The measurement of ultra-high radiation environments:

accelerators and nuclear fusion reactors

by B. Camanzi¹ and A. G. Holmes-Siedle², Senior IEEE Member

 

A keynote paper, presented at RADECS'06, Athens, September 26-9, 2006..

 

Abstract Accelerators and fusion reactors produce high levels of radiation. The dosimetry needs are summarised and suitable new sensors proposed. The new techniques are called dosimetry for "Ultra-High Dose/Fluence Measurement (UHDM/UHFM)". Some innovative approaches are suggested.

 

¹ Particle Physics Department, Rutherford Appleton Laboratory, Chilton Didcot, Oxon OX11 0QX, UK         

² R. E. M. Oxford Ltd, Eynsham, Witney, Oxfordshire, OX29 4PD, UK

 

1. Introduction

In this paper, we review the problem of radiation damage common to accelerators and nuclear fusion reactors. In the broadest sense, this is the disruption by various particles of key performance properties in the different materials used. Our primary call in this paper is that the time is right for attention to new dosimetry in future accelerator and fusion-device developments

.  Accelerators are used in the search for and study of the ultimate constituents of matter. In accelerators, a beam of elementary particles (mainly e^-, p and their anti-particles) is created, accelerated to velocities near the speed of light and brought into collision with another beam or a stationary target. Following these collisions new particles are created, which are then observed and studied by experiments placed nearby. Industrial nuclear reactors harness nuclear forces to produce power. The present power reactors (fission reactors) derive power from a massive, compact core of high-Z isotopes. Approaches based on fusion are not as advanced but promise to be a good future source of energy. In a fusion reactor, the reaction is achieved within a large volume of low-Z plasma. Unavoidably, accelerators and reactors produce very intense mixed radiation fields which inflict ionizing and displacement damage on components. The challenges for dosimetric measurement are greater than in previous activities such as space and medicine [1]. In both classes of machines studied here, the exposure of various components (sophisticated electronics, sensors, etc.) to penetrating, scattered radiation, of very high intensity is unavoidable. Such fields create damage and activate the materials used to build the machines. In this paper the radiation fields produced by accelerators and fusion facilities will be predicted. New dosimetric techniques will be needed. To describe them, we will employ two new terms which distinguish the levels of dose range and fluence range and two major classes of damage, ionization and displacement. The terms are: - UHDM Ultra-High Dose Measurement; - UHFM Ultra-High Fluence Measurement.

"Reactor dosimetry" is a term already used for the neutron-fluence studies in fission reactor which are essential in determining power-plant lifetime [2]. The primary aim of our proposed UHDM and UHFM is similar: to give warning of serious damage to the working system, especially the signalling of possible catastrophic failures.  However, in the large, new systems with which this article deals, the dosimetry and diagnostics will be very different from fission-reactor dosimeters. First, to keep the machines under control, the electronics must possess very fast response times. Secondly dosimetry over long periods gives us vital information on the route to developing radiation-tolerant systems. Based on ultra-high radiation measurements, we can avoid life limitations imposed by radiation damage and better plan the disposal of radioactive parts.

The accelerator physics community already understands the importance of having a dosimetry system [3,4,5,6,7]. For the fusion community, we encourage further studies like the present one, to persuade R&D departments of the similarities and differences in "diagnostics" and "dosimetry". Diagnostic measurements are designed for the fast-reacting control of machine processes and are integral in the feedback systems (e.g. in controlling neutron generation in fusion plasma). Dosimeters are for the long-term maintenance of components, an activity sometimes called "machine health monitoring" [1]. Following a discussion on the dosimetry needs of accelerators and fusion reactors, including the complex charged-particle equilibria caused by high-Z components, suitable advances on commercially available dosimetric devices will be proposed and justified. The discussion will point out the major differences between this "UHDM/UHFM" sphere of dosimetry for radiation effects and the well-known "light aerospace" and "personnel protection" technology. Our aim is to map a path for profitable development through these fields.

2. Accelerators

 

As a consequence of the very high energies and intensities of the beams used, the new accelerators being planned (see below) and their experiments will have to operate reliably for many years in radiation fields of intensity and complexity never faced before. The radiation field will indeed be composed of different particles (p, n, p, K, e, g) and may have large variations at different locations. Due to radiation damage accelerators and experiments will experience a degradation of performance in time that may eventually stop their capability of doing physics. Regular monitoring of the radiation accumulated will help by preventing from this to happen. Some of these machines and experiments may even be classified as nuclear installations, having to obey to specific rules for material handling and disposal. By measuring the accumulated dose and induced radioactivity levels a dosimetry system could help in implementing correctly such regulations.

The figures for dose and fluence ranges for each accelerator and their experiments are also summarised in Table 1.

 

2.1 The Large Hadron Collider (LHC)

 

The Large Hadron Collider is currently under construction at CERN, in Geneva, Switzerland and is due to start operation in 2007. LHC will collide two beams of protons each of which will have energy of 7 TeV. Four experiments (CMS, Atlas, LHCb and Alice) will be installed along LHC. The predicted annual values of dose and fluence in the sections of the accelerator [6] where the electronics will be placed reach 1 kGy and 1x10¹¹ part./cm².  In some part of the accelerator the radiation field is instead much more intense (up to 600 MGy per year), but in these areas the needs are for dose measurements only. The radiation field generated at the collision points tends to be quite high as well: for example for the CMS experiment the predicted values [8] over a lifetime of 10 years reach a maximum of 2 MGy for the dose and 5x10¹⁵ part./cm² for the fluence nearer the collision point. Both accelerator and experiments are already classified as nuclear installations.

An upgrade of LHC, called SLHC (Super LHC) is currently under consideration. In this more powerful collider the radiation field will be 10 times more intense.

 

2.2 The International Linear Collider (ILC)

 

Studies are underway for a linear collider that will accelerate and collide a beam of electron and one of positron to energies in the centre of mass up to 500 GeV.

Simulations of the expected radiation levels are at a very early stage and so far have been carried out only for the experiment and not for the accelerator, which is not included in Table 1. Also if the radiation levels are very much dependent on what final configuration will be chosen for the experiment, it is clear it will be the inner part of the experiment, the Vertex Detector (VD), that will see the highest levels of radiation, up to 200 Gy and 1x10¹⁰ part./cm² per year [9].  

 

2.3 Neutrino factories

 

Structures to create intense beams of neutrinos are currently being proposed. The neutrino factory proposed at CERN is the most powerful one assuming a 4 MW proton beam.  Based on a study carried out for a 1.5 MW proton beam [10], the target area for creating pions appears to be the zone with highest levels or radiation. The figures for the expected annual dose and fluence in the target area for a 4 MW proton beam reach values as high as 8x10¹⁰ Gy and 3x10²⁰ part./cm² in the hottest spot.

 

3. Nuclear fusion reactors

 

The major source of displacement and ionization once outside the first wall region are 14 MeV neutrons. However, dosimeters will also be needed for the measurement of isotope gamma emissions from heavily-activated metals and ceramics in the structure. Table 2 summarises the fluences and doses which may have to be handled by dosimeters in fusion facilities. These estimates make use of specific careful studies [11] on the materials prospects of DEMO and PROTO, two advanced fusion designs. For IFMIF the figures are based on the assumption that it will deliver 20 dpa per year of 14 MeV neutrons in metals. For full-power reactors over a lifetime, the doses may be ten times as high as the ones shown in Table 2, but prediction is uncertain until the engineering designs are complete.

 

3.1 ITER and DEMO

 

A large international R&D effort, ITER, has begun with the object of showing the feasibility of nuclear fusion as a commercially viable source of alternative energy. The ITER experimental fusion tokamak is to be constructed in France and its first plasma operation is expected in 2016. Although ITER is expected to produce 500 MW of fusion power, the cycle time will be such that radiation damage will not be a problem for such reactor and this is the reason why ITER does not appear in Table 2.

DEMO, the subsequent machine, will use ITER data to build a power generating device. The damage and the dose measurement problems in DEMO will be of startling severity compared with what we know now in power reactors. Our estimates in Table 2 show that bulk damage in the tokamak walls constitutes hundreds of displacements per atom (dpa) and the levels of ionization may be thousands of MGy of dose. These values set our upper targets for UHDM and UHFM dosimeters.

Other materials, sited further from the plasma, [12] set our lower targets.

 

3.2 International Fusion Materials Irradiation Facility (IFMIF)

 

Unavoidable technical factors mean that success in DEMO can only be achieved if there is a massive development of radiation-tolerant structures and electronics. Very accurate measurement of radiation dose and damage is needed when testing the "super-hardened" materials and components which will undoubtedly be needed. We maintain that new dosimetry principles must be developed in the research phase. The test facility, IFMIF [11] is the ideal facility for the development of those dosimetric principles. Our estimates of the required levels for dose and fluence in DEMO and IFMIF are shown in Table. 2.

 

3.3 Beyond ITER and DEMO

 

For the purpose of this paper we suppose that PROTO, the machine that may follow DEMO, and other commercial nuclear fusion reactors will be similar to DEMO but "scaled up". This means that the radiation damage problem will also be scaled up. Given good research now in the measurement of radiation damage in materials and devices, the UHDM/UHFM dosimeter technology we envisage can grow appropriately and be ready to manage the higher damage in a timely way.

 

4. Dosimetry

 

Typical dosimetric arrays for accelerators and fusion reactors present many resemblances as emphasised in Fig. 1, despite somewhat different scales. A hot core, that is the beams in an accelerator and the plasma in a fusion reactor, is surrounded by components prone to radiation damage (electronics, sensors, etc.). In Fig. 1, we distinguish between two radiation levels: ultra-high (dosimeters labeled UH) and high (dosimeters labeled H). Whatever the method, a dosimetry system will certainly have to possess the following features: 1. it must discriminate between ionizing and non-ionizing energy deposition; 2. it must have a dynamic range covering many decades; 3. it must go to a much higher upper dose and fluence limit than before (see Tables); 4. it must provide online real-time 2D and 3D mapping; 5. it should be small.

For an accurate measurement of the ionizing component of the energy loss or "dose", the average atomic weight of the dosimetric sensor needs to be in charged-particle equilibrium with the material in which "dose" value is required. To this end and considering the wide spread use of high-atomic-weight (high-Z) materials in the systems under consideration, some dosimeters will themselves have to be made from high-Z materials. Examples are “vanadium-equivalent”, “caesium iodide equivalent”, etc. We will describe candidates from the field of scintillators and II-VI and III-V semiconductors whose damage responses to gamma and neutrons are already understood [13]. This pushes also toward the use of amorphous materials with high Z that have the advantage of not being sensitive to displacement damage and can be found with a wide range of sensitivities to ionizing radiation; see e.g. studies on the charge  trapping properties of ZrO₂ and SiO₂   [14,15].

For the field of UHDM dosimetry there is a large selection of technologies. Old, well established technologies like gas tubes (air ionization chambers and electrometers), optical dosimeters (TLDs, OSLs, scintillators) can and will be refined for higher doses, in particular by choosing "hard" semiconductor and optical materials, both organic and inorganic [16,17]. Some other old techniques, placed in a new context, involve the possible use of thin films, fibres, Micro Electro Mechanical Systems [MEMS]. For dosimeters based on mechanical and micro-electromechanical measurements, structures in which the radiation-induced mechanical changes are sensible in small volumes in the desired dose and dose-rate range will be developed. There are definitely some useful advances to be made. Requirement No. 5 above is important because the space in the machines will always be limited. We plan to use our experience of the RadFET [2,3,4,5] to design micro and nano-size dosimeter devices. Some examples of suitable principles are: the bending of micron-sized metal-silica pairs due to differential expansion, elastic modulus measurements in filaments of material monitored by X-ray micro-beams. Another field with a very wide range of possibilities is dosimetry using measurements of new chemical species, analogous to Fricke dosimetry. A good name for this area of investigation is “micro-UHD dosimetry”. The end-product is a close-packed array of dosimeters, yielding a high granularity of data at high doses.

In the field of UHFM, we must look for principles based on detection of defect clusters (e.g. the counting of displacement clusters on the nanoscale) or the measurement of the special bulk properties known to be affected. Examples of the latter are: changes in resistivity or elasticity in metals, changes in carrier lifetime or mobility in semiconductors, generation of luminescent centres distinct from those created by ionization in transparent materials. Just as for ionization, we will consider many high-Z materials, given the fact that, for displacement, the heavier the atom, the larger the required exchange of momentum [1].

 

5. Conclusions

Our justification for this paper is that radiation sources of an intensity and complexity never faced before will soon be with us in the two fields described: - accelerators and fusion reactors. In these machines, it will be mandatory to perform regular mapping of the radiation damage and dose levels accumulated in structural materials and also in components placed near to the "hot spots”. We recommend that dosimetry research is carried out in partnership with the diagnostics designers - the two measurement systems are complementary. In commercial reactors, dosimetry systems develop directly into damage warning systems. The paper will map a path for appropriate development in these fields and propose a new generation of "ultra-high dose/fluence measurement (UHDM/UHFM)" devices, capable of ultra-high ranges. The first target will be online mapping of the doses and fluences reached in parts of the International Fusion Materials Irradiation Facility at a fine spatial resolution. Some innovative “micro” approaches are suggested in which dosimetric technologies are selected and "tailored" to the demands described here.

 

Acknowledgements

 

The authors wish to thank R. Edgecock, A. Ibarra, J. Palmer, E. Stassinopoulos, T. Wijnands, S. Worm and S. Zinkle for useful discussions. The support of PPARC and the British Council, UK is gratefully acknowledged.

 

References

[1] A. G. Holme-Siedle and L. Adams, “Handbook of Radiation Effects”, Oxford University Press, 2002; Chapter 15.3; N. Itoh and A. M. Stoneham, "Materials Modification by Electronic Excitation", Cambridge Univ. Press, 2001.

[2] A. G. Holmes-Siedle, J. M. Leffler, S. R. Lindgren and L. Adams "The RADFET System for Real - Time Dosimetry in Nuclear Facilities" 7th Annual ASTM -Euratom Symposium on Reactor Dosimetry, Strasbourg, August 27 - 31 1990, 851-859

[3] B. Camanzi, A. G. Holmes-Siedle and A. K. McKemey, “The Dose Mapping System for the Electromagnetic Calorimeter of the BaBar Experiment at SLAC”, Nucl. Inst. And Meth A 457 (2001) 476-486

[4] B. Camanzi, M. Glaser, E. Tsesmelis and L. Adams, “A Study on the Applicability of Solid State, Real-Time Dosimeters to the CMS Experiment at the Large Hadron Collider”, Nucl. Inst. And Meth A 500 (2003) 431-440

[5] F. Ravotti, M. Glaser, M. Moll, C Ilgner, B. Camanzi and A. G. Holmes-Siedle, “Response of RadFET Dosimeters to High Fluences of Fast Neutrons”, IEEE Trans. Nucl. Sci. NS-52 (4) (2005) 959-965

[6] T. Wijnands, “Radiation Monitoring for Equipment in the LHC tunnel”. Available at:

https://edms.cern.ch/file/565013/0.2/LHC-PM-ES-0006-00-10.pdf

[7] LHC Experiment Radiation Monitoring Working Group, RADMON. Available at:

http://www.cern.ch/lhc-expt-radmon/

[8] M. Huhtinen, “Radiation Environment Simulations for the CMS Detector”, CERN CMS TN/95-198, 1995

[9]  S. Worm, private communication.

[10] T. Anderson et al., “A Feasibility Study Of A Neutrino Source Based On A Muon Storage Ring”, N. Holtkamp and D. Finley Eds., 2000
[11] B. D. Wirth et al. "Multiscale Modeling of Radiation Damage in Fusion Reactor Materials" presented at DOE, March 12, 2002  S. Zinkle and A Kohyama, "Advanced Materials for Fusion Technology " presented at SOFE 2002, January 25, 2002; A. Moeslang, "Irradiation Devices and Testing (for fusion damage simulation)", Materials Assessment Meeting, Karlsruhe 5-8 June 2001. Available at:

 http://fire.pppl.gov/fusion_materials.html.  http://ww.iter.org/ITERpublic/ITER/1.7n.html;    http://www.machinedesign.com/ASP

[12] A. G. Holmes-Siedle, B. A. Engholm, B. A. Battaglia and J. F. Bauer, "Damage Calculations for Devices in the Diagnostic Penetration of a Fusion Reactor", IEEE Trans. Nucl. Sci. NS-31 (6) (1984) 1106-1112

[13] M. Decréton, T. Shikama and E. Hodgson, "Performance of Functional Materials and Components in a Fusion Reactor: the Issue of Radiation Effects in Ceramics and Glass Materials for Diagnostics", Jou. Nucl. Mater. 329-333, Part 1 (2004) 125-132

[14] W. Primak, "Threshold for [displacement] Radiation Effects in Silica ".  Phys. Rev. B (1971) 6.4846

[15] Estimates derived by the authors from private communications and unpublished reports including the European Fusion Development Association; Ciemat, Spain; Oak Ridge National Laboratories and Princeton University, USA.

[16] R. Van Nieuwenhove and L. Vermeeren, "Experimental Study of Radiation Induced Electromotive effects on Mineral Insulated Cables", Rev. Sci. Instr., 74 (2004) 4675-4682

[17] J. Bartko, B. O. Hall and K. F. Schoch Jr., "High Conductivity Poly(phenylene sulfide) Prepared by High Energy Ion Irradiation". Jou. Appl. Phys 59, (1986) 1111-1116

 

	Dose (Gy)	Fluence (part./cm2)
Accelerators: LHC SLHC Neutrino factories	1 to 6x108 10 to 6x109 3x107 to 8x1010	7x108 to 1x1011 7x109 to 1x1012 2x1018 to 3x1020
Experiments: CMS at LHC CMS at SLHC VD at ILC	1 to 2x105 10 to 2x106 Up to 200	1x1011 to 5x1014 1x1012 to 5x1015 1x109 to 1x1010

 

 

 

Tab. 1: Summary of the yearly figures for dose ranges in Gy and fluence ranges in part./cm² for the accelerators and some of their experiments. The damage is due to penetrating particles (including albedo neutrons and secondary photons)