Experiments according to the time-of-flight principle
HZB experts have many years of experience with time-of-flight experiments
Whilst research reactors produce neutrons continuously by nuclear fission, ESS will produce pulses of neutrons in a process called “spallation”. These pulses are bunches of neutrons with different speeds but produced at the same time. The time it takes a neutron to fly a certain distance may then be used to determine its speed. Neutrons travelling at different speeds are diffracted by an ensemble of nuclei – so this can be used to determine the arrangement (structure) of atoms. The motions of atoms may also be deduced because moving atoms (or electrons) collide with neutrons which either slow then down or speed then up hence arriving slightly before or after they should.
Time-of-flight instruments may also be used at reactor sources if a pulse is artificially introduced by “choppers” as shown in the figure below. One example at BER II is the Bioref reflectometer which was built at HZB in cooperation with Ruprecht-Karls-Universität of Heidelberg. It is especially suited for sophisticated studies of biological materials. BioRef works in a similar way to planned instruments at ESS. It uses a complex chopper system to split the neutron beam into suitable neutron pulses. The neutrons in these pulses travel at different speed and therefore reach the detector, which counts the neutrons, at different times. Knowledge of the neutron speed reveals for example the spacing between layers of biological materials, e.g. cartilage, fat and joint fluids in bone implant materials.
Time-of-Flight Neutron Scattering Experiment: BioRef
1 The neutron source emits a continuous stream of neutrons travelling at different speed. For ease of illustration, red dots represent fast neutrons and blue dots represent slow neutrons. In fact, neutrons exist at many different speeds (wavelengths) 2 Chopper1 only allows neutrons to pass through when its opening is precisely in line with the neutron beam. The result is small, individual neutron pulses (A), which are defined even more precisely by Chopper 2. The neutrons contained in these pulses (B) continue travelling at different speeds. 3 At diaphragm 1 (D), only some of the neutrons that strike the open window pass through. Some neutrons are caught here (C) and the beam becomes narrower (E). 4 The neutron pulse becomes stretched out because the faster neutrons (G) fly out in front while the slower neutrons (F) remain behind. 5 Chopper 3 (H) cuts off the ends of the pulse so that only neutrons travelling near the mean velocity remain. At diaphragm 2, the beam must traverse another small window, after which its direction is very precisely defined. 6 The neutron beam, in which the neutrons are still travelling at different speeds, strikes the sample (I) and is partially reflected (J) and directed through diaphragm 3. 7 The neutron pulses are tailored to have the slowest neutrons of one pulse arrive at the detector just before the fastest neutrons of the following pulse (K). A “time and space resolved” detector captures and counts the neutrons. Their speed can thus be calculated by their arrival time. Time resolution is important because neutrons of different speeds carry different information about a given sample structure. Together with the reflection angle and the number of neutrons measured this provides precise information about the surface of the sample and its nanometre scale structure. (Infographic: Ela Strickert)