Contact

Dr. Arnd Junghans
Project leader
Nuclear Physics
a.junghansAthzdr.de
Phone: +49 351 260 - 3589
Fax: +49 351 260 - 13589

Dr. Roland Beyer
PostDoc / Beamline scientist
Nuclear Physics
roland.beyerAthzdr.de
Phone: +49 351 260 - 3281

nELBE: Neutron time-of-flight experiments at ELBE

E. Altstadt1, C. Beckert1, R. Beyer2, H. Freiesleben3, V. Galindo1, M. Greschner3, E. Grosse2,3, A. R. Junghans2, J. Klug2,  D. Légrády1, B. Naumann4, K. Noack1, K.D. Schilling2, R. Schlenk5, S. Schneider5, K. Seidel3, A. Wagner2, F.-P. Weiss1,6

1Institut für Sicherheitsforschung, FZD
2Institut für Strahlenphysik, FZD
3Institut  für Kern- und Teilchenphysik, TUD
4Abteilung Sicherheit, Strahlenschutz, FZD
5Abteilung Mechanische Entwicklung, FZD
6Institut  für Energietechnik, TUD

The main components of the neutron time-of-flight source being developed at ELBE are described below and in [1,2]. The neutron time-of-flight source consists of  the liquid-lead neutron radiator utilising the pulsed electron beam, the collimator shaping the neutron beam, and the different detector setups for studying the neutron-induced reactions. In addition, a comparison is made with other neutron time-of-flight facilities.

The liquid-lead neutron radiator

When operating the liquid-lead radiator, the electron beam passes a beryllium window mounted on a stainless steel vacuum chamber and hits the radiator, consisting of a molybdenum channel confining the liquid lead. The Be window and the vacuum chamber constitute the radiator housing indicated in the upper part of the right figure. The drawing below shows the radiator in some detail. A lead-shielded, water-cooled aluminium beam dump absorbs the unused radiation (electrons, photons, neutrons). Apart from the molybdenum channel in the vacuum chamber, the liquid lead is transported in an isolated and electrically heatable stainless-steel tube circuit, and cooled through a heat exchanger using an InGaSn eutecticum as an intermediate heat transfer fluid.

The neutrons are emitted almost isotropically from the radiator, while a large part of the electrons and the bremsstrahlung photons mainly emerge in the forward direction. Thereby, using the neutrons emitted about 90° from the direction of the electron beam, a high suppression of the photon-to-neutron rate is obtained. The neutrons are shaped by a collimator into a well-defined beam, entering the experimental site in the adjacent room.

To take advantage of the picosecond electron beam pulse length, the radiator volume needs to be small enough to keep the time interval of beam passage and photoneutron production in the sub-nanosecond range. This also minimizes scattering inside the radiator, and the background of thermal neutrons. At the same time, the radiator volume has to be large enough to obtain a reasonable conversion efficiency. Simulations have shown that a good trade-off between source strength and energy resolution is obtained with a volume of approximately 1 cm3.

The electron beam will deposit up to 25 kW thermal load in the radiator – too high to be dissipated by gas cooling and heat radiation for a solid target of such small size. Since cooling with water is unfavorable due to its neutron-moderating properties, liquid lead was chosen as radiator material. This also has the advantages of a low-lying giant dipole resonance giving a high (³,xn) neutron yield, a large temperature interval (1420 K) between melting and boiling points, and less activation compared to other possible radiator materials (e.g., mercury). A lead channel with a cross section of (11.2 mm)2 was chosen, corresponding to two radiation lengths for electrons in lead. Molybdenum was selected for the channel walls (0.5 mm thick) due to its good strain tolerance and heat conductivity.

A spindle lifter was constructed to raise the whole radiator and beam dump assembly 3 m up from its rest position inside a lead shield, to the level of the electron beam line, as shown in the figure. The park position allows for the accessibility of the cave and the operation of the other neutron sources.

Monte Carlo simulations were performed using MCNP4C3 [3] to characterize neutron and photon intensities, time and energy distributions, and resolutions. The main parameters determining the neutron intensity at the sample position are the radiator dimensions, the energy of the beam electrons, the beam current, and the length of the neutron flight path. At present, the maximum average beam current is 1 mA with a repetition rate of 13 MHz. A superconducting RF photo electron injector (SRF gun) is being developed, allowing for a beam current of 1 mA at a repetition rate of 0.5 MHz [4]. For electrons with energy Ee = 40 MeV and a beam current of Ie = 1 mA, the simulations predict a neutron source strength of 2.7·1013 s-1 from the radiator and a neutron flux density of 1.5·107 cm-2 s-1 at the measuring position 3.9 m from the radiator (after the collimator). The table below shows a compilation of fluxes for different electron energies.

Electron energy / MeV

Radiator source strength / s-1

Flux density at measuring position / cm-2 s-1

20

7.9·1012

4.3·106

30

1.9·1013

1.0·107

40

2.7·1013

1.5·107

The figure below shows the energy distribution of the significant contributions to the neutron flux density at the measuring position, with Ee = 30 MeV and Ie = 1 mA. The total distribution is represented by black diamonds. Almost 92 % stem directly from the lead in the radiator (light squares), while about 8 % of the neutrons were scattered or created in the molybdenum channel (red circles an order of magnitude below the total). A very small fraction (< 0.1 %) was scattered in the steel housing accommodating the radiator (green triangles).

Neutronenflussdichteverteilung - Beiträge 2

The neutron beam collimator

The collimator between the neutron radiator and the experimental hall must eliminate neutrons and photons that are scattered or produced in the collimator, in order to create a well-defined beam of unscattered neutrons with a sharp edge, and a minimal background of neutrons and photons outside the beam. In addition to this, the component of slow neutrons coming directly from the radiator, overlapping into the next beam pulse and creating measurement ambiguities, as well as the direct photon intensity at the experimental site, must be minimized.

The figure to the right shows an example of an investigated collimator composition placed in the wall consisting of 1.2 m concrete (green) and 1.2 m of heavy concrete (blue). Grey collimator insertions are made from lead and yellow ones from borated polyethylene (CHB), while uncoloured regions do not contain any material. The neutrons travel from left to right, and the lead insertions are placed after the CHB sections in order to absorb photons created in (n,γ) reactions. An additional 10 cm thick lead shield is placed after the wall. The collimator opening (beam diameter) is 3 cm. The increasingly larger outer radii are 5, 6 and 7 cm, surrounded by a stainless steel tube with 1 cm thick walls.

Neutron beam profiles for different collimator compositions are shown below, given by detectors placed at an axial distance of 3.9 m from the radiator, and at increasing radial distances from the beam center. The collimators shape a neutron beam with a sharp edge, indicated by the intensity drop between r = 1.5 cm and r = 2.0 cm. At larger distances, the intensity is four orders of magnitude less than in the beam. The best result is obtained with a conical hole with an entrance diameter of 2 cm. A similar trend has been shown for photons.

Strahlprofil, Koll., n

Absorbers for slow neutrons

With an accelerator repetition rate of 13 MHz, the time interval between beam pulses is 77 ns. If the electrons hit the radiator at t = 0 ns, the photons arrive at the measuring position 13 ns later, and the fastest neutrons at t = 65 ns. For Ee = 30 MeV, the neutron time distribution has a peak at about 180 ns and a tail extending up to 1 µs. This means that most of the neutrons will overlap into the following pulses, making unambiguous measurements impossible. This problem can be overcome by lowering the repetition rate to 0.5 MHz, and by placing an absorber in the beam. The loss in beam intensity due to a lower repetition rate will be compensated for with the SRF gun. The effect of different absorbers is shown in this figure:

Neutronenflussdichteverteilung - Absorber 2

With a frequency of 0.5 MHz, the next pulse comes 2 µs later, as indicated in the figure. Thus, neutrons with energies less than En = 20 keV create a background in subsequent pulses, and must be suppressed. The plot shows that with a 5 cm thick slab of polyethylene (PE) and a thin disc of Cd (green triangles) placed at the collimator entrance, the neutron flux density drops two orders of magnitude at such low energies, compared to the situation with no absorber (light squares).

The cost of using the absorber is a decrease in peak intensity by a factor of 5, but the relative background reduction is considerable. This is illustrated in the figure below. The upper graph with black squares shows the time distribution of the neutron flux density from one beam pulse, accompanied by the background from the previous pulse (light grey squares). Below these, the graphs with triangles show the same situation with an absorber of 5 cm PE and 5 mm Cd. If a 2.5 MeV neutron is measured without absorber (arriving 180 ns after leaving the radiator; indicated in the figure), the background flux density from the previous pulse is 500 times lower. With the absorber in place, the signal-to-background ratio changes to 104.

Neutronenflugzeit - 2 Pulse 2

The correlation between neutron kinetic energy (En) and time of flight (ToF) is shown in in the following scatter plots. These show the major part of the energy and time ranges, and the region around the flux density maximum, respectively. The main ridge in the spectra is from neutrons with a correct energy to time-of-flight correlation. Neutrons that have lost their undisturbed correlation between En and ToF form a tail to the main ridge. They are suppressed by two to three orders of magnitude, and constitute only 4 % of all events. The low intensity ridge approximately 20 ns above the distribution of unscattered neutrons is due to scattering in the stainless steel housing around the radiator. The inset in the more detailed plot shows a horizontal cut through the spectrum at 230 ns. The energy width (FWHM) of the peak for neutrons with this ToF is 5 keV, which corresponds to an energy resolution less than 0.4 % at En = 1.5 MeV.

The corresponding spectra when inserting the before mentioned absorber into the collimator shows that the En-ToF correlation is maintained, which confirms the feasibility of using such an absorber. The only change is the intensity drop in the peak; by a factor of five.

Detector development

For measurements of neutron-induced reactions, different detector types are being developed. For neutron capture ³-rays, a BaF2 scintillation detector array of up to 60 crystals is being built, as shown in the figure. The sample position is in the center of the ring formed by the scintillators, which together cover 80 % of the total solid angle. The crystals are 19 cm long and have a hexagonal cross section with an inner diameter of 53 mm. They are read out by fast Hamamatsu R2059 PM tubes, which are UV sensitive to be able to measure both the slow and the fast component of the BaF2 scintillation light. Thereby pulse shape discrimination (PSD) can be utilised to separate photon signals from intrinsic ³-particle background. The time resolution attained with a 60Co ³-source is typically 650 ps (FWHM). The readout will be performed with dedicated ADC/TAC modules [5] that allow simultaneous measurement of timing and energy signals including PSD in VMEbus standard. The system will be controlled by a RIO3 real-time Unix computer.

For neutron detection, Li-glass scintillators and a plastic scintillator wall are being developed. The plastic scintillator wall will allow detection of fast neutrons from approximately 25 keV kinetic energy through proton recoils in the scintillation material, see ref. [6]. The wall will cover an active area of about 1 m2, consisting of 10 scintillation panels (100 cm x 12 cm x 1 cm) read out on both ends.

For neutron energies below 500 keV, Li-glass scintillators will be used. They are enriched with up to 18 % 6Li and allow detection of low-energy neutrons mainly through the 6Li(n,t)± reaction.

Comparison with other facilities

As can be seen in the following table, the setup is very competitive with all existing high-resolution neutron beams in its luminosity, including n_TOF at CERN-PS. The proton-accelerator based neutron sources at Los Alamos and the planned Oak Ridge Neutron Spallation Source lose a significant part of their intensity advantage over ELBE when they increase their flight path in order to reach an energy resolution of less than 1 %, as expected for ELBE.

Facility

CERN n_TOF

CERN n_TOF Phase-2

LANL NSC

ORNL SNS

FZK VdG

ORNL ORELA

IRMM GELINA

ELBE

ELBE with SRF

Pulse charge / nC

ca. 103

ca. 103

4·103

3·104

0.01

ca. 100

ca. 100

0.08

1.8

Power  / kW

10

10

60

1000

0.4

8

7

5

40

Pulse rate / s-1

0.4

0.4

20

60

2.5·105

500

800

1.6·106

5·105

Flight path / m

183

ca. 20

60

84

0.8

40

20

4

4

n pulse length / ns

> 7

> 7

125

100-700

ca. 1

> 4

> 1

< 0.4

< 0.4

Emin / eV

0.1

0.1

1

0.1

103

10

10

2·105

5·104

Emax / eV

3·108

3·108

ca. 108

ca. 108

2·105

5·106

4·106

7·106

1·107

Resol at 1 MeV / %

0.5

5

ca. 10

> 10

ca. 10

< 1

< 2

ca. 1

ca. 1

n flux dens /
s-1 cm-2 (E decade)-1

105

ca. 107

ca. 106

106-107

ca. 104

104

4·104

4·105

3·106

[1] A photo-neutron source for time-of-flight measurements at the radiation source ELBE
E. Altstadt, C. Beckert, H. Freiesleben, V. Galindo, E. Grosse, A.R. Junghans, J. Klug, B. Naumann,
S. Schneider, R. Schlenk, A. Wagner, and F.-P. Weiss;
Ann. Nucl. Energy 34 (2007) 36

[2] Development of a neutron time-of-flight source at the ELBE accelerator
J. Klug, E. Altstadt, C. Beckert, R. Beyer, H. Freiesleben, V. Galindo, E. Grosse, A.R. Junghans,
D. Legrady, B. Naumann, K. Noack, G. Rusev, K.D. Schilling, R. Schlenk, S. Schneider, A. Wagner,
F.-P. Weiss
Nucl. Inst. Meth. A 577 (2007)  641

[3] MCNP – a General Monte Carlo N-Particle Transport Code (Report LA13709) ed. J. F. Briesmeister 2000 (Los Alamos: Los Alamos National Laboratory) 

[4] J. Teichert, FZD, private communication, 2005 

[5] C. P. Drexler, U. Thöring et al 2003 IEEE Trans. Nucl. Sci. 50 969

[6] Proton-recoil detectors for time-of-flight measurements of neutrons with kinetic energies from some tens of keV to a few MeV
R. Beyer E. Grosse, K. Heidel, J. Hutsch, A.R. Junghans, J. Klug, D. Legrady, R. Nolte, S. Röttger,
M. Sobiella, A. Wagner
Nucl. Inst. Meth. A 575 (2007)  449


Contact

Dr. Arnd Junghans
Project leader
Nuclear Physics
a.junghansAthzdr.de
Phone: +49 351 260 - 3589
Fax: +49 351 260 - 13589

Dr. Roland Beyer
PostDoc / Beamline scientist
Nuclear Physics
roland.beyerAthzdr.de
Phone: +49 351 260 - 3281