by Winfried Petry

ABSTRACT

The construction of the new German high flux neutron source FRM-II is finished and FRM-II is is waiting for its license to start nuclear operation. With the beginning of the routine operation 19 instruments will be in action, including 5 irradiation facilities and 14 beam tube instruments, most of them use neutron scattering techniques for the purpose of material science. Some of these instruments are unique, others are expected to be the best of their kind, all instruments are based on innovative techniques. Further instruments are under construction.

Introduction

In 1957, the Bavarian state government took into operation the Federal Republic's first nuclear reactor, Forschungsreaktor München (FRM). Now, with the new Munich research reactor, FRM-II, is about to provide German scientists and industry with a powerful, scientifically attractive neutron source at the beginning of the new millennium. The concept of a compact, ²³⁵U-enriched core allows the provision of high neutron intensity combined with the highest possible levels of safety and environmental compatibility. Being operated by the Technische Universtität Munich and placed in the heart of a university campus its usage is mainly dedicated to basic and applied research and education. However, roughly 30% of its experimental capacities are foreseen for industrial use. Once the licence to start nuclear operation is granted a period of 10 - 12 months is necessary to tune FRM-II up to its full power of 20 MWatt and to achieve routine operation. Routine operation foresees 5 cycles of 52 days each per year.

Special moderators optimise the flux density of the neutrons for various uses. At 20 MWatt thermal power an unperturbed thermal neutron flux of 8 x 10¹⁴ n/cm²s is expected to build up in the D₂O moderator. In order to minimize the leakage of fast neutrons out of the biological shielding all 12 beam tubes have tangential orientations with respect to the core. A cold source containing about 26 lt. of liquid D₂ is located in the maximum of the thermal flux and feeds three beam tubes, one of them is particular wide and hosts six neutron guides of a cross section up to 6 x 17 cm² leading to the neutron guide hall. Another of these three cold beam tubes will contain a solid D₂ mini source at a temperature of 5 K. By down scattering ultra cold neutrons of typical wavelength of 1000 Å are produced. UCN densities of 10⁴ per cm³ are expected. Further a graphite hot source is placed in the maximum of the thermal flux and shifts the neutron spectrum to shorter wavelength. At the outer corner of the heavy water moderator thermal neutrons are converted to fast neutrons of MeV energy by fission reaction in a plate of about 270 g enriched ²³⁵U. One beam tube faces this converter and provides fast neutrons for tumor therapy and radiography with fast neutrons. One of the two inclined beam holes contains a Cd cladding. n,-reaction with the Cd provides an intense -radiation which converts by pair creation to positrons and electrons. The positrons are extracted electromagnetically, thermalised and yield an up to now unreached intensity of 10⁹ - 10¹⁰ thermal positrons/cm²s at the sample. One through-going beam tube penetrates the biological shielding from two sides. It will house 1 g of ²³⁵U in order to produce fission products. Those will be separated by mass and extracted to deliver an intense beam of fission products. The UCN source and the fission product beam are particular ambitious projects and will be finalized in the upcoming years. All other beams are ready for being taken into operation.

With the emphasis on commercial use, five irradiation facilities will be ready when routine operation begins. The increasing use of radioactive isotopes in science and technology entails great flexibility in output. The various irradiation facilities arranged in the moderator tank have thermal neutron flux densities of between 5 x 10¹² cm^-2s^-1 and 4 x 10¹⁴ cm^-2s^-1. A substantial advantage of these irradiation positions is the availability of a purely thermal neutron spectrum, in which the parasitic generation of undesirable radio-nuclides by threshold reactions or the occurrence of extended defect clusters through irradiation with fast neutrons is suppressed to the greatest possible extent.

Possible irradiation periods range from 1/10 second to 52 days of a cycle period for sample sizes from a few g up to silicon single crystal blocks 20 cm in diameter and 50 cm in length. The rapid, pneumatically operated rabbit system exhibits a conveying time from the irradiation to the measuring position of approximately 300 ms; here, it is possible to perform spectroscopy of short-lived species such as ²⁰F (T_1/2 = 11 s) and even ^207mPb (T_1/2 = 0.82 s). A second pneumatic rabbit system allows irradiation periods of between 30 s and 5 h. Long irradiation periods of up to several weeks are possible with the hydraulic capsule irradiation facility. As a result of the high flux density of fast neutrons, the position of the central control rod is the ideal production site for the positron emitter ⁵⁸Co. A particular example of future cooperation with industry is the production of homogeneously doped silicon; large Si blocks are irradiated in a homogeneous field of thermal neutrons, ³⁰Si being transmuted into stable ³¹P.

Table 1:Irradiation facilities at FRM-II

Neutrons are ideally suited to investigate the microscopic or atomic origin of modern functional materials. As a consequence most of the instrumentation at beam tubes or in the neutron guide hall are dedicated to material science. They are applicable to a large variety of materials and work pieces, ranging from metals to biomolecules, from engines to fuel cells.

Tumour therapy

The biological effect of high-energy radiation is based on the irreparable damage of the DNA in living cells. Neutrons display an outstanding efficiency in this respect compared to conventional X-ray or gamma-treatment: both cords of the DNA helix have to be cut in order to stop the self-repairing mechanism of a cell, and neutrons with their relatively high energy transfer to living matter are most effective for this purpose.

Neutron therapy is also advantageous in case of those tumours, where the supply of oxygen is highly reduced. Under hypoxic conditions neutrons are more efficient with regard to tumour cell kill than the X-rays used in conventional radiation therapy. Recurrent tumours in particular appear to be hypoxic.

In cooperation with the FRM, the department of radiation therapy of the Technische Universität München has been managing for several years a fast-neutron irradiation facility used for tumour therapy and basic research. There are very promising results for various types of near-surfaces tumours, especially in the region of the head and neck and for certain breast tumours. In cases of tumours with high resistance against X-ray treatment, such as malignant melanomas, neutrons often

show spectacular success. Until now, more than 700 patients were treated with fast neutrons at the FRM. In general this treatment is very well tolerated. For many patients a cessation of tumour growth has been achieved. Clinical application has proved that especially patients with highly differentiated salivary tumours benefit from neutron treatment.

The experience gained from the present neutron therapy at the FRM proved invaluable when it came to the design of the new facility at the FRM-II. Precise beam-handling, a large beam area, variation of the neutron energy and significantly higher intensity will now improve the therapeutic possibilities and will also furthermore promote research in this field.

Material science with positrons

The availability of thermal positrons with intensities in the order of 10⁹ - 10¹⁰ p/cm²s will enable new types of experiments. By positron life-time measurements it is expected to detect the defect structure of surface layers of metals and polymers. A positron microbeam should enable a positron microscope in order to reveal plastic deformations in the vicinity of micro cracks or electrical transport damage of conductor tracks on microchips. Positron induced Auger electron spectroscopy allows to detect the electronic structure of the upper most monolayer of solids. Contrarily to the well established Auger spectroscopy by electron beams this new method detects its signal free of background.