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Neutron Activation

For rabbit system

Absorption and Scattering

Neutron activation and scattering calculator

This calculator uses neutron cross sections to compute activation on the sample given the mass in the sample and the time in the beam, or to preform scattering calculations for the neutrons which are not absorbed by the sample.

  1. Enter the sample formula in the material panel.
  2. To perform activation calculations, fill in the thermal flux, the mass, the time on and off the beam, then press the calculate button in the neutron activation panel.
  3. To perform scattering calculations, fill in the wavelength of the neutron and/or xrays, the thickness and the density (if not given in the formula), then press the calculate button in the absorption and scattering panel.

Chemical formula

The chemical formula parser allows you to specify materials and mixtures. Formulas are parsed with periodictable python package.

simple formula
A basic formula consists of elements and their quantities.
CaCO3
represents the chemical CaCO3
multi-part formula
Formulas can be built from parts by separating them with "+" or space, with a number before the part representing repeats. Using parentheses, a formula is treated as if it were a single unit.
CaCO3+6H20
,
CaCO3 6H2O
and
CaCO3(H2O)6
all represent ikaite, CaCO3·6H2O
isotopes
Isotopes are represented by element[nuclide index]. Special symbols
D
and
T
can be used for 2H and 3H. Isotopes can be mixed within a formula, such as
DHO
for partially deuterated water.
O[18]
represents the 18O
density
Mass density is needed to compute scattering factors for the material. The density can be entered in the density field, or it can be given in the formula by adding @value to the end. Densities for the pure elements are already known.
H2O@1
indicates that water has a density of 1 g/cm3
isotopic density
If the formula uses a mixture of isotopes, you can still use the density of the material assuming natural abundance, but add an "n" to the value to scale it to the isotope specific density. If you already know the isotopic density, use the value by itself and it will not be scaled.
D2O@1n
,
D2O@1.11
, and
D2O@1.11i
all give the density of D2O as 1.11 g/cm3
mole fractions
Using non-integer quantities, arbitrary concentration ratios can be be constructed.
78.2H2O[16] + 21.8H2O[18] @1n
represents water with 78.2% 16O and 21.8% 18O
mass fractions
Formulas can be mixed by mass, with each part starting with a percentage followed by formula followed by "//". The first part must use "%wt" to indicate that it is a mass fraction. The final part is the base, and it does not need a percentage since it makes up the rest of the material.
50%wt Co // Ti
is more descriptive than Co0.552Ti0.448
33%wt Co // 33% Fe // Ti
builds a 1:1:1 mixture by mass of cobalt-iron-titanium
volume fractions
Volume fractions are like mass fractions, but they use "%vol" instead. Each component of the volume fraction must specify the density.
20%vol (10%wt NaCl@2.16 // H2O@1) // D2O@1n
is a 10% saline solution by weight mixed 20:80 by volume with D2O, which is the same as
NaCl(H2O)29.1966(D2O)122.794@1.10i

Material mass

Units: g, kg, mg or ug

The total neutron activation depends on the mass of the individual isotopes in the sample and the total time in the beam. All activation calculations assume a thin plate sample, with all parts of the sample exposed to full flux during activation, and no self-shielding when estimating the activation level outside the beam.

Neutron activation is a function of isotope, not element. When an element is used in a formula, the natural abundance of the individual isotopes is used to determine the total activation. By default, the activation calculator uses values from the IAEA handbook1, and the scattering calculator uses the NIST atomic weights and isotope composition database.2

Parameters are controlled by URL:

isotope abundance
The NIST database can be selected for isotope abundance using: index.html?abundance=NIST
activation cutoff
The cutoff values for displaying activation data are set to 0.0001 μCi by default. The full activation levels can be displayed using: index.html?cutoff=0
decay cutoff
The activation calculator determines the amount of time for the activation to decay to the cutoff level, or to 0.0001 μCi if cutoff is 0. This can be set to a value such as 0.1 μCi using: index.html?decay=0.1

  1. IAEA (1987) Handbook on Nuclear Activation Data. TR 273 (International Atomic Energy Agency, Vienna, Austria, 1987). http://cds.cern.ch/record/111089/files/IAEA-TR-273.pdf
  2. Bölke, et al. Isotopic Compositions of the Elements, 2001. J. Phys. Chem. Ref. Data, Vol. 34, No. 1, 2005 http://www.nist.gov/pml/data/comp.cfm

Mass density

Units: g/cm3 or A3

Density is used to compute absorption, transmission and scattering.

default from formula
Leave the density field blank and add
@
plus the density to the end of the formula, where density is in g/cm3. For compounds with specific isotopes, you can use the density of the naturally occurring compound as
@
plus density plus
n
and the isotope specific density will be computed. Density defaults to 1 g/cm3, or for pure elements, the natural density given in the periodic table is used.
D2O@1n
or
D2O@1.11
g/cm3
Enter the density by itself, which will be interpreted as g/cm3, or equivalently, kg/L. No units are needed. If the value ends with an 'i', then the density used will be the isotopic density, otherwise the natural density will be used, and the material density will be computed by substituting the isotope ratios given in the formula.
D2O has a natural density of
1n
and an isotopic density of
1.11
cell volume
Enter a number followed by A3 for Å3. Be sure that your formula contains the correct number of atoms for the unit cell, possibly by using n(formula), where n is 6 for hexagonal close packed, 4 for face centered cells, 2 for body centered and base centered cells, or 1 for simple cells.
4NaCl has a cell volume of
179.4 A3
crystal lattice parameters
Enter lattice parameters "a:n b:n c:n alpha:n beta:n gamma:n" where a, b, c are in Å and α, β, γ are in degrees. If not specified, b and c default to a. Ratios can also be used, so that "b/a:n" gives b=n*a, and "c/a:n" gives c=n*a. Angles α, β, and γ default to 90°. Be sure that the formula contains the correct number of atoms for the unit cell.
4NaCl has a lattice of
a:5.6402

Thickness

Units: cm

The material thickness in cm is used to determine sample transmission, or how much beam will be absorbed by the sample or scattered incoherently. Leave it at 1 cm if you do not need this information.

Thermal flux

Units: n/cm2/s

Provide the thermal flux equivalent for the pre-sample beam configuration for the instrument. This is only need for computing the neutron activation1 from the experiment, and is not used for computing scattering cross sections.

Within the NCNR, you can access a list of instrument fluxes, but this is not available from outside.

  1. IAEA (1987) Handbook on Nuclear Activation Data. TR 273 (International Atomic Energy Agency, Vienna, Austria, 1987). http://cds.cern.ch/record/111089/files/IAEA-TR-273.pdf

Cadmium ratio

Units: none

Samples in the rabbit tubes can be shielded with cadmium to reduce the thermal flux while leaving the epithermal flux mostly unchanged. The cadmium ratio determines the degree of reduction in the scattering cross sections, corresponding to the reduced flux. This value is unitless. Use a value of 0 for beamline experiments.

Thermal/fast ratio

Units: none

When performing neutron activation analysis in a rabbit tube, the additional fast neutron activations need to be determined. The thermal/fast ratio is used to determine the fast neutron flux from the thermal flux equivalent for the given rabbit tube. The resulting fast flux is (thermal flux)/(thermal/fast ratio). This value is unitless. Use a value of 0 for beamline experiments.

Time on beam

Units: h m s d w y

Time on the beam is the length of the exposure at the given flux. Activation will be accumulated over time. Time defaults to hours, but can be set to hours, minutes, seconds, days, weeks or years by adding h, m, s, d, w, or y to the value respectively.

Time off beam

Units: h m s d w y OR yyyy-mm-dd hh:mm:ss

The sample begins to decay immediately, even while it is being activated. The time off beam field allows you to specify how long since the sample was removed from the beam. The default is hours, but can be set to hours, minutes, seconds, days, weeks or years by adding h, m, s, d, w, or y to the value respectively. We always compute the activation level when the sample is removed from the beam, and at 1 hour, 1 day and 15 days post activation.

Instead of saying how long ago the sample was removed from the beam, you can specify when it was removed. Times are given as year-month-day hour:minute:second. Approximate times are allowed, such as 2010-03 for March, 2010. This is equivalent to 2010-03-31 23:59:59, which is the end of March so that the activation estimate will be conservative. That is, this is the most activation consistent with the sample being on the beam sometime in March, 2010. Times are specified in US/Eastern. Add "Z" after the time of day to indicate universal coordinated time (UTC), or add a timezone offset such as "+01" for +1 hours in France in winter, when daylight savings time is not in effect.

Examples

If you type:This is equivalent to:
2 m2 minutes ago
11 hour ago
2.5w2 and a half weeks ago
3 y3 years ago
2015-01-02 21:45:00January 2, 2015 at 9:45 PM US/Eastern
2010-03March 31, 2010 at 11:59:59 PM US/Eastern
2010-7-5 12:23July 5, 2010 at 12:23:59 PM US/Eastern
2015-01-02 21:45:00ZJanuary 2, 2015 at 9:45 PM UTC
2015-01-02 21:45:00-0600January 2, 2015 at 9:45 PM US/Central
2015-08-02 21:45:00-0500August 2, 2015 at 9:45 PM US/Central

Source neutrons

Units: Ang, meV or m/s

The energy of the source neutrons will affect the absorption cross section and hence the penetration depth and sample attenuation. Energy can be expressed as wavelength in Å, as energy in meV, or as neutron velocity in m/s.

Neutron cross sections are tabulated1 at 1.798 Å = 25.3 meV = 2200 m/s, with an assumed 1/v dependence for the absorption cross section. For heavier isotopes (Cd, Hf, rare earths) and/or shorter wavelengths (below 1 Å) there are neutron resonances which are not accounted for in the scattering calculator. There is also a 1/v dependence for single phonon interactions which gives rise to significant inelastic scattering for lighter isotopes (H, D) and/or longer wavelengths (above 5 Å). This factor is both temperature and material dependent and will not be included in the scattering calculations.

  1. Rauch, H. and Waschkowski, W. (2003) Neutron Scattering Lengths in ILL Neutron Data Booklet (second edition), A.-J. Dianoux, G. Lander, Eds. Old City Publishing, Philidelphia, PA. pp 1.1-1 to 1.1-17.

Source X-rays

Units: Ang, keV or Ka

X-ray absorption and scattering are computed from the energy dependent atomic scattering factors.1 Energy can be expressed as wavelength in Å, as energy in keV, or using an element name for the Kα emission line2 for that element.

  1. B.L. Henke, E.M. Gullikson, and J.C. Davis. X-ray interactions: photoabsorption, scattering, transmission, and reflection at E=50-30000 eV, Z=1-92, Atomic Data and Nuclear Data Tables Vol. 54 (no.2), 181-342 (July 1993). http://henke.lbl.gov/optical_constants/
  2. R. D. Deslattes, E. G. Kessler, Jr., P. Indelicato, L. de Billy, E. Lindroth, and J. Anton. Rev. Mod. Phys. 75, 35-99 (2003). http://www.nist.gov/pml/data/xraytrans/index.cfm