A cosmic muon veto system (CMVS) has been developed at CTBTO Headquarter to suppress muons induced background for a broad energy hyper-pure germanium detector. The system consists of five plastic scintillation plates operating in anti-coincidence mode with

the germanium detector . The set-up is using an advanced, full-featured Multichannel Analyser (MCA) which is Web-browser-based controlled and interacts with the acquisition software. By using Ethernet connection to the MCA, all setup and acquisition parameters

can be remotely set and controlled. The steps performed in optimizing the CMVS for future use at certified stations of the IMS network are presented, as well as the results of this project.

expected,

Commissariat à l’énergie atomique et aux énergies alternatives

DAM Ile-de France Bruyères-le-Châtel

Conclusion and future plans: The advantages of a simplified design of CMVS to increase the sensitivity of radionuclide laboratories have been shown and its applicability for Radionuclide stations of the CTBTO monitoring network has been proved.

I. Radulescu, B. Nadalut, K. Khrustalev, M. Auer

Comprehensive Nuclear-Test-Ban Treaty Organization, Vienna, Austria

SnT2015 Poster No. T3.2 P9

Disclaimer

The views expressed on this poster are those of

the author and do not necessarily reflect the

view of the CTBTO

Project objective: Improvement of System Sensitivity

The International Monitoring System oversees a network of

80 radionuclide monitoring stations that perform high-resolution

gamma-spectrometry on air filter samples. These stations are

designed to detect any of the CTBT relevant radionuclides

indicative of nuclear events. For these stations one of the

prerequisite conditions is achieving low limits of detection of

any of the radionuclides.

To improve the sensitivity of a radionuclide monitoring station a highly efficient experimental setups have been developed for

minimizing the background contributions in gamma-ray spectrometry by means a cosmic muon veto system.

Generally, if a gamma-ray detector is placed above ground its background is consisting of:

environmental radioactivity, including radon, originating from building materials (consisting of -radiation from naturally

occurring radioactive elements 238U, 235U, 232Th and 40K) in the space where the detector is placed and from airborne

radioactivity, neutrons from natural fission and from the (α,n) reaction;

radioactive impurities in the detector and shielding materials;

cosmic rays, with relevant contributions from muons and neutrons;

electronics and microphonics noise

The considered muon interaction mechanisms are:

the direct ionisation in the detector sensitive volume by the incident muon;

the production of -electrons (knock-on);

the direct production of electron-positron pairs;

the muon decay delivering an electron or a positron with energy of up to 53 MeV;

the muon bremsstrahlung.

• BE5030 (SN8643): • planar p-type detector • 55% relative efficiency • 2.2 keV at 1332.5 keV for 60Co • 10cm lead shielding • 1mm of cadmium • 1mm of copper

CMVS Hardware integrated at CTBTO test station in Vienna:

• 2 x Muon Shield V-151 4 EJ200 Plastic Scintilator Plates, 53x563x255mm

• 2 x Muon Shield V-151 4 EJ200 Plastic Scintilator Plates, 53x563x365mm

• 1 x Muon Shield V-151 4 EJ200 Plastic Scintilator Plates, 53x613x613mm

• AMP1000(5-)-E2-X High Voltage Distribution Box and Preamplifier

• 2 x Lynx-MCA Digital Signal Analyzer

CMVS system at CTBTO: initial installation

Using the Cosmic Muon Veto System 53.6% reduction has been obtained for

the integral count rate of the HPGe detector.

The highest reduction (85.7%) was obtained for the net count rate of

annihilation peak, due to direct production of electron-positron pairs by cosmic

muons.

The MDA has been calculated for some CTBT relevant radionuclides and the

improvement has been higher than 30%.

Lynx MCA controls all plates and operates in conjunction with the Lynx MCA

controlling the HPGe. The new firmware allows record of both unsuppressed

and suppressed spectra.

The simplified design is considered more suitable for implementation in

radionuclide laboratories and monitoring stations.

The cosmic veto gamma-spectrometer provides improved sensitivity for

CTBT relevant radionuclides.

Nuclides Energy (keV)

Ratio of Net peak Area of

unsuppressed to supressed

spectra

Pb-Ka2-X-ray 72.8 0.89

Pb-Ka1-X-ray 75.0 0.84

Bi-Ka1-X-ray 77.1 -

Pb-Kb1-X-ray 84.8 1.17

Pb-Kb2-X-ray 87.3 0.97

71mGe 198.3 -

Pb214 295.2 1.32

Pb214 351.9 1.20

annihilation 511.0 6.98

Cd114 558.5 1.10

Bi214 609.8 1.26

Pb206 803.3 1.91

Ac228 911.1 0.94

Bi214 934.1 -

Bi214 1120.3 1.20

Bi214 1238.1 -

K40 1460.8 0.97

Bi214 1729.6 -

Bi214 1764.5 1.19

Bi214 2204.2 1.48

Tl208 2614.5 1.06

CMVS System at CTBTO: Background reduction results

Conclusions

CMVS System at CTBTO: Lynx MCA

Data processed to produce suppressed and unsuppressed spectra.

Measurements were made of a detector background to evaluate the minimum

detectable activity (MDA) for a number of fission products.

5-day background spectra reduced by a mean of 53.6% the integral count

rate.

Peaks at 558.4, 669.3, 693.0 and 803.3 keV attributable to (n, n’) reactions

and recoil of 114Cd, 63Cu, 72Ge and 206Pb atoms were reduced.

MDA reduction results

114Cd(n,n’)114Cd activation

peak

511 keV annihilation

peak

Calculation of MDA for the unsuppressed spectrum and the suppressed after 5

days of measurement showed: 140Ba improved from 25.5mBq to 17.2mBq, which represents 32.7%,

expected,

CMVS Improvements and implementation plans

Delay Gate: 12.5µs

Gate width: 2.5µs

• The system improvements are related to the new firmware of the Lynx that

embedded the delay gate in the MCA; this feature has been tested and

validated.

• A new Lynx upgrade allowing simultaneous records of unsuppressed and

suppressed spectra is under testing at CTBTO test station in Vienna

• Implementation of cosmic veto system in a remote location. Sample

transfer for reanalysis may be difficult for monitoring stations situated at

very remote locations. Remote stations often have an auxiliary detector

system, which could be improved by adding a CMVS and thus could be

used for high sensitivity sample re-measurements.

137Cs - 36.9% 134Cs - 37.1%

95Zr - 38.6% 141Ce - 31.6%