT with the -d option, an it will acquire (alpha,n) data automatically. Otherwise, the user should install TALYS and run with the -t option.

The output provides the total neutron yield, calculated by integrating all of (alpha,n) cross sections over the tracks of the alphas as they slow down. The units used here are the number of neutrons produced per decay of the entire decay chain or list of alpha energies being simulated. So if a chain has three alpha-emitting isotopes, NeuCBOT tells you the expected number of neutrons after all three isotopes have decayed with their given branching ratios.

NeuCBOT also gives a breakdown of the contribution of each isotope in the target material to the total neutron yield.

Due to effects of binning the neutron energy spectrum and inaccuracies of Riemann integration, it is possible that the integral of the neutron energy spectrum will be slightly different from the total neutron yield. These differences are typically under ~1%. We therefore include the integral of the neutron energy spectrum in the output.

Lastly, the neutron energy spectrum is printed. By default, the spectrum is binned into 100 keV bins and the spectrum is given in units of neutron/decay/100 keV.

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5. Citations

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0. A. Koning, et al. TALYS-1.6. http://www.talys.eu/
1. J. Ziegler, et al. SRIM-2008. http://www.srim.org/
2. J. K. Tuli, et al. Evaluated Nuclear Structure Data File (ENSDF), 1996.
3. P. De Bievre and P.D.P. Taylor, Int. J. Mass Spectrom. Ion Phys. 123, 149 (1993).