HSE
Occupational Health & Safety and Environmental Protection
Technical Note
CERN-RP-2013-050-REPORTS-TN
instrument intercomparison in the high energy mixed field at the cern-eu reference field (cerf) facility
Marco Caresana1, Manuela Helmecke2, Jan Kubancak3,4, Giacomo Paolo Manessi5,6, Klaus Ott2, Robert Scherpelz7, Marco Silari5,*
1Politecnico of Milan, Department of Energy, Via Ponzio 34/3, 20133 Milan, Italy
2Helmholtz-Zentrum Berlin, BESYY II, 12849 Berlin, Germany
3Nuclear Physics Institute of the ACSR, Department of Radiation Dosimetry, Na Truhlářce 39/64, 180 00 Prague, Czech Republic
4Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Břehová 7, 115 19 Prague, Czech Republic
5CERN, CH-1211 Geneva 23, Switzerland
6University of Liverpool, Department of Physics, L69 7ZE Liverpool, UK
7Pacific Northwest National Laboratory, Richland, WA 99352, USA
Abstract
This paper discusses an intercomparison campaign performed in the well-characterized mixed radiation field at the CERN-EU Reference Field (CERF) facility at CERN. Various instruments were employed: conventional and extended-range rem counters including a novel instrument called LUPIN, a bubble detector using an active counting system (ABC 1260-1 Neutron Dosimeter), and two Tissue Equivalent Proportional Counters (TEPC). The results show that the extended range instruments (WENDI-2, LINUS, LUPIN, BIOREM + Pb converter, ABC1260 + Pb converter) agree well within their uncertainties and within 1σ with the H*(10) FLUKA value. The conventional rem counters (LB6411, two BIOREM units and ABC1260) are in good agreement within their uncertainties and underestimate H*(10) as measured by the extended range instruments and as predicted by FLUKA. The TEPCs slightly overestimate the FLUKA value but they are anyhow consistent with it when taking the comparatively large total uncertainties into account, and indicate that the non-neutron part of the stray field accounts for about 30% of the total H*(10).
CERN, 1211 Geneva 23, Switzerland
Geneva, 12 June 2013
9
INTRODUCTION
Monitoring of stray radiation at workplaces characterized by mixed fields with radiation spectra extending over a wide energy range is a difficult task. These mixed fields are usually dominated by neutrons with a more or less pronounced photon contribution, but other radiation components (electrons, muons, pions and protons) cannot always be neglected. Measurements in such complex radiation environments can lead to huge variations in detector readings, due to differences in their energy response function and sensitivity. With the aim of evaluating the performance of various active monitors in a well-characterized mixed field, an intercomparison campaign was carried out in 2012 at the CERN-EU Reference Field (CERF) facility(1), employing a wide range of instruments in a reference exposure location outside a concrete roof-shield.
CERF is a reference radiation facility in operation at CERN for almost 20 years, providing a neutron spectral fluence typical of that encountered outside high-energy hadron accelerator shieldings and similar to the radiation environment at commercial flight altitudes. The stray radiation field is generated by a positive hadron beam (2/3 protons and 1/3 positive pions) with momentum of 120GeV/c impinging on a copper target placed inside an irradiation cave (Figure 1a). The secondary particles produced in the target traverse an 80cm concrete shield on top. This roof-shield produces an almost uniform radiation field over an area of 2x2m2 located at approximately 90º with respect to the incoming beam direction, divided in 16 squares of 50x50cm2. Each element of this ‘grid’ represents a reference exposure location (concrete top, CT, Figure 1b). The energy distributions of the particles (mainly neutrons) at the various exposure locations were obtained in the past by Monte Carlo simulations performed with the FLUKA code(2,3).
Figure 1. a) Axonometric view of the CERF facility. The side shielding is removed to show the inside of the irradiation cave with the copper target set-up. b) The reference grid with the 16 exposure locations used on the concrete roof-shield.
The measurements were performed in CT7. At this location the neutron spectral fluence is characterized by a low-energy peak with energy around 0.4eV, an intermediate region between the thermal and the evaporation peak located at about 1MeV, and a high-energy peak centred at about 100MeV(1). Both commercial and prototype detectors were employed: various conventional and extended-range rem counters including a prototype called LUPIN (designed in particular to work in pulsed fields), a bubble detector using an active counting system (ABC 1260-1 Neutron Dosimeter), and two Tissue Equivalent Proportional Counters (TEPC). Table 1 lists the detectors employed in the intercomparison and the institutes participating in the experiment.
Table 1 – Summary of detectors employed and institutes participating in the intercomparison.
Detector name / Detector type / InstituteWENDI-2 / Extended range rem counter / CERN
LB6411 / Conventional rem counter / CERN
Biorem FHT 752 / Conventional rem counter / CERN
LINUS / Extended range rem counter / CERN
LUPIN / Prototype extended range rem counter / CERN/POLIMI
RadEye NL / Pocket meter / CERN
ABC 1260-1 / Bubble detector / CERN
TEPC / TEPC / PNNL
HAWK FW-AD2 / TEPC / ASCR
BIOREM FHT 752 / Conventional rem counter / HZB
BIOREM FHT 752 + Pb shell / Extended range rem counter / HZB
BEAM MONITORING
The mixed proton/pion beam is delivered to the CERF facility from the Super Proton Synchrotron (SPS) with a typical intensity of 108particles per SPS spill. The spill duration (beam extraction time) is about 10s over an SPS cycle of about 45s. The beam spot is approximately rectangular, about 30mmx40mm. The beam monitoring is provided by a custom-made, air-filled Ionization Chamber (IC) placed in the beam a few meters upstream of the target. The IC is a parallel-plate, transmission type ionization chamber with a diameter of 185mm. The chamber has five parallel electrode plates made of Mylar, of 2.5mg/cm2 thickness and 16mm inter-plate spacing. The central plate is the collector and the ones on either side are the polarity electrodes. The polarization voltage on these plates (300V) is supplied by an external battery. The beam traverses 32mm of air at atmospheric pressure in the sensitive part of the chamber. The signal coming from the IC is fed into a charge digitizer, which requires a 24V voltage provided by an external power supply. The output pulses are fed to a National Instrument USB 6342 DAQ connected to a desktop computer (PC). The DAQ is interfaced to the PC via the LabVIEW graphical language. The data are saved in a log-file that records the differential and integrated IC readings (expressed in IC-counts) every second.
INSTRUMENTATION
Rem counters
LINUS
The Long Interval NeUtron Survey meter (LINUS)(4-7) in use at CERN is the original extended range neutron rem counter developed about 20 years ago from an Anderson-Braun type device. The detector consists of a 3He proportional counter embedded in a spherical polyethylene moderator, which incorporates a boron-doped rubber absorber and a 1cm thick lead shell to extend its response function to several hundred MeV by the generation of intermediate-energy neutrons via (n,xn) inelastic scattering reactions.
Thermo Scientific WENDI-2
The Wide Energy Neutron Detection Instrument (WENDI-2) is an extended range rem counter designed to measure the H*(10) rate within an energy range from thermal to 5GeV(8). It consists of a 3He proportional counter surrounded by a cylindrical polyethylene moderator assembly and a layer of tungsten powder. As in the LINUS, this additional layer of high-Z material enhances the detector response to high-energy neutrons(9,10).
Berthold Technologies LB6411
The neutron probe LB6411 is a commercial rem counter designed to measure H*(10) in different radiation fields up to 20MeV. It consists of a 3He/methane proportional counter surrounded by a spherical moderator of polyethylene with density of 0.95g/cm3.
Thermo Scientific BIOREM FHT 752
The BIOREM is a commercial neutron dose rate meter for stationary and portable use, especially suited for environmental measurements. It employs a BF3 proportional counter in a cylindrical moderator made of polyethylene and boron-carbide. As in the design of the LINUS, its response to high-energy neutrons can be enhanced by adding an external lead shell.
LUPIN
The Long Interval Ultra-wide dynamic Pile-up free Neutron rem counter (LUPIN)(11-13) is a prototype of extended range rem counter available in two versions, using either a 3He or BF3 proportional counter. The counter is inserted in a spherical or cylindrical moderator with lead and cadmium inserts and uses a front-end electronics based on a logarithmic amplifier. The working principle is very simple: the current generated inside the proportional counter is amplified with a current to voltage logarithmic amplifier and the output voltage is acquired with an ADC. The current is integrated over a user settable time window. The integrated charge divided by the charge expected by a single neutron interaction gives the number of neutrons occurring in a defined time.
Other radiation detectors
Thermo Scientific RadEye NL
The Thermo Scientific RadEye NL is a commercial neutron radiation detector. It uses a 3He proportional counter with 2.5bar filling pressure and is equipped with a small-size polyethylene moderator to increase the efficiency to fast neutrons. Its main purpose is to detect radiation sources more than estimating a neutron dose rate(14).
Bubble detector (ABC 1260-1 neutron dosemeter)
The ABC 1260-1 Neutron Dosimeter from Framework Scientific is a portable neutron area monitor that can be controlled via a PC. It is based on the superheated drop technology(15). Bubble formation events are recorded in real time by means of piezoelectric transducers picking up the acoustic pulses emitted during drop vaporization. If used with special emulsions, this detector provides a direct measurement of H*(10) and H*(10) rate. Following an approach similar to that used in the development of the LINUS, its response can be extended to several hundred MeV with the addition of a 1cm thick cylindrical lead shell placed around the detector cap(16,17).
TEPCs
PNNL TEPC
The TEPC from PNNL(18) consists of a 12.7cm diameter hollow sphere of A150 plastic inside a stainless steel pressure vessel filled with a methane-based tissue equivalent (TE) gas. The gas is at a very low pressure, 1506Pa (11.3torr), so that the stopping power of the TE gas inside the sphere matches the stopping power of a sphere of tissue 2µm in diameter. The detector acts as a proportional counter, and the signal is fed into a multichannel analyzer, which records a pulse-height spectrum that is essentially proportional to the lineal energy deposited inside the sphere by particles (protons, alphas) knocked out of the plastic by neutron interactions. The pulse height spectrum can be analyzed to find the total energy deposited in the detector (and converted to dose) and a representative quality factor. The quality factor is chosen to give an estimate of Hp(10).
Far West Technology HAWK FW-AD2
The HAWK FW-AD2 TEPC environmental monitor(19) is a detector intended for research purposes primarily aboard commercial and military aircrafts. It consists of a 12.6cm diameter hollow sphere of A150 TE plastic inside a cylindrical stainless steel container. High voltage is applied to the spherical TE plastic, while the central anode-wire is electrically isolated and held at virtual ground potential by a charge-sensitive preamplifier. The sphere is filled with 933Pa of 99.7% propane gas and operated between -600 and -900V, which provides a gas gain from 200 to 400. The specific voltage is set during calibration. The HAWK records two lineal energy spectra in the TE material: the first/low gain spectrum for the LET range 0-1535keV/μm, the second/high gain spectrum for 0-25.55keV/μm (the HAWK contains two pulse-shaping circuits, one with a gain 60 times greater than the other and an A/D hardware to collect both spectra). The collected data are stored every minute on a flash memory card. The data can be used to recalculate the HAWK readings into REM500 equivalent readings. This procedure(21) is based on the substitution of the quality factor table and trim of events <8.2 keV/μm and >425keV/μm.
Table 2 sums up the calibration details for each detector. For commercial detectors it gives the operating range in terms of dose rate and neutron energy as declared by the manufacturer.
Table 2 – Summary of the calibration details and the operating range of the detectors. The calibration coefficient takes into account the uncertainty on the calibration source.
Detector / Calibration coefficient [nSv/count] / Calibration spectrum / Measured operational quantity / Declared operating range (for commercial detectors)Dose rate / Neutron energy
WENDI-2 / 0.32±0.03 / Pu-Be / H*(10) / 10 nSv/h - 100 mSv/h / 0.025 eV - 5 GeV
LB6411 / 0.72±0.06 / Pu-Be / H*(10) / 10 nSv/h - 100 mSv/h / 0.025 eV - 20 MeV
BIOREM (CERN) / 0.67 ±0.05 / Pu-Be / H*(10) / 10 nSv/h - 400 mSv/h / 0.025 eV - 10 MeV
LINUS / 0.88 ±0.07 / Pu-Be / H*(10) / n.a. / n.a.
LUPIN / 0.48 ±0.04 / Pu-Be / H*(10) / n.a. / n.a.
RadEye / 3.00 ±0.24 / Pu-Be / H*(10) / n.s. / n.s.
ABC 1260 / 195.0±15.6 / Pu-Be / H*(10) / n.s. / 0.025 eV - 20 MeV
(200 MeV with Pb)
PNNL TEPC / 6.80±0.27 / 252Cf / Hp(10) / n.a. / n.a.
HAWK / 0.39±0.04 / 252Cf / H*(10) / n.a. / n.a.
BIOREM (HZB) / 0.67 ±0.05 / Pu-Be / H*(10) / 10 nSv/h - 400 mSv/h / 0.025 eV - 10 MeV
While for the rem counters the calibration procedure and the related coefficient is unambiguous, for the TEPCs one can refer to different calibration processes: using an internal 244Cm alpha source (for the calibration of the linear energy scales, to convert channel numbers from the pulse-height spectrum, and to check the gas gain), an external photon source (typically 137Cs or 60Co, for the calibration of the low-LET component) or an external 252Cf source (to verify the response to the high-LET component). Since in these measurements the interest is focused on the neutron component of the stray field, the values in Table 2 refer to the calibration with the 252Cf source.
MEASUREMENTS
Experimental
The aim of the measurements was to intercompare the response of the detectors amongst them and with the FLUKA value. All measurements were performed by placing each detector in turn in the CT7 reference location. The integrated counts of each detector were normalized to the ICcounts. The internal clock of each detector was synchronized with the internal clock of the PC used for data acquisition before the start of each measurement, so as to allow an off-line normalization of the data. Data acquisition for all the detectors was controlled remotely.
Results
Table 3 shows the measured quantity (H*(10) for all the detectors, except for the PNNL TEPC which measured Hp(10)), normalized to the beam intensity expressed in ICcounts. For the HAWK, the data have been recalculated into REM500 equivalent readings. The FLUKA value for the reference exposure location CT7 is also shown. The values always refer to the neutron component of the radiation field. For the TEPCs the value refers to the high-LET contribution of the field (>8.2keV/μm for the HAWK, >10keV/μm for the PNNL TEPC). We assume that the neutron contribution to the radiation field coincides with the high-LET component, even if it cannot be excluded that other particles contribute to the high-LET component, causing an overestimation of the H*(10). At the same time, it cannot be excluded that neutrons may contribute to the low-LET part of the spectrum. The HAWK also provides an estimation of the total H*(10), 351.9 ± 36.7 pSv/IC count, determined according to the procedure explained in ref.(22).