Zahlavi

HK3 - Thermal neutron analytical methods

Three instruments HC3-a HC3-b and HC3-c operate at the thermal neutron beam formed by short neutron guide tube. The neutron guide ensures an efficient transport of thermal neutrons from a Ć 100 mm horizontal channel of the reactor to small target areas of the HC3-a, HC3-b and HC3-c spectrometers. The neutron guide is built of a mirror type tube of rectangular cross-section, cylindrically bent in the vertical direction. It consists of 15 mirror sections made of glass plates of float type. The surface of these plates is coated with Ni reflecting layer with thickness of 2000 Å. The internal cross-section of each mirror section is 4×150 mm2. The overall length of the guide is 5.63 m, the curvature radius being 825 m. In order to suppress a background due to a direct beam of gamma rays and fast neutrons, the guide is tightly surrounded by a combined shielding consisting of lead and polyethylene pellets. Unlike thermal neutrons, gamma rays and fast neutrons are not subject to reflections from the Ni coating and penetrate the guide walls. In the shielding around they are scattered and absorbed and only collimated thermal neutrons pass through. In addition, a biological shielding, formed by boron-doped polyethylene and lead bricks, is built along the whole guide. The shape of the incoming neutron beam at the entrance of the guide matches the cross-section of the guide. This has been achieved using a 90 cm long collimator made of lead with a rectangular aperture. Immediately behind the guide exit, the neutron beam is tailored by an additional collimator made of 6Li2CO3 to reduce the beam cross-section to 4×60 mm2. The flux of thermal neutrons at the guide exit averaged over the beam cross-section is (1.5±0.2)·107n cm-2 s-1. The cadmium ratio is equal approximately to 105. The above mentioned fluxes refer to the reactor power of 8 MW. Beyond the guide exit, the vertical divergence of neutron trajectories is characterized by angular deviations below 0.5°.

Neutron Depth Profiling

Thermal Neutron Depth Profiling (NDP) facility was set up just behind the neutron guide at HK3. The multidetector spectrometer consists of a large vacuum chamber, automatic target holders and several different data acquisition systems which can be used at the same time. NDP is the nuclear analytical technique available to profile light elements in solids. It utilizes the existence of isotopes of elements that produce prompt monoenergetic charged particles upon capture of thermal neutrons.

From the energy loss spectra of emitted products the depth distributions of light elements can be reconstructed. The NDP method is an excellent tool for studies of numerous problems in solid-state physics (diffusion, sputtering), material science (corrosion), electronics, optronics, life sciences etc. Its applicability and efficiency has steadily expanded.

List of nuclides exploited in NDP analyses


Nuclide Natural abundance 
or activity
Nuclear reaction Energy of reaction products Cross section Detection limit

  [atoms/mCi] %   E1 [keV] E2 [eV] [barns] [at/cm2]
3He 0.00013 3He(n,p)3H 573 191 5326 3.1·1013
6Li 7.42 6Li(n,a)3H 2051 2734 94 1.8·1014
7Be* 2.5·1014 7Be(n,p)7Li 143 207 48000 3.5·1012
10B 19.6 10B(n,ag)7Li 1471 839 3606 4.3·1013
10B 19.6 10B(n,a)7Li 1775 1014 230 6.7·1014
14N 99.64 14N(n,p)14C 584 42 1.81 9.1·1016
17O 0.037 17O(n,a)14C 1415 404 0.24 7.1·1017
22Na* 4.4·1015 22Na(n,p)22Ne 2247 103 31000 4.7·1012
33S 0.76 33S(n,a)30Si 3091 412 0.14 1.2·1018
35Cl 75.5 35Cl(n,p)35S 598 17 0.49 3.4·1017
40K 0.012 40K(n,p)40Ar 2231 56 4.4 3.8·1016
59Ni* 1.3·1020 59Ni(n,a)56Fe 4757 340 12.3 1.4·1016
209Bi 100.0 209Bi + n -> 210Bi     0.02 1.2·1019
    210Bi -> b + 210Po        
    210Po -> a + 206Pb   5300    


Instrument Parameters

Beam cross-section   4 x 60 mm
Neutron flux   1·107 n/cm2s
Sample size   50 - 1000 mm 2
Detector systems:  
- standard arrangement
(single detector facing the sample)
  4
- sandwich arrangement
large angle coincidence spectroscopy (2-dimensional data processing)
  1
- dE-E telescope arrangement   1
- pulse shape discrimination analysis   2
Solid angle of the detector-sample system   0.001% - 0.1%
Detectors   PIN photodiode - HAMAMATSU
Charged particle detector - CANBERRA
Detector areas   50 - 300 mm
Depletion depth   10 - 100 µm
Detection limit  
- standard arrangement - N, Cl   10-3 % at.
- He, Li, B   10-5 % at.
- sandwich arrangement - Li, B   10-8 % at.
Depth resolution   10 - 50 nm
Maximum detectable depth interval   3 - 70 µm

 Spectrometer NDP

Prompt Gamma Activation Analysis

The facility for the measurements of 10B concentrations in biological samples includes HPGe detector with 25% relative efficiency and associated Pb - 6Li2CO3 shielding. Described facility is installed at the distance of 1 m from the exit of the neutron guide. At the target position the neutron flux is approximately 3×106 n cm-2 s-1 at the reactor power of 8 MW. This facility makes it possible to determine 10B concentration of 1 ppm in 1 ml samples with statistical uncertainty of 5% within 15 min. This instrument is in a common property with Nuclear Research Institute, plc.

Applications

The experimental set-up was mainly developed for the on-line determination of 10B concentrations of ppm-order in biological samples. This facility can also be used for determination of other isotopes with a sufficiently large (n,g) cross-section.

Instrument Parameters

Neutron flux   3·106 n/cm-2 s-1
Beam cross section at target position
  25 x 5 mm2
Detector
  HPGe (25% relative efficiency)
Sample enclosure for liquid and powder sample   Teflon cylinder vial: Diameter - 10mm & Height - 10mm

Expected interference-free detection limit for PGAA instrument at research reactor LVR-15 assuming 24h irradiation.

Element Det. limit  (µg) Eγ (keV) Element Det. limit  (µg) Eγ (keV)
 Hydrogen     20  2223  Cobalt      20  230, 556
 Boron       0.06  478  Nickel    200  283, 465
 Nitrogen  4000  1885, 5298  Copper      10  159, 278
 Sodium     70  472, 869  Zinc    700  115, 1077
 Magnesium  2000  585,1809  Selenium      40  239
 Aluminium   500  1779, 7724  Molybdenum    150  720, 778
 Phosphorus  2000  637, 1072  Silver      30  192, 236
 Sulfur   300  840,  2379  Cadmium        0.1  559, 651
 Chlorine     10  517, 786  Samarium        0.03  333, 439
 Potassium    100  770, 7771  Gadolinium        0.02  182, 1186
 Calcium    600  519, 1943  Gold      30  215
 Titanium     40  342, 1381  Mercury        1.5  368
 Chromium   150  749,834  Lead  40000   7368
 Manganese     30  847, 1811  Neodymium        10  619, 697
 Iron   300  352, 7631  Indium          5  162, 186

Application Example

The deterimantion of boron concentration in graphene powder.

Electronic properties of graphene can be changed by doping with electron-donating (nitrogen, phosphorus) or electron-withdrawing (boron) groups. Level of doping (concentration of doped group) is crucial for for tuning the electrochemistry performance of graphene. Boron-doped were prepared by thermal exfoliation of graphite oxide in an atmosphere with boron trifuoride diethyl etherate at high temperatures. The effect of exfoliation temperature as well as of hydrogen content in atmosphere were investigated using PGAA at LVR-15 Řež.
Special software was developed for precise fitting of Doppler-broadened boron peak at 478 keV.

 

Fig. 1 Example of Doppler-broadened boron peak at 478 keV in gamma spectrum of graphene powder.

Thermal Neutron Capture Facility

Two-germanium-detector system is employed for the study of γ-γ coincidences from (n,g) reactions. The horizontally oriented detectors are placed above and bellow the horizontally oriented target, their axes being mutually parallel. At present, the bottom detector is of the HPGe type with the 20% relative efficiency and the energy resolution of 1.9 keV at Eg= 332 keV, while the Ge(Li) detector with the 12% relative efficiency and 2.1 keV energy resolution at 1332 keV is located above the target. These detectors can be easily replaced by another ones. The distance between cylindrical surfaces of the germanium crystals is 4 cm only. The neutron flux at the target position is (2.8 ± 0.5)·106 n cm-2 s-1 at the reactor power of 8 MW. The coincidence efficiency for a g-cascade from the 60Co source is 3·10 -5, including the effect of the solid angle. The detectors are shielded from g-ray and neutron background in the reactor hall by a combined shielding. The electronic system is based on the standard fast/slow coincidence arrangement. The events are accumulated by a PC computer.

Applications

The facility was designed primarily for the study of photon–strength functions, but it can also be used for nuclear structure studies via g-g coincidences following the thermal neutron capture.

Instrument Parameters

Neutron flux at the target position
3·106 n/cm2 s
Cadmium ratio
105
Beam cross section at target position
25 x 5 mm2
Detectors HPGe (20% relative efficiency)
Ge(Li) (12% relative efficiency)
Coincidence full-energy-peak efficiency 3·105 for 60Co (1173 keV+1332 keV)