The ideal specimen for use at BT-1 is a powder, contains <1% hydrogen (by atom), does not contain any elements with high absorption cross sections for neutrons (see below) and is available in sufficient quantity to fill a 5-10 ml container. For a sample meeting this description, a dataset may be collected in 2-4 hours. Actual samples often differ from this ideal.
The ideal sample size for BT-1 is 10 cm3 (typically 5-15 g) of material but much less is often sufficient.
Vanadium sample cans are used and can be sealed using various thickness' of either Indium or Lead wire. Sample can sizes and volumes are detailed in the table below:
|Size||I.D. (mm)||Length (mm)||Volume (c.c.)|
Samples must have a sufficient number of crystallites (grains) so that the powder diffraction approximation is good. For accurate measurements the crystallites must be randomly oriented. The sample can be oscillated or spun to improve particle statistics and randomization, but this is rarely needed. In most cases, samples that have been pressed into pellets can be used without grinding. Cast ingots or machined parts can exhibit significant amounts of preferred orientation. (A separate instrument, BT-8, can be used to make texture measurements.) Conversely, samples with extremely small crystallites (<1 micron) exhibit significant diffraction peak broadening and many not profit from measurements made with a high-resolution instrument.
Samples are typically sealed into vanadium containers, though other containers such as aluminum and steel are used for some measurements. If the sample is to be cooled to a temperature below 77 K, we typically load the sample in a He glove bag or glove box. These cans are typically sealed using an indium gasket, but other sealing materials can be used for high-temperature measurements. Small (<100 ml) irregular ceramic samples have also been used for data collection without modification. Due to the wide angular range of the detector bank, the diffractometer cannot be used in reflection mode for flat-plate samples or transmission mode with large thin samples.
NIST has vanadium sample containers with volumes of 1.5, 3.4, 5, 6 and 10 ml, where 10 ml is effectively the largest sample volume that can be studied. Where necessary, samples much smaller than 1 ml can be accommodated. Signal-to-noise decreases for a given data collection time as the sample size decreases. Use of long data collection times to accommodate smaller samples must be justified with respect to the value of the scientific work and the scarcity of the sample.
Hydrogen (as opposed to deuterium) has the effect of increasing the background scattering and thus decreases the instrumental sensitivity. Deuterium does not cause this, so use of deuterium-exchanged materials is encouraged for materials where hydrogen is present in amounts greater than a few percent of the total number of atoms in the material. Particular care is needed with hygroscopic materials to keep them dry, or to exchange the water with deuterated water.
In cases where preparation of a deuterated material is impossible or impractical, data collection and structure refinement may be performed on materials that contain more than a few percent hydrogen, provided that the extra time needed is warranted by the value of the scientific work. Prompt-Gamma Neutron activation analysis, also available at the NCNR, can be used to quantify hydrogen levels.
Most elements become radioactive when placed in a neutron beam due to neutron capture. In most cases, this activation is minimal or the residual radioactivity is minimal after a wait of a few hours or days. Elements that will tend to have significant activity, depending on the amount and time in the beam are:
Other elements that tend to neutron activate, though at lower levels are:
Materials containing the above elements are frequently studied at BT-1, but if the sample is of great value or is needed for other experiments outside the NCNR, the potential for radioactivity should be considered in advance. The expected activation level can be estimated from the elemental composition and the anticipated duration of the measurement. The BT-1 instrument scientists can help you to get in touch with a Health Physicist to do this.
Note that the previous list is not comprehensive, so do not assume that you will be able to get your samples back from NIST immediately after your experiment, unless you have been told so by a Health Physicist. All samples that have been irradiated at NIST, for example in neutron diffraction measurements, must be cleared by a member of the Health Physics staff before removal from the NCNR. NRC regulations may restrict the storage or shipping of a sample that has been exposed to a neutron beam. The vast majority of samples used at BT-1 can be removed from NIST within a day or two of measurements, but if your sample is considered radioactive according to NRC regulations, NIST can transfer it only to an institution that is licensed to receive it. If your institution does not have such a license and the activity level in your sample is high, it may be necessary for NIST to store it until the activity level decreases or in rare cases dispose of it.
The following elements can cause difficulty with neutron diffraction. If you want to study a material containing such an element contact one of the BT-1 instrument scientists to discuss the experiment further.
Many other elements have moderately high neutron absorption cross sections. You can consult a table of neutron scattering lengths and cross sections via the web for more information.
If any of the following are present in your sample, an extensive safety review will be required:
Note: in many cases, use of isotopically purified materials, for example samples containing 11B, 112Cd or 114Cd (rather than the elements at natural isotopic abundance) can avoid these problems. Many isotopes are separated for medical or other applications, so costs vary dramatically.
The above information is not an exhaustive list of all concerns, but does list the items that are most frequently encountered. The BT-1 instrument scientists are the primary contact for more information.