# SPDX-License-Identifier: BSD-3-Clause
# Copyright (c) 2023 Scipp contributors (https://github.com/scipp)
"""Parameters for neutron interactions with atoms."""
from __future__ import annotations
import dataclasses
import importlib.resources
from functools import lru_cache
from typing import TextIO
import scipp as sc
[docs]
def reference_wavelength() -> sc.Variable:
"""Return the reference wavelength for absorption cross-sections.
Returns
-------
:
1.7982 Å
"""
return sc.scalar(1.7982, unit='angstrom')
[docs]
@dataclasses.dataclass(frozen=True, eq=False)
class ScatteringParams:
"""Scattering parameters for neutrons with a specific element / isotope.
Provides access to the scattering lengths and cross-sections of neutrons
with a given element or isotope.
Values have been retrieved at 2024-02-19T17:00:00Z from the list at
https://www.ncnr.nist.gov/resources/n-lengths/list.html
which is based on :cite:`sears:1992`.
Values are ``None`` where the table does not provide values.
The absorption cross-section applies to neutrons with a wavelength
of 1.7982 Å.
See :func:`reference_wavelength`.
"""
isotope: str
"""Element / isotope name."""
coherent_scattering_length_re: sc.Variable | None = None
"""Bound coherent scattering length (real part)."""
coherent_scattering_length_im: sc.Variable | None = None
"""Bound coherent scattering length (imaginary part)."""
incoherent_scattering_length_re: sc.Variable | None = None
"""Bound incoherent scattering length (real part)."""
incoherent_scattering_length_im: sc.Variable | None = None
"""Bound incoherent scattering length (imaginary part)."""
coherent_scattering_cross_section: sc.Variable | None = None
"""Bound coherent scattering cross-section."""
incoherent_scattering_cross_section: sc.Variable | None = None
"""Bound incoherent scattering cross-section."""
total_scattering_cross_section: sc.Variable | None = None
"""Total bound scattering cross-section."""
absorption_cross_section: sc.Variable | None = None
"""Absorption cross-section for λ = 1.7982 Å neutrons."""
def __eq__(self, other: object) -> bool | type(NotImplemented):
if not isinstance(other, ScatteringParams):
return NotImplemented
return all(
self.isotope == other.isotope
if field.name == 'isotope'
else _eq_or_identical(getattr(self, field.name), getattr(other, field.name))
for field in dataclasses.fields(self)
)
[docs]
@staticmethod
@lru_cache
def for_isotope(isotope: str) -> ScatteringParams:
"""Return the scattering parameters for the given element / isotope.
Parameters
----------
isotope:
Name of the element or isotope.
For example, 'H', '3He', 'V', '50V'.
Returns
-------
:
Neutron scattering parameters.
"""
with _open_scattering_parameters_file() as f:
while line := f.readline():
name, rest = line.split(',', 1)
if name == isotope:
return _parse_line(isotope, rest)
raise ValueError(f"No entry for element / isotope '{isotope}'")
def _open_scattering_parameters_file() -> TextIO:
return (
importlib.resources.files('scippneutron.atoms')
.joinpath('scattering_parameters.csv')
.open('r')
)
def _parse_line(isotope: str, line: str) -> ScatteringParams:
line = line.rstrip().split(',')
return ScatteringParams(
isotope=isotope,
coherent_scattering_length_re=_assemble_scalar(line[0], line[1], 'fm'),
coherent_scattering_length_im=_assemble_scalar(line[2], line[3], 'fm'),
incoherent_scattering_length_re=_assemble_scalar(line[4], line[5], 'fm'),
incoherent_scattering_length_im=_assemble_scalar(line[6], line[7], 'fm'),
coherent_scattering_cross_section=_assemble_scalar(line[8], line[9], 'barn'),
incoherent_scattering_cross_section=_assemble_scalar(
line[10], line[11], 'barn'
),
total_scattering_cross_section=_assemble_scalar(line[12], line[13], 'barn'),
absorption_cross_section=_assemble_scalar(line[14], line[15], 'barn'),
)
def _assemble_scalar(value: str, std: str, unit: str) -> sc.Variable | None:
if not value:
return None
value = float(value)
variance = float(std) ** 2 if std else None
return sc.scalar(value, variance=variance, unit=unit)
def _eq_or_identical(a: sc.Variable | None, b: sc.Variable | None) -> bool:
if a is None:
return b is None
return sc.identical(a, b)
