Concepts of UCN sources for the FRM-II

doi:10.1016/S0168-9002(99)01059-1

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Volume 440, Issue 3, 11 February 2000, Pages 666-673

https://doi.org/10.1016/S0168-9002(99)01059-1 Get rights and content

Abstract

Three concepts for sources of ultra-cold neutrons (UCN) for the reactor FRM-II at Garching near Munich are studied: one, Mini-D₂, is a source with 170 cm³ of solid deuterium in the beam tube SR4 and the second one a large solid-deuterium source (volume about $30 dm^{3}$ ), mounted in the beam tube SR5 as an advanced cold source with a number of neutron guides. The third one, Mark 3000, uses superfluid ⁴He at a cold-neutron guide. A UCN density of up to $7×10^{4} cm^{−3}$ may possibly be achieved in the storage volumes of Mini-D₂ yielding more than 10⁹ UCN for extraction to an attached experimental setup. The usable UCN flux at the periphery of the large deuterium source is predicted to be $2×10^{7} cm^{−2} s^{−1}$ . Mark 3000, finally, is expected to yield a UCN density of about $10^{5} cm^{−3}$ .

Introduction

For a long while ultra-cold neutrons (UCN) have been successfully employed to study different aspects of fundamental physics. The precision of current experiments is mostly limited by the number of UCN available for storage or in-flight experiments. With the upcoming of the new neutron source FRM-II at Garching near Munich different concepts for improved UCN sources have been discussed. They partly require a dedicated design incorporated into the presently foreseen structure of the new research reactor.

In the beam tubes close to the core of FRM-II the thermal neutron flux will be about $8×10^{14} cm^{−2} s^{−1}$ , which can be used to produce high UCN fluxes and densities. Three types of UCN sources are studied for the new facility,

•
a storage source, Mini-D₂ [1], with $170 cm^{3}$ of solid deuterium at about 5 K and $20 dm^{3}$ storage volume that is expected to reach a UCN density ρ about eight-hundred times larger than that achieved up to now (at ILL with $ρ≈90 cm^{−3}$ [2]),
•
a large source with about $30 dm^{3}$ of solid deuterium with beam guides for cold neutrons (CN), very cold neutrons (VCN) and UCN, and
•
the Mark 3000 storage source with superfluid helium, developed in Japan [3].

Section snippets

The Mini- $D_{2}$ source

The small storage source with solid deuterium, Mini-D₂, is based on the idea [4] that UCN produced in solid deuterium should leave this material into vacuum as quickly as possible in order to avoid up-scattering and absorption. In the Mini-D₂ source a small cylindrical volume of solid deuterium (length about 7 cm) – called converter – is positioned very near to the cold source of FRM-II in an evacuated tube – called storage tube – of about 6 cm diameter and 7 m length. The far end of the tube with

The calculation of UCN densities

At equilibrium the rate of UCN generation in the converter is equal to the rate of losses by absorption and up-scattering in D₂, escape through holes in the tube walls and by beta decay. There is a continuous flow of UCN through the area $A^{∗}$ to the storage volume and back. The losses in this (evacuated) volume are dominated by losses during wall collisions.

Two equations describe gain and loss of UCN in the converter and storage volumes, $NPV_{c} Δ E_{u} +Φ_{ex} A^{∗} =Φ_{in} A^{∗} +ρ_{c} V_{c} /τ_{c}$ $Φ_{in} A^{∗} =Φ_{ex} A^{∗} +ρ_{v} V_{v} /τ_{v} .$ The fluxes Φ_in

Discussion of the density formulae

ρ₀ is reached in the converter if it is completely enclosed by the walls of a vessel and hence no neutrons escape into the storage volume. ρ₀ may only be approached if the flux Φ₀ of incoming cold neutrons is constant in the converter volume, which means that d_c (converter thickness) $⪡ L_{D}$ (diffusion length) with L_D=N⁻¹(3σ_sσ_abs)^−1/2, where σ_s and σ_abs are the scattering and absorption cross-sections, respectively. For deuterium the diffusion length is as large as L_D≈2.2 m at 9 meV neutron energy.

Possible layout for a UCN source

A possible layout of the planned UCN source is shown in Fig. 2. The ultra-cold neutrons are produced in a hollow cylinder of solid deuterium with an inner diameter of 1.5 cm, an outer diameter of 6.2 cm and a length of 7.0 cm (total volume $170 cm^{3}$ ). The source is situated at one end of a 7 m long vacuum tube inside the beam tube SR4, near the cold source of FRM-II. The solid deuterium will be deposited on the cold (5 K) surface directly from the gas phase. This technique should reduce the

Other sources

In the large solid-deuterium source [18], [19] a 40-cm-diameter sphere of solid deuterium shall be cooled by super-critical helium to temperatures of about 6 K. Fig. 4 presents vertical and horizontal cuts through the source. The arrangement of 15-cm-diameter rings serves to keep the temperature inside the source low. Thermal neutrons are injected from the left, the cooled neutrons leave the source to the right. Three neutron guides transport CN, VCN and UCN to the experimental hall. The

Conclusion

Three concepts for UCN sources have been presented, each with a different field of use. The Mini-D₂ source would operate in pulsed mode with a loading time of several hundred seconds. The large solid-deuterium source is foreseen to work in continuous mode. The source with super-fluid helium, finally, is a pure storage source. Whereas the design for the Mini-D₂ source is well advanced and detail studies are in progress, only a rough scheme has been set up so far for the large D₂ source. The

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Concepts of UCN sources for the FRM-II

Abstract

Introduction

Section snippets

The Mini-D2 source

The calculation of UCN densities

Discussion of the density formulae

Possible layout for a UCN source

Other sources

Conclusion

Nucl. Instr. and Meth. A

Nucl. Instr. and Meth. A

Cryogenics

Z. Phys. B

Springer Tracts Mod. Phys.

Z. Phys. B

Z. Phys. B

JETP Lett.

The Mini- $D_{2}$ source