Ultracold neutron facility
Very Cold Neutron guide tube
straight vertical and dips into the deuterium of the vertical cold source
curved: 12.8 m length and 13 m radius of curvature
about 7 x 7 cm2 with wavelengths 20 < λ < 400 Å
one half bypasses the turbine wheel and supplies the VCN experiments,
Neutron turbine (Doppler shifting device)
1700 mm in diameter
blades on periphery
690 cylindrically shaped (height 160 mm, 158 deg.arc with a radius of curvature of 65 mm)
high quality Ni
low velocity range from 0 to 15 m/s
230 rpm, receding speed of about 25 m/s
VCN (1 port)
20 - 400 Å
7 cm high and 3.4 cm wide.
v < 40 m/s: the spectrum varies according to the height in the beam
v > 40 m/s: the spectrum is fairly homogeneous
flux at v = 40 m/s (100 Å) : about 105 cm-2s-1(m/s)-1 (= 0.4 x 105 cm-2s-1Å-1)
UCN (4 ports)
beam sizes: 4 x 4, 7 x 7, 10 x 10 and 14 x 10 cm2
total flux: 2.6 x 104 cm-2s-1 for vz < 6.2 m/s (3.3 x 104 for vz < 7 m/s) calculated density : 87 and 110 cm3 for v < 6.2 and v < 7 m/s, respectively
measured density at the position of experiments 4 m away from the turbine house: about 50 cm3 for v < 6.2 m/s (storage in a stainless steel bottle)
UCN beam distributor
(within the turbine housing) it guides the full UCN beam to just 1 of 3 beam ports at a time
In more detail the source consists of a lower straight vertical neutron guide the bottom end of which dips into the deuterium of the vertical cold source. The upper part is a curved neutron guide of 12.8 m length and 13 m radius of curvature. The neutron mirrors are high quality Ni surfaces. These neutron guides transmit a beam of about 7 x 7 cm2 with wavelengths 20 < λ < 400 Å to level D. In the turbine vessel the beam is split into 2 halves: one half bypasses the turbine wheel and supplies the VCN beam, the other half is Doppler shifted into the UCN region under the action of the neutron turbine.
The neutron turbine consists of a wheel of 1700 mm diameter. On its periphery 690 cylindrically shaped blades are mounted (height 160 mm, 158 deg. arc with a radius of curvature of 65 mm). The reflecting surfaces are again high quality Ni surfaces. A receding speed of these Ni surfaces of about 25 m/s (for 230 rpm) transforms neutrons into the low velocity range from 0 to 15 m/s with an associated enlargement of the beam from 7 x 3.4 to 8 x 16 cm2 and wide divergence.
The turbine housing is equipped with 5 exit ports, 1 for the VCNs (20 - 400 Å), and 4 for UCNs. As many experiments work in storage mode, there is a beam distributor within the turbine housing, which guides the full UCN beam to just 1 of 3 beam ports at a time.
The characteristics of the different beams are:
The VCN beam is 7 cm high and 3.4 cm wide. The spectrum varies according to the height in the beam for v < 40 m/s, but is fairly homogeneous for v > 40 m/s. The flux at v = 40 m/s (100 Å) is about 105 cm-2s-1(m/s)-1 (= 0.4 x 105 cm-2s-1Å-1).
The 4 UCN beams have cross sections as follows: 4 x 4, 7 x 7, 10 x 10 and 14 x 10 cm2. Their spectra (measured by TOF experiments at the outlet of the turbine blades) are given in the figure. The total flux is 2.6 x 104 cm-2s-1 for vz < 6.2 m/s (3.3 x 104 for vz < 7 m/s) resulting in calculated values for the density of 87 and 110 cm3 for v < 6.2 and v < 7 m/s, respectively. At the position of experiments 4 m away from the turbine house the density has been measured by storage in a stainless steel bottle to be about 50 cm3 for v < 6.2 m/s.
These values are presently the highest ones worldwide and exceed those obtained on the former PN5 by a factor 50 to 100. It is necessary to mention that all experimental UCN equipment has to be housed within vacuum containers to reduce UCN losses; for VCN use, this is not an absolute necessity.
Reference: A. Steyerl et al., Phys. Lett. A 116 (1986) 347