atw 2018-04v6


atw Vol. 63 (2018) | Issue 4 ı April

| | Fig. 2.

Cross sections of vanadium reactions and photon production under neutron irradiation.

Reaction 51 V (n, p) 51 Ti 51 V (n, γ) 52 V

Threshold Neutron Energy 1.72 MeV 0 MeV

14 MeV Cross-section 30 mb (approx.) 0.6 mb (approx.)

Beta Emitter, Half-life 51 Ti- 5.76 m 52 V- 3.74 m

Average Beta Energy 51 Ti- 0.87 MeV 52 V- 1.07 MeV

SPND Current (14 MeV) 3.46 × 10 -12 A 6.92 × 10 -14 A

SPND Current (TBM) 7.97 × 10 -9 A 3.44 × 10 -8 A

| | Tab. 1.

Beta-emitters and corresponding currents from fast neutron reactions in vanadium based SPND.

14 MeV source are shown. Neglecting

the self-shielding of electrons in emitter

material, effect of other materials

and taking a saturation condition

(considering the short half-lives of

daughter nuclides), one can ascertain

the orders of magnitude of currents

possible with V-SPND, as reported.

For this estimation, vanadium density

of 6.1 g cm -3 , and a typical volume of

1 cm 3 are assumed. For a 14 MeV

neutron source, a flux intensity of

1 × 10 10 cm -2 s -1 is considered, which

is achievable with state of the art

14 MeV neutron generators. For TBM,

activation calculation was done [5]

with the HCLL neutron spectrum

and typical flux intensity (up to 1 ×

10 14 cm -2 s -1 ) using EASY-2007 [6].

With high-sensitivity ammeters,

currents down to the order of 1 ×

10 -14 A can be reliably measured [7].

Values in Table 1 show that a vanadium

emitter based SPND will produce

measurable signals in TBM. Due

to its high neutron threshold energy,

the (n, p) reaction can be utilized to

measure fast neutron flux exclusively.

Fast neutron reactions lead to

high-energy gamma production. This

phenomenon competes with the neutron

absorption reactions (Figure 2).

Photoelectric and Compton electron

emission from emitter causes a prompt

current which is expected to form the

major component of the signal of

V-SPND towards 14 MeV neutrons.

Secondly, vanadium being a medium-

Z nucleus can be a potential

emitter for SPGD also. With optimized

dimensions and choice of collector

material, a vanadium SPD can be

envisaged for monitoring of photon

flux in TBM.

Instead of the usual coaxial type

cylindrical geometry, we designed

our test SPD in sandwich-type flat

geometry. This provides a relatively

higher cross section area to the incident

neutrons, and ease of access for

testing various materials in the same

device. Thin foils (0.5 to 2 mm) of

emitter, insulator and collector are

arranged to form an assembly in an

aluminum case, which also serves as

an electromagnetic shield. Central

conductor of the signal cable is linked

to the emitter plates of the detector.

The collector plates, case and the

cable sheath are shorted and securely

connected to the ground. Schematic

sketch and photograph of the test

detector are shown in Figure 3 (left).

With comparable cross sections of

reactions in different materials, the

insulator and collector materials also

play an important role in SPD

response. Behaviors of different

material combinations are experimentally

tested. Alumina (Al 2 O 3 ) or

beryllia (BeO) is used as insulator and

Inconel-600 or graphite is used as

collector in our experiments. Effects

of the change of geometry and dimensions

are also studied. A Keithley 6485

Picoammeter (sensitivity range -20 fA

to 20 mA) is used as the measuring

device. A low-noise triax cable (Belden

9222) is used to reduce the interferences

in low-current measurement.

The tests are conducted at the

14 MeV neutron generator of Technical

University of Dresden (TUD-NG),

shown in Figure 3 (right). Here,

deuteron beams are impinged on a

tritiated titanium target causing D-T

reaction which leads to production of

neutrons with peak energy of approx.

14 MeV. TUD-NG offers neutron flux

intensities up to 1 × 10 10 cm -2 s -1 . The

detector is placed in front of the

tritium- target of TUD-NG and tested

under different conditions by varying

flux levels and irradiation times.

3 Results

The irradiation tests of flat sandwichtype

vanadium SPD were performed

at TUD-NG, with neutron flux intensities

around 1 × 10 9 cm -2 s -1 . DC

signals in the range of 100 fA to 100 pA

were measured. In Figure 4, a plot

shows variation of SPD signal with

change in neutron flux. The detector

was composed of 1 mm thick layers of

vanadium emitter and Inconel-600

collector. The signal was found to be

proportional to the incident flux, with

approx. 90 pA at the highest flux level.

At low fluxes and low currents,

the measurements have high uncertainties.

Interference from electromagnetic

sources of stray currents,

| | Fig. 3.

(Left) internal design of the sandwich-type flat SPD: (top)- an engineering sketch of the geometry

having sandwich of foils of emitter (green), insulator (grey) and collector (red), and (below) a photograph

of the assembly with vanadium SPD.

(Right) experimental setup showing TUD-NG beamline, tritium target, mounted SPD, and the lead cable.


Research and Innovation

Irradiation Tests of a Flat Vanadium Self- Powered Detector with 14 MeV Neutrons ı Prasoon Raj and Axel Klix

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