Nuclear Instruments and Methods in Physics Research B 223–224 (2004) 333–338

www.elsevier.com/locate/nimb

The old carbon project: how old is old?

R.P. Beukens a, H.E. Gove b,*, A.E. Litherland a, W.E. Kieser a, X.-L. Zhao a

a IsoTrace Laboratory, University of Toronto, Toronto, ON, Canada M5S 1A7b Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627-0171, USA

Abstract

The goal of the old carbon project is to measure the ratio of 14C to 12C in methane at levels below 10�18 by

accelerator mass spectrometry (AMS). This research is chiefly motivated by the need for very low background raw

materials (methane, petroleum) to manufacture scintillator fluids for large volume neutrino detectors, particularly for

solar neutrinos. The 14C activity in such material, introduced by the radioactivity of the reservoir rocks, cosmic rays or

later handling, limits the low-energy sensitivity of the neutrino detector. This paper reviews the scintillator requirements

for low-energy neutrino observation in terms of the 14C/12C ratio, as well as earlier AMS and decay counting mea-

surements of this ratio at the 10�18 level. Recent experiments to determine the limitations of the heavy element line on

the IsoTrace spectrometer for these ratio measurements are reported; analysis of the data obtained to date indicate

a maximum interference limit of 14C/12C� 10�19. This progress report will also mention some methods for reducingthis interference further.

� 2004 Elsevier B.V. All rights reserved.

PACS: 07.75.+h

Keywords: Accelerator mass spectrometry; Old radiocarbon; Neutrino detectors

1. Introduction

This is the 25th anniversary of accelerator mass

spectrometry (AMS), which was introduced in

1977 [1] to measure the radioactive isotope ofcarbon, 14C, in milligram samples of organic car-

bon to establish when the organism died.

In the case of 14C dating by AMS the present

limit on the measurable ages of organic material is

somewhere between 45 000 and 70 000 years. This

is due to the contamination of the sample either in

* Corresponding author. Tel.: +1-585-275-4943; fax: +1-585-

273-3237.

E-mail address: gove@pas.rochester.edu (H.E. Gove).

0168-583X/$ - see front matter � 2004 Elsevier B.V. All rights reser

doi:10.1016/j.nimb.2004.04.066

the field or in the laboratory during sample prep-

aration. 60,000 years corresponds to a ratio of 14C

to stable carbon of about 10�15. The 10th life of14C is most useful in the context of this paper and

is approximately 19 000 years based upon a half-life of 5730 years. The level of 10�15 is far above

the level of radiocarbon found in natural gas,

which was measured at near or below about

2 · 10�18 [2] during the work that preceded theconstruction of the large 5Mg test counter at the

Borexino Laboratory. The value found at Borex-

ino after the large counter had been constructed

was 1.6 · 10�18 [3]. These measurements have beendiscussed in a preliminary way at a conference [4].

The present research is part of the program

designed to discover what factors limit this ratio in

ved.

334 R.P. Beukens et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 333–338

existing AMS systems, to correct them and then to

extend this ratio to well below 10�18(the equivalent

of a 14C age of 114,000 years or more). At the

present time the research to be reported here haslittle to do with normal radiocarbon dating be-

cause quite different samples are chosen. Radio-

carbon dating involves using samples that are

vulnerable to extraneous contamination in the

field. The chief motivation for the research relates

to the construction of scintillation detectors for

neutrinos. This paper represents a brief progress

report on what we refer to as the old carbonproject (OCP). A subsequent paper will discuss

more details [5].

2. Neutrino detectors

In the case of organic scintillation detectors for

neutrinos, particularly solar neutrinos, the pres-ence of 14C in the scintillation liquid limits the

ability to detect the lowest energy neutrinos. These

are the 7Be, pep, carbon–nitrogen cycle and espe-

cially the pp neutrinos from the sun, which are by

far the most abundant. The neutrino flux from the

pp process extends to neutrino energies of only

about 400 keV [6]. Although the maximum b en-ergy for 14C is 156 keV, finite energy resolutionand pile-up can create a tail in the b-energy spec-trum that extends to higher energies. Thus the

intrinsic level of 14C in the liquid scintillator

becomes important in setting the low-energy

threshold for detecting neutrino events. For this

reason it turns out that the 14C/C ratio should

ideally be well below 10�18. At a 14C/C ratio of

about 10�18the 14C b-rays still obscure the recoilelectrons from pp neutrinos although the obser-

vation of 7Be should be possible [3]. If the 14C/C

ratio in the scintillation fluid were as low as 10�22

or so, then the pp neutrinos could be detected.

Experiments involving the lowest energy neutrinos

are important to more accurately establish the

solar neutrino flux and the parameters of the

neutrino mixing. The recent breakthrough [7] atthe Sudbury Neutrino Observatory (SNO) in

Canada employed a clean heavy water detector to

measure neutrinos of energies above 5 MeV using

the Cherenkov photons. Such measurements open

the door to future neutrino measurements, per-

haps for decades to come, particularly of lower

energy neutrinos observable by scintillation

detectors.Methane or petroleum is used to manufacture

the megagrams of scintillator fluids needed for

large volume neutrino detectors. The 14C activity

in a sample of gas or oil depends mainly on the

natural radioactivity of the surrounding rock in

which the sample was stored for long periods and

from which it was extracted. These rocks, inevi-

tably, contain neutron and alpha particle emittingisotopes of uranium, thorium and their daughters.

The reactions expected [3], in order of impor-

tance, to contribute most to the production of14C in deep underground geological formations

are: 17O(n,a)14C, 14N(n,p)14C, 13C(n,c)14C and11B(a,n)14C.As the levels of U and Th fluctuate widely it

may be possible to find methane or oil with 14C/Cratios as low as 10�21 [3] and maybe as low as 10�22

(190 000 years). An AMS method of measuring14C/C ratios in small samples to 10�18and prefer-

ably to much less will be very advantageous. An

AMS facility having a current of 3 mA of 12C�

(currents higher than this have been obtained)

yields one count per hour for a sample with a 14C/

C of 10�18. 200 times enriched material is availablein quantity so a level of 10�20 could be reached in

a few hours if contamination can be controlled.

3. Previous measurements of 14C/C at low levels

There have been two measurements of 14C/C

ratios at very low levels. The first is an upper limitof 1.6 · 10�18 obtained at the IsoTrace Laboratoryof the University of Toronto [2]. In the second a

value of (1.94 ± 0.09) · 10�18, or an apparent age of110 000 years, was measured at the Borexino

Counter Test Facility (CTF) in the Gran Sasso

underground laboratory in Italy [3].

In the first measurement two CO gas samples,

derived from purified methane from a natural gaswell, were used. One of the samples was enriched

in 13C by Isotec, Inc. [8] by liquid-CO distillation.

Such a process also enriches 14C. Isotec produces

such samples for the 13C labelling of biomedical

R.P. Beukens et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 333–338 335

compounds. One sample contained the normal12C/13C ratio of 98.9/1.1. In the second the ratio

was 0.9/99.1. The above upper limit of 1.6 · 10�18was obtained at IsoTrace [2] when these sampleswere compared by AMS.

In the second measurement [3] a liquid scintil-

lator, the (CTF), at the Gran Sasso National

Laboratory in Italy was employed. This detector

located at an underground depth of about 3500 m

water-equivalent contained 4.8 m3 of liquid scin-

tillator. A 14C/12C ratio of (1.94± 0.09) · 10�18 wasfound in the particular scintillator investigated.This is the smallest 14C/12C ratio ever measured by

the decay counting method. The shape of the 14C bspectrum was also consistent with the theoretical

prediction [3].

An AMS method for measuring 14C to stable

carbon ratios at levels below 10�18, that does not

require the large enrichments employed in the

above measurements [2], is desirable. It would thenbe possible to identify sources of natural gas or

petroleum most suitable for the manufacture of

very low background scintillation liquids for the

detection of low energy or even pp neutrinos. It

would also reduce the volume needed to do so by

the CTF at Gran Sasso [3] by a factor of a billion

or so or a factor of a few million if enrichment

is used.

4. Current research on the OCP

Research into the AMS radiocarbon back-

grounds was first carried out 20 years ago [9] and

the results, together with some of the more modern

work with a second-generation tandem, are sum-marised in the previous paper [10].

Research on the OCP at IsoTrace is extending

that work and is proceeding along the following

lines: (1) The two present high-energy beam lines,

the 14C line and the heavy element line, at the

IsoTrace facility, University of Toronto, are being

compared for their background properties. These

should be quite different. Both are used primarilyfor income producing measurements (14C and 129I

respectively). Results from the 14C line have been

reported [2,9] and preliminary results of measure-

ments on the heavy element line are discussed

below. (2) Apparatus is under consideration to

directly convert methane to acetylene [11] and to

crack the acetylene to solid carbon targets. This

should almost completely exclude the possibility ofintroducing CO2 into the carbon targets. An

alternative is to use a gas ion source, which uses

the methane directly. (3) Effort will be devoted to

reducing the background in the 14C beam from all

possible sources in the accelerator system itself. (4)

Methods will be explored to reduce 14C contami-

nants in the materials of the ion source. Some

progress has been made in the first of these goalsas is outlined below.

5. Some results of the current research

The IsoTrace high-energy heavy element line

was used in the current study. Compared with the

radiocarbon line, the use of a high-resolutionelectric analyzer ðE=DE ¼ 900Þ in the heavy ele-ment line dramatically reduces the ME=Q2 inter-ference so that only the E=Q interference is left asthe primary concern [5]. The task is then to analyze

in detail how much each contaminating beam is

likely to accompany the 14C3þ beam, emerging in

the final energy spectrum as an E=Q interference,and how much each analyzer in the whole AMSsystem can contribute to its reduction.

While many of the experimental details are yet

to be published, the undoubted dominant source

of E=Q interference is found to be the 12C3þ fromthe scattering of 12C� from the low-energy magnet

box and by the 13C3þ from the spontaneous decay

of 13CH� into 13C� after mass analysis and before

reaching the entrance of the accelerator, as shownin Fig. 1. The ME=Q2 interference and other as-pects of the OCP will be discussed in a subsequent

paper [5].

The injection of 12C� and 13C� ions into the

tandem when the injection magnet is set for mass

14 is clearly illustrated in a series of scans, as is

shown in Fig. 1. The upper curve is the normalized

intensity of 12C3þ, measured in the final detector orthe Faraday cup in front of the detector as a

function of the magnetic field of the injection

magnet. The rest of the AMS system was kept on

the settings for measuring 12C� fi 12C3þ. The slits

Fig. 1. Normalized intensities of 12C3þ and 13C3þ are shown

versus the magnetic field of the injection magnet, measured with

the rest of the AMS system held constant, selecting12C� fi 12C3þ and 13C� fi 13C3þ, respectively. The peak below

the main peak comes from those CH� ions that loose an H

atom before the low-energy magnetic analyser and the others

are discussed in the text. A terminal voltage of 1.81 MV was

used for all measurements.

336 R.P. Beukens et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 333–338

before and after the injection magnet are tightly set

to the beam size, so the phase space of the injectionmagnet is minimized without beam loss. A similar

measurement was done for 13C� fi 13C3þ, as shown

by the lower curve in Fig. 1.

The cause of the fractional injection of 12C� and13C�, while measuring 14C�, has been confirmed to

be in part due to the scattering of 12C� and 13C�

beams by the walls of the injection magnet box, as

has been suggested by the previous work [9]. Butwith the phase space restriction around the mag-

net, only 10�8 of the 12C� and 6 · 10�10 of the 13C�

ions is injected when measuring 14C�, as a result of

wall scattering. The injection due to the sputter tail

is at even lower levels because of the use of an

electric analyzer ðE=DE ¼ 400Þ after the ion source

and before the magnet. However, in the case of the12C� and 13C� injection, there are clear mass-13

and 14 peaks due to the CH� ions. Because the rest

of AMS system in each case is set for C� fiC3þ,the mass-13 and 14 peaks can only be the result of

CH� (after the injection magnet but before the

accelerator)fiC� (into the accelerator not neces-

sarily at the normal injection energy)fiC3þ, when

the injection magnet is selecting masses-13 and 14.

The 13C3þ from this source is about 2.5 · 10�8 ofthe 13C� beam. This will, as we shall see below,

clearly be another important source of the E=Qinterference for 13C from both the normal and

enriched samples, and it cannot be easily reduced

by further improvement in the injection system.

The 12CH�2 ions do not contribute as can be seen

from the figure.

With our present system, the only effective

analyzer left to remove these 12C� and 13C� beams

is the high-energy magnet ðM=DM ¼ 2600Þ. Yetcharge changing +3fi+2 and then +2fi+3, as

well as possibly elastic scattering from the magnet

box, would still allow part of the resulting 12C3þ

and 13C3þ beams to pass the high-energy magnet

even when it is set to select 14C3þ [10]. To deter-

mine the performance of the high-energy magnet

on reducing the E=Q interference, it was scanned

with a steady 13C3þ beam, with the rest of AMSsystem selecting 13C� fi 13C3þ. The measurement

procedure is similar to that of the injection magnet

scans, and the normalized result is shown in Fig. 2.

With the slits wide-open at the focal plane of the

high-energy magnet the E=Q interference level at

mass-14 is found to be 10�10 of the full mass-13

beam intensity. Closing the slits down to the

minimum opening without cutting down the beamreduces this ratio by a factor of 10 as shown in the

insert.

The above tests are based on the use of regular

graphite targets. When cracked graphite is used,

the mass-14 peak at the injection magnet due to13C hydride would be some 10 times more intense

and so the estimates given in this paper will for

cracked acetylene samples be some 10 times larger.If the E=Q interference reduction factor for 13C3þ

by the high-energy magnet is taken from Fig. 2

inset to be 10�11, an estimate can then be made of

the E=Q interference on 14C measurement by the

Fig. 2. Normalized intensities of 13C3þ versus the field of the

high-energy magnet, measured with the rest of the AMS system

selecting 13C� fi 13C3þ. The shoulder on the peak is the remains

of the continuum observed when no magnetic analysis is used

[5].

R.P. Beukens et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 333–338 337

present system due to this source. This is, for

graphite samples, limited at the level of �2.5 ·10�21 for the ratio 14C/12C. For samples enriched

in 13C by a factor of 100 the ratio would be for14C/13C� 2.5 · 10�19 but as the 14C is also enriched,a similar ratio limit for 14C/12C is also reached.In a similar manner, the 12C-induced E=Q

interference limit is estimated to be at the apparent

level of 14C/12C� 5 · 10�20, with magnet box-wallscattering being the main reason for the injected12C� (which can be largely eliminated with a

new magnet box design). Even with hydrides that

are 10 times as intense as those from graphite the

limit of the present apparatus is somewherenear 10�19, which is quite useful for further OCP

work.

6. Conclusions and outlook

The result we have obtained so far allow us to

estimate that the heavy element line at the Iso-Trace AMS facility can, in the absence of ion

source material and sample preparation contami-

nation, measure 14C/12C ratios from graphite to

the limit of �5 · 10�20 without any further modi-fication. This limit, due to the E=Q interference, is

in fact close to the value of the 14C line [9] but with

the added advantage of virtually no ME=Q2

interference in the final 14C3þ-energy spectrum. In

contrast the 14C line has a level of 14C/12C P 10�14

for the ME=Q2 interference, although the final

ionisation counter can separate this interference

for radiocarbon dating. The 13C3þ and 12C3þ

ME=Q2 peaks in the spectrum (estimated at presentto be lower than 10�21) from the heavy element line

can easily be resolved in the final counter and are

of no concern. Simple modifications to the system,

such as a new magnet box for the low-energymagnetic analyser and an additional small mag-

netic analyser in the high-energy system, will

reduce the 14C/12C-ratio limit still further.

In the coming years, while the appropriate

modifications to the system are being made, we

will also concentrate on the problems of ion source

contamination, sample preparation, and the crea-

tion of larger C� beams.

Acknowledgements

This work is supported by a grant from

the National Science Foundation, USA and the

Natural Science and Engineering Research Coun-

cil of Canada. We are indebted to R.S. Raghavanand Yu. Kamyshkov for early advice on the

OCP.

References

[1] For references to an account of the historical development

of accelerator mass spectrometry see H.E. Gove, From

Hiroshima to the Iceman – The Development and Appli-

cations of Accelerator Mass Spectrometry, Institute of

Physics Publishing, Bristol, 1999.

[2] R.P. Beukens, Nucl. Instr. and Meth. B 79 (1993) 620 (The

upper limit quoted in this reference is 10�18 but a later

analysis gave the number quoted above.).

[3] G. Alimonti et al., Phys. Lett. B 422 (1998) 349.

[4] A.E. Litherland, K.H. Purser, in: S.H. Connell, R. Tegen

(Eds.), Proceedings of the International Conference on

Fundamental and Applied Aspects of Modern Physics,

L€uderitz, Namibia, 2000, 2001, p. 457.

[5] X.-L. Zhao, A.E. Litherland, H.E. Gove, W.E. Kieser,

to be submitted.

338 R.P. Beukens et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 333–338

[6] J.N. Bahcall, M. Pinsonneault, Rev. Mod. Phys. 64 (1992)

885.

[7] Q.R. Ahmad et al., Phys. Rev. Lett. 89 (2002) 1.

[8] Isotec, Inc., Miamisburg, OH, USA.

[9] H.W. Lee, Ph.D. Thesis, University of Toronto, 1987.

[10] M.-J. Nadeau, H.W. Lee, A.E. Litherland, K.H. Purser,

X.-L. Zhao, Nucl. Instr. and Meth. B, these Proceedings.

doi:10.1016/j.nimb.2004.04.065.

[11] S. Kado, Y. Sekine, K. Fujimoto, Chem. Commun. 24

(1999) 2485.