1. The principle of the Mini-D₂ source

A new, powerful source of UCN shall be installed in the beam tube SR-4 of the Munich high-flux reactor FRM-II. It employs a small amount of solid deuterium at a temperature of about 5 K, positioned very near to the cold-neutron source inside a long evacuated tube. The UCN are produced by down-scattering the cold neutrons in the D₂ converter and accumulated in the tube. Within a few minutes a UCN density of about 10⁴ n/cm³ builds up in the storage tube. By opening the valve at the end of this tube the experiment will be periodically supplied with up to 2×10⁸ UCN. Operated in continuous mode the source will provide a UCN flux of about 5×10⁵ n/cm²s. The principle of the Mini-D₂ source is illustrated in Fig.1.

Fig. 1 Scheme of the Mini-D₂ UCN source at FRM-II.

• The temperature of the converter for the production of UCN by down-scattering of faster neutrons is kept low enough to let the equilibrium of UCN production and losses no longer be determined by up-scattering, but by absorption in the converter (super-thermal source). Solid deuterium at 5 K temperature meets this requirement [1,2].

• The Mini-D₂ source is best suited for storage experiments. The experimental volume is refilled with UCN periodically every few minutes. In the meantime the continuously produced UCN are accumulated in the storage tube, which then delivers the UCN to the experiment Accumulation takes place as the losses in the storage tube are negligible compared with those in the converter (storage source).

• The converter is placed as close as possible to the cold-neutron source. Feeding the converter with cold neutrons reduces its optimum size down to a few centimeters – roughly the UCN self-absorption length (Mini-D₂)[3].

2. Technical design

The end piece of the in-pile part of the source is shown in Fig. 2. The storage tube is made from an aluminum alloy and covered at the inside with Ni and Be. It is cooled by helium gas at 25 K. The end part of the storage tube, which contains the converter, has a separate cooling circuit operated with super-critical helium at 4.7 K. By this condition the deuterium is solidified only in the end part. The central cold finger serves to improve cooling of the solid D₂.

Fig. 2 Cut-away view of the source head with the converter.

The in-pile cryostat is a strong double-wall construction made of zircaloy. It is cooled by helium gas of room temperature. Figure 3 shows a cut-away view of the source in the SR-4 beam tube. The length of the in-pile part is 4 meters; the total length of the storage tube from the converter to the attached experiment exceeds 8 meters, the diameter is about 6 cm.

Fig. 3 Cut-away view of the SR-4 beam tube with the in-pile part of the source.

Detailed information on the Mini-D₂ UCN source may be found in the internal report [4]. It includes a description of the physics background, associated calculations and detailed technical proposals for cooling circuits and mechanical design for all systems of the source. Special attention is paid to safety problems.

3. Status of the project

The project was initiated in 1998. Currently it is in an advanced stage of the technical design and of the official approval. The start of the source operation is scheduled for 2003.

References

[1]. B. Golub and K. Böning: Z. Phys. B51, 95 (1983)

[2]. Z-Ch. Yu, S.S. Malik, R. Golub: Z. Phys. B - Cond. Matter 62 137 (1986)

[3]. U. Trinks, F.J. Hartmann, S. Paul, W. Schott: Nucl. Instr. Meth. A 440, 666 (2000)

[4]. I. Altarev, F.J. Hartmann, S. Paul, W. Schott, U. Trinks: Internal Report, TUM- E18 (2000)