Cold Chopper Spectrometer

CSPEC will used in a wide variety of scientific applications, spanning the life sciences, functional materials and chemistry. Its key capability is to follow kinetic events in situ or in operando, enabled by very high flux.

CSPEC is a direct geometry time of flight spectrometer developed as a German/French collaboration between FRM II and LLB. While both institutions are strong in soft matter and biophysics research, LLB additionally focuses on physical chemistry, magnetism, superconductivity, and structural and phase transition studies. FRM II specializes in materials science, structure research, quantum phenomena, nuclear and particle physics, and neutron methods. The result of this partnership is a workhorse spectrometer that will deliver results for both an academic and industrial user base.

CSPEC will deliver a high cold neutron flux across a wavelength band of 1.72 Å at the sample, positioned 160 m from the ESS target. The high brilliance of the ESS in conjunction with the long distance of the sample from the moderator will result in a cold time of flight spectrometer that will outperform all other spectrometers of its kind in the world, thereby leading to new scientific capabilities and possibilities that are currently not accessible due to flux limitation.

Instrument Class


Beam Port


Lead Scientist

P. P. Deen

Lead Engineer

J. Guyon Le Bouffy

CSPEC will be used in a wide variety of scientific disciplines focusing on, for example, functional and battery materials, quantum materials that may lead to spintronic applications and  biological macromolecules.

Its key capability is to follow kinetic events in situ or in operando, enabled by very high flux. It will probe the structures, dynamics, and functionality of large hierarchical systems as they change or operate. Hierarchical systems include liquids, colloids, polymers, foams, gels, and granular and biological materials as well as the ever complex low-energy dynamics of energy materials and emergent magnetic behaviour. As we probe the dynamics of these systems, we unlock the principles that steer the organization of atoms into complex matter, and we can develop and improve functional materials for solving societal challenges.

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Schematic drawing of ESS CSPEC instrument.

Schematic drawing of ESS CSPEC instrument. 

The instrument length is 160 m from moderator to sample. The CSPEC guide will view the coldest part of the moderator with neutrons guided using a s-shaped curved guide that removes the direct field of view from the sample to the moderator thus eliminating the possibility of spurious high energy particles reaching the sample.

Overview of CSPEC from moderator to experimental cave

Engineering drawing of multiple access to the CSPEC sample environment pot within the detector tank surrounded by the detector cave shielding

The wavelength bandwidth, Δλ = 1.72 Å, of the instrument is defined using bandwidth choppers at 15 and 20 m while the monochromatic pulses are defined using the pulse shaping and monochromating choppers positioned at approximately 105.6 and 158.5 m from the moderator. Finally, the monochromatic pulses are well separated in time using a repetition rate multiplication chopper close to the monochromatic chopper that separates each ESS time period, 71 ms, into 10 – 15 incident pulses on the sample.

The final guide piece can be chosen by the user to provide flux on a large sample area, 4 x 2 cm2, or a focussed sample area, 1 x 1 cm2. The scattering from the sample will pass through a radial oscillating collimator and reach a B10 multigrid detector, 3.5 m from the sample, with a scattering angle of 170o in the equatorial plane and ±26.5o vertical coverage. 

Figure 3 shows an example of the the range of incident wavelengths on the sample within the ESS time period on CSPEC. The flux of each incident wavelength on sample as a function of energy resolution, at the detector, for a range of wavelengths is shown in Figure 4. 

Figure 3 (l). Time-Distance diagram of CSPEC with a range of incident wavelengths within the ESS time period.

Figure 4 (r).  Unfocussed flux of a single incident wavelength on sample as a function of energy resolution and incident wavelengths, ESS Power = 5 MW and with complete detector coverage. Comparisons with CNCS and LET are made. Additional gains will be made through the cumulative use of RRM.

These simulations are derived from McStas 2.3 simulations with ESS 5 MW power, scattering from an incoherent sample (0.01 m radius) at the sample positioned and probed at the detector position. In this figure comparisons with current high profile cold chopper spectrometers are made and this shows that an order of magnitude in flux will be gained for the complete and finalised instrument. In the day 1 scope of the instrument only 50 % of the detector coverage will be readable and it is expected that ESS will run at 2 MW thereby providing a 2-3-fold improvement, as opposed to an order of magnitude, for each incident wavelength. These improvements are valid for the case of inelastic scattering experiments.

cspec energy resolution

Figure 5. Range of energy resolution across the various bandwidths (different colours) on CSPEC.

In the case of quasielastic measurements it will be possible, in certain cases, to sum the incident pulses across the ESS time period. Figure 5 shows the energy resolutions across the bandwidth as the CSPEC chopper are phased to wavelengths ranging from 2 to 13 Å. The science case for CSPEC is focussed on in-situ kinetic measurements. In these experiments the dependence on momentum transfer, Q, is often neglected, to date, in favour of intensity of the energy transfer by integrating the complete detector area in Q. Information obtained from the Q-dependence is vital to understand the spatial behaviour of the relevant dynamics and is not available for many measurements on current chopper spectrometers. For wavelengths > 7 Å the energy resolution across several of the incident pulses will be comparable to the variation in Q resolution across the detector range. On CSPEC it will be possible to integrate some of the incident pulses across the CSPEC bandwidth and therefore provide a substantial increase in flux.

The design of the CSPEC sample environment pot will provide an optimized facility to perform experiments ranging from in-operando studies of electrolytes, pump-probe experiments on photosensitive materials, and the investigation of small samples in extreme sample environments. 

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Some members of the CSPEC team at IKON13 in Lund, September 2017


  • P. Deen
    Lead Scientist
    European Spallation Source
  • J. Guyon Le Bouffy
    Lead Engineer
    Laboratoire Léon Brillouin
  • W. Lohstroh
    Scientific Coordinator
    FRM II, Technical University of Munich
  • S. Longeville
    Scientific Coordinator
    Laboratoire Léon Brillouin