Academia.eduAcademia.edu

Figure 5 – uploaded by K. Habicht

See full PDFdownloadDownload figure
Fig. 5. Schematic TOF sketch visualizing the effect of reduced bandwidth per WFM sub-frame with increasing pulse width. The increase in pulse width may be required either because of reduced resolution needs or longer wavelength at same resolution. As a consequence the mean starting time (dashed line) of the pulse of a specific wavelength shifts towards the middle of the source pulse (b, c). Hence, keeping the band (i.e. angle between the lines defining Amin and Amax in the TOF diagram) constant the pulse shaping chopper(s) would need to be closer to the source (b, red lines), as the efficient source pulse width projected on to the detector position is smaller as seen by the difference in the starting points of the dashed lines. As the chopper(s) have the same position for all subframes and their minimum distance to the source has a lower limit the only solution is to reduce the angular width between the lines (c, red lines), which corresponds to a reduction of the wavelength band with growing pulse widths required for either case, longer wavelength or lower resolution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) We have investigated the potential to combine constant resolution with WFM by graphical TOF diagrams, which are a

Figure 5 Schematic TOF sketch visualizing the effect of reduced bandwidth per WFM sub-frame with increasing pulse width. The increase in pulse width may be required either because of reduced resolution needs or longer wavelength at same resolution. As a consequence the mean starting time (dashed line) of the pulse of a specific wavelength shifts towards the middle of the source pulse (b, c). Hence, keeping the band (i.e. angle between the lines defining Amin and Amax in the TOF diagram) constant the pulse shaping chopper(s) would need to be closer to the source (b, red lines), as the efficient source pulse width projected on to the detector position is smaller as seen by the difference in the starting points of the dashed lines. As the chopper(s) have the same position for all subframes and their minimum distance to the source has a lower limit the only solution is to reduce the angular width between the lines (c, red lines), which corresponds to a reduction of the wavelength band with growing pulse widths required for either case, longer wavelength or lower resolution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) We have investigated the potential to combine constant resolution with WFM by graphical TOF diagrams, which are a

Related Figures (17)

Fig. 1. Schematic view of the guide system installed at HZB and the planned chopper cascade for WFM of the ESS test beamline: (a) cold source moderator, (b) in-pile part, (c) rotary shutter, (d) beamline shutter, (e) ESS source chopper (double), (f) WFM PSC1, (g) WFM PSC2, (h) frame overlap chopper (FOC) 1, (i) wavelength bandwidth chopper (WBC) (double), (j) concrete supports, (k) FOC2, and (1) guide end at 46.2 m distance to the cold source. The guide system in the neutron guide hall 1 at HZB has been subject to an extensive upgrade program, including an upgrade of several instruments [23,24]. In the course of this program, instruments were relocated, optimizing their positions, which also allowed installing an additional neutron guide. This guide now hosting the ESS test beamline directly views the center of the new HZB cold source and is located between the neutron guides for the renewed cold neutron TOF spectrometer (NEAT) [23] and the SANS instrument V4 [25]. The beamline consists of a 46.17 m long super-mirror coated guide providing 3 times larger angles of total reflection (m=3) than a Ni coated guide. The guide has a large cross-section of 60 x 125 mm/?. The guide system (see Fig. 1) starts at a distance of 1.53 m from the cold source with an in-pile section feeding in total 6 guides (1.87 m length). From there the guide separates from the common section with its final width of 60 mm being straight for the following 1.53 m, where a rotary shutter is installed. The following 5 m are curved with a radius of curvature Rc;=1500 m followed by another 30.45 m long curved section with a radius of Re7=2300 m.The final 5 m long section is straight and ends at 46.17 m from the cold source. In order to host choppers the guide system provides several gaps: a 15 cm gap at 21.7 m, a 25 cm gap at 31.5 m and 10 cm gaps each at 30.4 m and 37.6m from the cold source. An additional large 0.6m gap provides space for the WFM pulse shaping choppers (PSC) at 28.3 m-28.9 m from the cold source. The curvature and the m=3 coating result in a spectrum with a cut-off wavelength of about 2A (Fig. 2). The guide ends close to the SANS detector vessel providing approximately 1 m space between the vessel, and the shielding of the NEAT guide system, which increases to 2 m at the end of the guide hall 14m downstream (60m from the cold source). In addition, a frame overlap chopper is required and will be installed in order to separate pulses at any possible measuring position from 46.17 m to 60 m from the cold source, i.e. between 24.5 m and 38 m from the source pulse chopper system. Accord- ingly, wavelength bandwidths between 6.1 A and 10.2 A width are available within the repetition time. This frame overlap chopper, i.e. the wavelength band defining chopper (WBC), is a double chopper system as well. It allows selecting different
Fig. 1. Schematic view of the guide system installed at HZB and the planned chopper cascade for WFM of the ESS test beamline: (a) cold source moderator, (b) in-pile part, (c) rotary shutter, (d) beamline shutter, (e) ESS source chopper (double), (f) WFM PSC1, (g) WFM PSC2, (h) frame overlap chopper (FOC) 1, (i) wavelength bandwidth chopper (WBC) (double), (j) concrete supports, (k) FOC2, and (1) guide end at 46.2 m distance to the cold source. The guide system in the neutron guide hall 1 at HZB has been subject to an extensive upgrade program, including an upgrade of several instruments [23,24]. In the course of this program, instruments were relocated, optimizing their positions, which also allowed installing an additional neutron guide. This guide now hosting the ESS test beamline directly views the center of the new HZB cold source and is located between the neutron guides for the renewed cold neutron TOF spectrometer (NEAT) [23] and the SANS instrument V4 [25]. The beamline consists of a 46.17 m long super-mirror coated guide providing 3 times larger angles of total reflection (m=3) than a Ni coated guide. The guide has a large cross-section of 60 x 125 mm/?. The guide system (see Fig. 1) starts at a distance of 1.53 m from the cold source with an in-pile section feeding in total 6 guides (1.87 m length). From there the guide separates from the common section with its final width of 60 mm being straight for the following 1.53 m, where a rotary shutter is installed. The following 5 m are curved with a radius of curvature Rc;=1500 m followed by another 30.45 m long curved section with a radius of Re7=2300 m.The final 5 m long section is straight and ends at 46.17 m from the cold source. In order to host choppers the guide system provides several gaps: a 15 cm gap at 21.7 m, a 25 cm gap at 31.5 m and 10 cm gaps each at 30.4 m and 37.6m from the cold source. An additional large 0.6m gap provides space for the WFM pulse shaping choppers (PSC) at 28.3 m-28.9 m from the cold source. The curvature and the m=3 coating result in a spectrum with a cut-off wavelength of about 2A (Fig. 2). The guide ends close to the SANS detector vessel providing approximately 1 m space between the vessel, and the shielding of the NEAT guide system, which increases to 2 m at the end of the guide hall 14m downstream (60m from the cold source). In addition, a frame overlap chopper is required and will be installed in order to separate pulses at any possible measuring position from 46.17 m to 60 m from the cold source, i.e. between 24.5 m and 38 m from the source pulse chopper system. Accord- ingly, wavelength bandwidths between 6.1 A and 10.2 A width are available within the repetition time. This frame overlap chopper, i.e. the wavelength band defining chopper (WBC), is a double chopper system as well. It allows selecting different
Fig. 2. Simulated differential neutron flux density d®/d/ at the in-pile entrance and at the end position of the guide system of the ESS test beamline. The inset shows the same spectra normalized by the total neutron flux density Dip...
Fig. 2. Simulated differential neutron flux density d®/d/ at the in-pile entrance and at the end position of the guide system of the ESS test beamline. The inset shows the same spectra normalized by the total neutron flux density Dip...
Fig. 3. Left: opening profiles of the two counter rotating chopper discs of the ESS source mimicking choppers. Right: a comparison of the expected ESS pulse shape and the corresponding pulse shape of the test beamline ‘source choppers”.
Fig. 3. Left: opening profiles of the two counter rotating chopper discs of the ESS source mimicking choppers. Right: a comparison of the expected ESS pulse shape and the corresponding pulse shape of the test beamline ‘source choppers”.
Chopper specifications for the main WFM chopper system for six-fold frame multiplication and the corresponding wavelength frames. Table 1
Chopper specifications for the main WFM chopper system for six-fold frame multiplication and the corresponding wavelength frames. Table 1
However, there are clearly cases in which WFM at the future ESS source is highly desirable, as instruments require flexible resolutions among which the loosest can be realized most efficiently through the length of the instrument without pulse Fig. 4. Schematic TOF sketch of straightforward pulse shaping for a “natura ength” instrument: (a) versus 2-fold wavelength frame multiplication at half that ength (b) a “natural length” instrument with pulse suppression anc c) underlining the equivalence of all these approaches in terms of efficiency illing the detector with neutrons at all times. Note that in (b) the choppe windows have half the width of that in (a) and (c) in order to achieve the same wavelength resolution at half the instrument length, while (a) just achieves hal: ‘he bandwidth in one go and (c) suppresses every second source pulse.
However, there are clearly cases in which WFM at the future ESS source is highly desirable, as instruments require flexible resolutions among which the loosest can be realized most efficiently through the length of the instrument without pulse Fig. 4. Schematic TOF sketch of straightforward pulse shaping for a “natura ength” instrument: (a) versus 2-fold wavelength frame multiplication at half that ength (b) a “natural length” instrument with pulse suppression anc c) underlining the equivalence of all these approaches in terms of efficiency illing the detector with neutrons at all times. Note that in (b) the choppe windows have half the width of that in (a) and (c) in order to achieve the same wavelength resolution at half the instrument length, while (a) just achieves hal: ‘he bandwidth in one go and (c) suppresses every second source pulse.
Fig. 6. TOF diagram displaying the chopper set-up as well as the disc profiles; the insets A and B provide magnified views of the detector position and the pulse shaping respectively. The dark colored areas together with the non-colored next to them display the mean of the desired wavelength bands of each sub-frame, while the consecutive semi-transparent areas display potential overlap fractions originating in the pulse blur. These are partly stopped at the frame overlap choppers or otherwise arrive in non-used time fractions at the detector.
Fig. 6. TOF diagram displaying the chopper set-up as well as the disc profiles; the insets A and B provide magnified views of the detector position and the pulse shaping respectively. The dark colored areas together with the non-colored next to them display the mean of the desired wavelength bands of each sub-frame, while the consecutive semi-transparent areas display potential overlap fractions originating in the pulse blur. These are partly stopped at the frame overlap choppers or otherwise arrive in non-used time fractions at the detector.
Fig. 7. Simulation of time resolution (burst time at pulse shaping choppers over TOF) for the shortest wavelength used , i.e. 2 A, as a function of the beam width at the chopper position as defined by a slit between the two pulse shaping choppers for a nominal resolution defined by the chopper distance and the distance to the detector of 2% (left) and 0.5% (right). The results imply that the beamwidth has to be limited to 2.5 cm and 0.6 cm at the chopper pair, in order to reach the nominal resolution for the shortest used wavelength.
Fig. 7. Simulation of time resolution (burst time at pulse shaping choppers over TOF) for the shortest wavelength used , i.e. 2 A, as a function of the beam width at the chopper position as defined by a slit between the two pulse shaping choppers for a nominal resolution defined by the chopper distance and the distance to the detector of 2% (left) and 0.5% (right). The results imply that the beamwidth has to be limited to 2.5 cm and 0.6 cm at the chopper pair, in order to reach the nominal resolution for the shortest used wavelength.
Time of Flight at detector position for 2% resolution Fig. 8. Results of the simulations for 2% wavelength resolution which show that the sub-frames are sufficiently separated in their arrival time at the detector (top) and at the same time sufficiently overlapping in wavelength range in order to cover the full bandwidth by stitching sub-frames together (bottom). The lines in the diagrams mark the corresponding positions of equal wavelength of subsequent frames; regions containing overlap, i.e. no unique TOF to wavelength solution are cut out and not used in the lower diagram.
Time of Flight at detector position for 2% resolution Fig. 8. Results of the simulations for 2% wavelength resolution which show that the sub-frames are sufficiently separated in their arrival time at the detector (top) and at the same time sufficiently overlapping in wavelength range in order to cover the full bandwidth by stitching sub-frames together (bottom). The lines in the diagrams mark the corresponding positions of equal wavelength of subsequent frames; regions containing overlap, i.e. no unique TOF to wavelength solution are cut out and not used in the lower diagram.
Fig. Al. First the lowest desired wavelength has to be defined as well as the position of the first WFM PSC (WFM PSC1). In the TOF diagram the chopper position is represented by a horizontal line. The wavelength defines the inclination of the corresponding line representing such wavelength in the TOF diagram. For this graphical approach the line for the shortest desired wavelength originates from the latest point in time of the neutron pulse, which is intended to contribute to the system. In the given case it is 2 A neutrons at 2.86 ms (the nominal pulse width at ESS). The intersection of this line with the first WFM chopper position line (horizontal) defines the closing point of the first WFM pulse shaping chopper (WFM PSC1). From this intersection point a vertical line to the second WFM pulse shaping chopper position, which is defined by the desired wavelength resolution and the length of the instrument [29] (horizontal line), is drawn (red line) which defines the opening point of the second WFM pulse shaping chopper (WFM PSC2). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. Al. First the lowest desired wavelength has to be defined as well as the position of the first WFM PSC (WFM PSC1). In the TOF diagram the chopper position is represented by a horizontal line. The wavelength defines the inclination of the corresponding line representing such wavelength in the TOF diagram. For this graphical approach the line for the shortest desired wavelength originates from the latest point in time of the neutron pulse, which is intended to contribute to the system. In the given case it is 2 A neutrons at 2.86 ms (the nominal pulse width at ESS). The intersection of this line with the first WFM chopper position line (horizontal) defines the closing point of the first WFM pulse shaping chopper (WFM PSC1). From this intersection point a vertical line to the second WFM pulse shaping chopper position, which is defined by the desired wavelength resolution and the length of the instrument [29] (horizontal line), is drawn (red line) which defines the opening point of the second WFM pulse shaping chopper (WFM PSC2). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. A2. As the opening point of WFM PSC2 is defined by the vertical line betweer the two chopper positions the second line for the same wavelength (Amin) defining the pulse width for this wavelength can be drawn which of course has the same inclination (i.e. wavelength) and is hence parallel to the first line. By the intersection point (the opening point of the second chopper) and the fixec inclination this line is fully defined. These lines represent the pulse width definec by the chosen chopper distance of the lowest wavelength Zin of the sub-frame as initially chosen.
Fig. A2. As the opening point of WFM PSC2 is defined by the vertical line betweer the two chopper positions the second line for the same wavelength (Amin) defining the pulse width for this wavelength can be drawn which of course has the same inclination (i.e. wavelength) and is hence parallel to the first line. By the intersection point (the opening point of the second chopper) and the fixec inclination this line is fully defined. These lines represent the pulse width definec by the chosen chopper distance of the lowest wavelength Zin of the sub-frame as initially chosen.
Fig. A4. Intersection of the longest wavelength with the position (horizontal line) of the WFM PSC1 defines the required opening time of this chopper (as the closing point is already defined). The drawing is completed by a line for the longest wavelength intersecting WFM PSC1 at the closing point, and hence also WFM PSC2 at the closing point.
Fig. A4. Intersection of the longest wavelength with the position (horizontal line) of the WFM PSC1 defines the required opening time of this chopper (as the closing point is already defined). The drawing is completed by a line for the longest wavelength intersecting WFM PSC1 at the closing point, and hence also WFM PSC2 at the closing point.
Fig. A3. On the other hand the longest desired wavelength of the corresponding frame (Amax) is now determined by the start of the source pulse (time zero, or any other minimum time chosen as the earliest time to contribute to the instrument) and the opening point of WFM PSC2. This line which represents the lower time limit of the Amax pulse width can be drawn by connecting these two points. The wavelength Amax is defined by the inclination of this line and hence depends on the choice of the chopper distances, the chosen minimum wavelength and the assumed starting and end points of the source pulse.
Fig. A3. On the other hand the longest desired wavelength of the corresponding frame (Amax) is now determined by the start of the source pulse (time zero, or any other minimum time chosen as the earliest time to contribute to the instrument) and the opening point of WFM PSC2. This line which represents the lower time limit of the Amax pulse width can be drawn by connecting these two points. The wavelength Amax is defined by the inclination of this line and hence depends on the choice of the chopper distances, the chosen minimum wavelength and the assumed starting and end points of the source pulse.
Fig. B1. Zero divergence, one slit between WFM PS choppers with 0.05 cm x 10 cm window.
Fig. B1. Zero divergence, one slit between WFM PS choppers with 0.05 cm x 10 cm window.
Fig. B3. Divergence defined by the in-pile part geometry; one slit between WFM pulse shaping choppers with 3 cm x 10 cm window; frame overlap chopper 1 and 2 windows were reduced by 15% (subframes 3-5) and 7.5%, respectively (sub- frame 6). Fig. B2. Divergence defined by the in-pile part geometry; one slit between WFM choppers with 3 cm x 10 cm window.
Fig. B3. Divergence defined by the in-pile part geometry; one slit between WFM pulse shaping choppers with 3 cm x 10 cm window; frame overlap chopper 1 and 2 windows were reduced by 15% (subframes 3-5) and 7.5%, respectively (sub- frame 6). Fig. B2. Divergence defined by the in-pile part geometry; one slit between WFM choppers with 3 cm x 10 cm window.
Fig. B4. Divergence defined by the in-pile part geometry; three slits, between WFM-PS-choppers with 5 cm x 10 cm, in front of the FOC 1with 5 cm x 10 cm and in front of FOC 2 with 2 cm x 10 cm windows.
Fig. B4. Divergence defined by the in-pile part geometry; three slits, between WFM-PS-choppers with 5 cm x 10 cm, in front of the FOC 1with 5 cm x 10 cm and in front of FOC 2 with 2 cm x 10 cm windows.
Fig. B5 Final configuration; divergence defined by the in-pile part geometry; three slits, between WFM PS-choppers with 5cm x 10cm, in front of FOC 1 with 5cm x 10cm and in front of FOC 2 with 2 cm x 10cm windows; FO chopper 1 and 2 windows were reduced by 15%, respectively (subframes 3-5) and 7.5% (subframe 6).
Fig. B5 Final configuration; divergence defined by the in-pile part geometry; three slits, between WFM PS-choppers with 5cm x 10cm, in front of FOC 1 with 5cm x 10cm and in front of FOC 2 with 2 cm x 10cm windows; FO chopper 1 and 2 windows were reduced by 15%, respectively (subframes 3-5) and 7.5% (subframe 6).
Related topics:
PhysicsNeutron Physics
580 California St., Suite 400
San Francisco, CA, 94104
© 2025 Academia. All rights reserved