The Nuclear Research Reactor No. 1 “RECH-1” destined to support scientific and technological development activities has been operating since 1974 in the Center of Nuclear Studies La Reina.

RECH-1 is located in a reinforced concrete building, which maintains a slightly lower pressure that the atmospheric one, hence assuring a confined environment.  Within this building there is a structure of reinforced concrete that contains the reactors pool, of 250 m3 of demineralised water that is surrounded by 1, 5 meters of heavy concrete that acts as a shield.

The main component of the reactor is the nucleus that generates a power of 5 million Watts.  The nucleus is mainly formed by fuel elements, that contained enriched uranium in the ²³⁵U isotope, and it is located near to the bottom of the pool at depth of 10 meter.  The total mass of ²³⁵U in the nucleus is around 5 kilograms.

The fuel element is formed by sixteen flat plates, each one formed by the dispersion of a uranium compound in aluminum with a thin coating of aluminum in both sides.  The plates are separated from each other and supported in a type of box.  Both sides of the fuel element are open in order to allow the circulation of water by the channels left between the plates and to extract the heat generated during the operation of the reactor.  The heat extracted from the nucleus is then transferred to a second cooling circuit, through a heat exchanger, being finally unloaded to the environment using a cooling tower.

The neutrons, which assure that the fission reaction in the nucleus of the reactor is self sustained, are produced during the fission process of ²³⁵U.  However, these must diminish its initial energy that in average is of two million electron-volts to an energy level of around twenty five mili-electron-volt.  This energy moderation of neutrons is achieved by the clash of the water molecules that circulate between the fuel plates. The low energy electrons are called thermic neutrons and, even though, they are moving at a speed of 2.2 kilometer per second.

The trustworthiness and safety of the reactor is mainly based in its operations procedure, the ones that prevents the occurrence of any potentially dangerous situation, and in the existence of automatic circuits, that prevent actions that may lead the reactor to unsecure conditions.  The existence of protection systems limits the consequences of an incident or accident.

Analysis by prompt-gamma spectroscopy

The prompt-gamma spectroscopy is a non-destructive instrumental technique considered as complementary to the Neutronic activation analysis. This method is particularly useful to determine elements that do not generate radioactive products after the absorption of a neutron, but that do emit gamma rays in the instant of the Neutronic capture.  The base of this technique is in the measurement of the gamma radiation that is characteristic of each element issued during the formation of the composed nucleus in the first stage of the nuclear reaction of Neutronic capture. Since this disintegration takes place in an extraordinary short period of time, the exposition of the sample to the Neutronic beams and the measurement of the gamma radiation issued must be simultaneously.

This technique has proved to be a useful and trustful analytical method to simultaneously measure the concentration between 15 to 25 elements in several matrixes. The C, N, Si, S, Ca, Fe, etc. may be measured in concentrations of 1%.  The elements that can be measured in intermediate concentrations (approximately 100 ppm) are H, F, CI, K, V, etc.  The maximum sensitivity is obtained for B, Cd, Sm, Gd and Hg, the ones that are measured at a trace level (less than 1 ppm).  The main applications are in agriculture, biology, environment and geology.

The prompt-gamma facility, located in the TN tube of the RECH-1 reactor, uses a collimated beam of thermic neutrons that has an incidence on the sample.  A hyper pure germanium detector of high resolution, joint to a multi-channel pulse analyzer and to the electronic associated to it, measures the energy and intensity of the prompt-gamma radiation issued.  A computer and last generation analysis software is used for the data acquisition and the processing of the spectrums. The energy allows identifying the elements, while the intensity of the pulse of that energy reveals its concentration.

Scattering of Neutrons.

This instrument is complementary to the neutrons diffractometer, allowing determining average deviations of the regular crystal structure and the eventual organization of defects in simple solids (formed by few atomic species). It also allows the observation of some effects of the vibrational structure of the solid over the scattering.

The instrument measures the incoherence scattering intensity of neutrons on the cylindrical samples in a horizontal arc of 110º.  It is located in tube TR1 of the reactor al it uses the flight time technique to provide an intensity distribution of the scattering in relation to the wave length and the observation angle.  It has a Bismute-Berilium filter cooled to the temperature of liquid nitrogen (77K) to eliminate the wave lengths that are less than 3,99 Å (Bragg wave longitudes).  As a beam pulse a disc with 6 transparent windows to the neutrons is used, a square section of 2 cm on each side is located close to the periphery. The disc turns at a velocity of 6000 rpm, the neutron pulses appear every 1,75 mili seconds. At 60 cm of the disc pulse the sample is located and at 100 cm of the sample the detector banks.

The electronic control of the instrument was updated and it allows collecting and analyzing simultaneous information of 14 neutron detectors installed in a measurement bank, plus two fission chambers for calibration and a frontal detector type BF3.  This is a multi-architects scale of 17 ways and 128 channels of time per way, of synchronized cycle with the neutron beamer and of a residence time of 10 micro-seconds per channel.

Neutron Diffractometer

The diffraction of neutrons is the best way to obtain detailed structural information at an atomic level in many types of materials.  In mono-crystals provides more precise data, however, big enough samples of mono-crystals are not always available and, frequently the materials of technological interest are in a dust or poly crystal form.  Instrumentation development and analysis techniques have made it possible to obtain comparatively precise structural information of the neutron diffraction on dust samples.

The more generalized application of this diffractometer is the determination of the crystalline structure of the materials and to investigate some of the phenomena that affect the structure. This structural information can also be obtained at a variable temperature between -245 a 20ºC the one that, is often, enough to understand the properties of the material that has scientific and technological interest, including among these magnetic materials, super-conduits of high temperature, ceramic, minerals and ionic conductors.
The neutron dispersion mainly occurs in the atomic nucleus, and the effective area of coherent dispersion is of the same order of magnitude for light and heavy atoms, and its dependence with the atomic number is completely irregular, contrary to what occurs with x-rays, where the dispersion factor depends on the square of the atomic number, thus implying that light atoms are hidden by heavy atoms.  This advantage has driven the interest toward the diffraction of neutrons using dust samples.

The diffractometer RECH-1, located in TR2 tube, has two axes, one in the germanium monochromator crystal where the wave length of the neutrons is selected (l), and the other in the sample, where variable dispersion angles are obtained. A detector sweeps an arc centered in the dust sample recording the intensity of the diffracted neutrons with respect to the angle (2q) formed by the incandescent beam and the dispersed one. The spectrum of the diffracted intensity in respect to the angle (2q) contains the basic information to obtain the structural parameters of the pulverized crystalline material sample.

Deep Neutron Profile

The NDP (Neutron Depth Profiling) instrument uses the neutron beam of the TR3 tube for the non-destructive evaluation, of the distribution and concentration of light elements in respect of the depth of the solid ones. With this technique it is possible to analyze the first micrometers of almost any condensed material, with detection limits of 10-100 ppm and a depth resolution of around 20nm. The elements that are most easily observable are lithium, boron, beryllium and sodium.
The application of this technique is in optics and in the field of material science, such as polymers, metallic alloys and in micro electronic materials, being also possible to obtain profiles trough inter facial frontiers.  As examples of the applications we can mention: measurement of boron profile implanted in silicon, profile of superficial boron in glass, profiling of lithium concentrations in aluminum and nitrogen alloys implanted in steel.

To obtain a depth profile, a beam of collimated thermical neutrons is used with which the sample is illuminated.  Some of these neutrons will interact with the nucleus of the elements of interest causing the emission of mono-energetically charged particles. These particles (alpha or protons) escape from the material in routes that are mainly lineal with a loss of energy through the numerous interactions of the electrons and the material.  The difference between the initial energy of the particle and its residual energy when emerging to the surface of the sample is directly related with the depth of the origin of the particle, this is, in relation to the position of the father atom. An aluminum vacuum chamber gathers the sample up to 15 cm of the diameter and a full arrangement of the electronic components is enabled for the acquisition and data processing.