Development of a new neutron mirror made of deuterated Diamond-like carbon

We developed a new neutron mirror made of Diamond-like carbon (DLC). DLC is a film of amorphous carbon that has characteristics of both diamond and graphite. We produced DLC mirrors by ionization deposition method which is one of the chemical vapor deposition (CVD). Generally, DLC made by CVD contents a few tens of percentages of hydrogen. It decreases the Fermi potential of the DLC coating because hydrogen has negative Fermi potential. In order to increase the Fermi potential of the coating, we deuterated the DLC by using deuterated benzene for the source gas. The characteristics of the deuterated DLC(DDLC) coating was evaluated by RBS, ERDA, x-ray reflectivity, AFM. As a result, DDLC coating has 243neV due to deuteration, which is the same level as Ni. The RMS of height of the DDLC was 0.6nm so that the DDLC coating can be applied for a focusing mirror or specular transportation of pulsed neutron. Besides, we also develop Hydrogen/Deuterium DLC multiple layer mirror. So far, 4 layers mirror has been succeeded.


Introduction
With the development of various neutron mirrors, neutron application is spreading in particle physics and even industrial society. Therefore, better performance of the neutron mirror is getting more important. Generally, it is said that good neutron mirror has high Fermi potential and low off specular reflectivity. The Fermi potential determined the limit of neutron reflection energy and how much amount of neutrons can be collected. The off-specular reflectivity determined how controllable neutron beam is. Ni alloy is well-used as a neutron mirror because of high Fermi potential and low off-specular reflectivity , for example, NiC, NiV. Other typical Figure 1. Schematic view of ionized evaporation method. The deposition mechanism is that (a) benzene or deuterated benzen is flowed in the chamber, (b) source gas is ionized by reacting the electron generated from a filament, (c)supettering ionized benzene to substrate by voltage,V b . Ar gas is used to make substrate's surface smooth before the deposition of DLC layer materials are Fe for a magnetic mirror, Be and Diamond-Like Carbon(DLC) for an alternative to Ni mirror. However, Be is difficult to handle because of poisonous nature. On the other hand, as the technology of making a film of DLC has recently been developing, DLC is gradually getting attention as a new neutron mirror. The critical energy of the reflection of neutrons is given by the Fermi V F potential of the material, which is written as whereh is plank constant divided by 2π, m n is the neutron mass, N i is the number density of the elements in the material and b i is their scattering lengths. Ni mirror has 243neV in this parameter, which is well-known as m = 1. The diamond is known as the best potential in all materials. Therefore, it is said that DLC is possible to be a mirror with a large Fermi potential. DLC is a film of amorphous carbon that has characteristics of both diamond and graphite. It can be classified by its three components, which are diamond, graphite and hydrogen. At recent work [1], hydrogen free DLC is selected for a neutron guide. This is because hydrogen has a negative factor of it. However, if all hydrogen is replaced by deuterium, its Fermi potential could be large enough to be applicable because deuterium has a positive potential and the small absorption cross-section for neutrons. We report about fabrication and evaluation of our new DLC(DDLC) neutron mirror, comparing with previous researches [2][3].

Fabrication
Techniques for producing diamond coatings have changed dramatically over the last few years.
It is now possible to coat large areas with a diamond-like material by means of chemical vapor deposition(CVD) [4]. We selected an ionized evaporation method to make a film of DLC, which is classified as a means of CVD. A reason to select this method is to be able to coat on a large and curve area. Another reason is that it is possible to make DDLC and HDLC multiple layer mirror, if hydrogen benzene and deuterium benzene are used. Hydrocarbon gas, benzene, is used for the source material. DDLC has been deposited by using deuterated benzene as source gas to replace hydrogen in the layer. Figure.1 is schematic view of ionized evaporation method. The deposition mechanism is that (a) benzene or deuterated benzen is flowed in the chamber, (b) source gas is ionized by reacting the electron generated from a filament, (c)supettering ionized benzene to substrate by voltage,V b . Ar gas is used to make substrate's surface smooth before the deposition of DLC layer. Silicon wafers with 3 inch diameter for a substrate. Surface of the wafer was sputtered by argon gas to make its surface smooth before the deposition of DLC layer. The parameters to deposit DLC are voltage to substrate, pressure of source gas, and temperature in the chamber. We produced DLC mirrors with changing these parameters for optimization. The range of parameters were 0 to 3.0kV for substrate voltageV b , 0.001 to 0.1Pa for the pressure of the source gas, and 200 • C to 400 • C for the temperature in the chamber.

Evaluation of the DDLC mirrors
The characteristics of DDLC as a neutron mirror was evaluated by Rutherford Backscattering Spectrometry(RBS), Elastic Recoil Data Analysis(ERDA), X-Ray Reflectivity(XRR), Atomic Force Microscope(AFM) and Neutron Reflectivity(NR) measurements at J-PARC BL16 SOFIA. From now we report results of these measurements respectively.

Neutron Reflectivity
Neutron reflectivity(NR) was measured at neutron reflectometer in J-PARC BL16 SOFIA [5]. This is the direct measurement for the Fermi potential. The wavelength of the incident neutron is 0.2 -0.9nm. By changing two slits, it is able to select the range of neutron beam at a film. The result of typical neutron reflectivity is shown in Figure. Figure.2(right) shows that the correlation between film-forming condition and neutron reflectivity, where high pressure is 0.1Pa, the filled circle, low pressure is 0.01Pa, the open circle. The best condition for the Fermi potential was 0.5kV for the substrate voltage. There was no temperature in the chamber dependence between 200 • C to 400 • C. Therefore, temperature in chamber is set at about 300 • C. On the other hands, Fermi potential of DDLC is high when pressure condition is low.

Measurement of mass density and chemical compositions
As futher evaluations, mass density and chemical compositions were measured by XRR in RIKEN[6] and RBS/ERDA at Tsukuba university [7]. XRR is a method to measure electron number density and thickness of a film from a spectrum of x-ray reflection. When X-rays are applied to a material's surface, a portion of x-rays is reflected at every interface. Interference of these partially reflected x ray beams creates a reflectometry pattern. By analyzing it, the parameters are determined. The source of x-ray is CuKα with wavelength of 1.54Å. The two slits set to measure the same position, which was measured by the NR. The evaluation of thickness in XRR is consistent with NR within 3%. RBS/ERDA is a method to measure chemical composition of a film. It is possible to determine the chemical composition and amount of the targets by measuring counts of scattered ions caused by He ions irradiation on the sample. Figure. Figure.4 shows the result of relation between (right)Deuterium components and film-forming condition, (left)density of film. The mass density of film was calculated by electron number density measured by XRR and chemical compositions measured by RBS/ERDA. There is a tendency in this figure that deuterium components and mass density of film is high with low pressure, which is reason why Fermi potentital is high when pressure is low. For the sample of 243neV, the parameters are respectively, the mass of density is 2.07(8) g/cm 3 , the components are 43(4)% of deuterium and 3.3(6)% of hydrogen in DDLC. Additionally, DDLC still contents 1-5% of hydrogen components.

Consistency of Fermi potential between neutron reflectivity and RBS/ERDA,XRR
From the equation (1), we get the equation where N A is Avogadro constant, A i is the atomic mass of i material, R i is the material of chemical composition. NR is the direct measurement of Fermi potential, whereas XRR and RBS/ERDA have the relation with Fermi potential. Therefore, substituting two parameters, mass density and chemical compositions, for Eq.2, Fermi potential is calculated. Figure.5 shows that y axis is Fermi potential of NR measurement, x axis is Fermi potential calculated by ρ M R i . The open circle is hydrogen DLC(HDLC) and closed circle is DDLC. In HDLC data, the value measured by NR tend to be larger than the value calculated by ρ M R i , while DDLC data has opposite trend of it. There is the consistency with error of 8%. The error of cross section of H/He, D/He caused this error.

roughness of DDLC
There are two types of refection; one is a specular reflectivity whose incident angle is same as refection angle, the other is off specular reflectivity whose angle not. As a model recently studied [10], roughness causes off-specular reflectivity. As generally, roughness causes off-specular reflectivity. Therefore, firstly, we measured the surface roughness by AFM to evaluate it. The result shows that DDLC surface is very smooth, rms of height is 0.6(2)nm in DDLC, on the other hand, 0.3(1)nm in Si substrate in Fig.6, where b is rms of height. Therefore, low off-specular reflectivity of DDLC can be expected.

a multiple layers of DDLC/HDLC
As mentioned above, it is possible to produce a DDLC/HDLC multiple layer by changing the source gas between hydrated and deuterated benzene. a multiple layers is a method of exceeding the critical angle of total reflection. It provide reflections from multiple interfaces between different indexes of refraction. Therefore, on trial, we made the forth multiple layer. Figure.7 shows a spectrum of neutron reflectivity of DDLC/HDLC 4 layer(solid line) with a single layer mirror of DDLC(dotted line). There are some sign of multiple reflection at figure.7.

conclusion
We fabricated a DDLC coating for a new neutron mirror, and measured its characteristics. Fermi potentials measured by NR were 190-243 neV. The maximum Fermi potential of DDLC gained 10% from the previous report[2] [3], which is the same as Fermi potential of NiC, 243neV.
The typical surface roughness of DDLC measured by AFM was 0.6 nm as a RMS of height. Therefore, low off-specular reflectivity of DDLC can be expected. The chemical composition was measured by RBS/ERDA. The typical components of hydrogen and deuterium was 3% and 43%, respectively. These results show that performance of DDLC as a neutron mirror is satisfactory. Therefore, DDLC will be useful for neutron guides with complicated shapes because DDLC can be deposited on any surface by CVD. In addition, we successfully made DDLC/HDLC multiple layer mirror.