Journal of Nuclear and Radiochemical Sciences, Vol. 7, No.1, pp. R31-R36, 2006

Radioactive Airborne Species Formed in the Air in High Energy Accelerator Tunnels
K. Kondo*
High Energy Accelerator Research Organization (KEK) 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
Received: March 31, 2006
The particle size distributions of radioactive nuclides formed in accelerator beam line tunnels were measured byusing a diffusion battery and were found to be log-normal with a geometric mean radius of about 28 nm. The radioactive aerosol formation was well explained by a simple physical adsorption of radioactive nuclides to the ambient
non-radioactive aerosols. The fractions of aerosol and gaseous components were measured for individual nuclides
formed in the air of a high-energy accelerator tunnel. The formation of these 11C and 13N species was explained bytheir hot-atom reactions and subsequent chemical reactions.

1. Introduction
High-power and high-energy proton accelerators have been
proposed for basic scientific research in several countries, andsome of them are under construction. In Japan, the high intensity proton accelerator (J-PARC), consisting of a 200-MeV
Linac as well as 3-GeV and 50-GeV synchrotrons, of which the
maximum beam power is about 1MW, is to be completed at theend of 2007. This accelerator was designed for a broad rangeof frontier sciences using pulsed-intense neutrons from the 3GeV synchrotron and neutrino and K beam from the 50-GeV
synchrotron.1
At these accelerators, the air in the beam line tunnel is
exposed to extremely high radiation fields during machine
operation, where primary and secondary high-energy particles
coexist. A number of radioactive nuclides are abundantly produced in the air, mainly through a variety of nuclear reactions
between these high-energy particles and the elements of air.
These nuclides generally behave as hot atoms with high kinetic
energies and charges, and after completion of hot-atom reactions, part of them form radioactive aerosols and others arepresent in a gaseous form. Studying the physico-chemical
properties of these radioactive airborne species is an interesting subject from radiochemical standpoint.
Moreover, airborne radioactive nuclides become a source of
external and internal exposures for workers when entering theaccelerator tunnels after a shutdown of machine operation. To
evaluate internal doses from radioactive nuclides induced in
the air, information on their particle sizes and chemical forms
is indispensable. However, thus far, there is little information
available on these physical and chemical properties of airborne
radioactive nuclides formed in the air in accelerator beam-line
tunnels.
An extensive study has been conducted on the physicochemical properties of airborne radioactive nuclides. In thispaper, a review is given of recent results.
2. Experimental Procedure(a) Measurement of the particle size distribution ofradioactive aerosols. The air in the proton beam line of the
KEK (High Energy Accelerator Research Organization) 12GeV synchrotron was sampled from an open space between theD1 magnet and the iron beam dump. Figure 1(a) shows a cross*Corresponding author. E-mail: Kenjiro.Kondo@kek.jp

Figure 1. (a) KEK-slow extracted proton beam line (EP2) and (b)schematic diagram of measurement. CNC: condensation nuclei
counter, DB: parallel plate diffusion battery, F1,F2; filter, A1,A2:
flow-meter, P1,P2: pump, M1,M2: mercury manometer, IC: ionization chamber, A,A0 : radioactivities collected on the filter, N,N0 : aerosol number.

sectional view around the sample position. The sample inlet isabout 50 cm above the beam axis and ~3.5 m downstream fromthe target, which is used for the production of secondary particles, such as K, π –mesons, and electrons. Figure 1(b) shows aschematic diagram of measurements at a measurement stationthat was placed outside of concrete shield. The details of thesampling conditions of the air are described in References 2–5.
For determining the size distribution of the radioactive aerosols, a diffusion method was applied by using a diffusion battery with parallel-side channels (PPDB).
The diffusion losses of aerosol particles from an aerosol© 2006 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 06/30/2006

R32 J. Nucl. Radiochem. Sci., Vol. 7, No. 1, 2006

Kondo

undergoing a laminar flow through the channels of a PPDB
depend on the diffusion coefficient of the particles, the mean
linear rate of the aerosol flow, and the dimensions of the channel. For mono-dispersed aerosols, the penetration fraction (P)with this type of battery is given by the DeMarcus’ equation,6
P= nn =0.9149exp(−3.77ν )+0.0592exp(−44.6ν )+0.026exp(−302ν). (1)0Here n and n 0 are the particle concentrations at the exit andentrance of the battery, respectively, and ν is an experimental
parameter. The diffusion coefficient (D) and ν are defined by
the following equations:ν = NDbl ,(2)tQ
D= kT 1+ A L + C L exp(− Br ) ,
6πη r
r
r
L

[

]

(3)

where N is the channel number of a diffusion battery, r theaerosol radius, t the half width between the plates, b the heightof the battery, l the length of the battery, Q the flow rate, kBoltzman’s constant (1.38×10 -16 erg K-1), T the temperature (K),L the mean-free path of air molecules (0.653×10 -5 cm) and η
the viscosity of the air (1.83×10 -4 dyne cm-2 at 296 K and 101.3kPa). A, B, and C are coefficients for Cunningham’s correction(A = 1.246, B = 0.87, and C = 0.42). The dimensions of the
PPDB used in the present work were N = 20, t = 0.05 cm, b =20 cm, and l = 57 cm.
Generally aerosols are polydisperse and of a log-normal distribution. The following generalized DeMarcus’ equation isassumed for aerosols with a log-normal distribution of particle
sizes7:0.9149P= —√ 2π logσg∫−∞ exp(−3.77ν)exp[+∞− (logr−logrg)2) dlogr2log2σg](4)0.0592 +∞− (logr−logrg)2+ —exp(− 44.66ν)exp) dlogr,2log2σg√ 2π logσg −∞

∫

[

]

where rg and σg are the mean geometric radii and the standard
geometric deviations, respectively. The third term in eq 1 canbe neglected in view of its smallness.
In the present work, n/n 0 can be replaced by the ratio of theradioactivity collected on the filter (A) through the PPDB to
the total radioactivity (A0). The radioactivities, A and A0, werecollected at the same time, as Figure 1 shows. The relation of
P vs Q was obtained experimentally, and then the rg and σg values were evaluated from curve fitting method using a leastsquares-fitting computer code (SALS).8(b) Radioactive aerosol formation using Ar gas containing dioctyl phthalate aerosols. In order to investigate more
precisely the formation mechanism of radioactive aerosols
under well defined experimental conditions, argon gas containing dioctyl phthalate(DOP) aerosols was irradiated 65 MeV
qusai-monoenergetic neutrons produced from the 7 Li(p, n)reaction by bombarding a 5.2 mm thick 7Li-target with 68 MeV
protons from an AVF cyclotron. The experiment was carriedout at Takasaki Ion Accelerator for Advanced Radiation
Application (TIARA) of the Japan Atomic Energy ResearchInstitute. Figures 2(a) and 2(b) show a schematic view of themonoenergetic neutron source and a block diagram of theexperimental setup. All charged particles were swept out by aclearing magnet, and the chamber was irradiated by puremonoenergetic neutrons. The argon gas served as a target forthe production of radioactive nuclides as well as a gaseous
medium for suspending the DOP aerosols. As predominantradioactive nuclides, 38 Cl(T1/2 =37.2 min) and 39 Cl(T1/2 =55.6
min) are produced through the (n, 2np) and (n, np) reactions of
40
Ar. High-purity Ar gas was flowed into the irradiation chamber at a flow rate of 20 L/min for 5 min. Then, the DOP aerosols were generated from liquid DOP using an atomizer (Model

Figure 2. (a) Schematic view of the mono-energetic neutron source
facility at TIARA. (b) Block diagram of the experimental setup.

3075,TSI Inc.) and the thus-obtained poly-disperse DOP aerosols were evaporated and condensed in the aerosol conditioner(Model 3072,TSI Inc.). The volume fraction of DOP in 2-propanol was 10% and the flow rate of argon was 1 L/min. The
size distribution of the DOP was measured with a scanningmobility particle sizer (SMPS Model 3936L22,TSI Inc.) consisting of an electrostatic classifier and a condensation particle
counter, which can measure the size distribution of aerosols in
the particle diameter range from 0.01 to 1 µm over a wide rangeof concentrations (from 2 to 108 particles/ cm3). After confirming the particle size distribution, the aerosol suspended in argon
were introduced into the argon-filled irradiation chamber for 5min, and then the inlet and outlet of the chamber were closed.
The concentration of the DOP aerosols in the chamber was on
the order of 10 4 particles/ cm3. The prepared sample-gas wasirradiated with 65 MeV qusai-monoenergetic neutrons of 2.3 ×
10 4 cm-2 s-1 for 60–120 min. After irradiation, the sample gas
was collected with a membrane filter at a flow rate of 20 L/min.
The size distribution of the aerosols was measured using a electrical low-pressure impactor (Model ELPI-2000, Dekati.). For
the activity-weighted size distribution, the collection substratesof the impactor stages were removed after sampling and theirradioactivities were measured using a gas-flow counter.
(c) Measurement of chemical forms of 11C and 13N airborne species. Measurements of 11C and 13N chemical species
were carried out by using a chamber consisting of a cylindricaland trapezoidal vessels made of aluminum having a volume of
0.67 m 3 at the slow extracted proton beam line (EP-2) of the
KEK 12-GeV synchrotron. A measurement station consistinggas-sampling and analyzing systems was placed in an area outside of the accelerator shielding. Atmospheric air free fromaerosols or high-purity mixed gas (N2 80% and O2 20%) froma gas cylinder was continuously supplied into the irradiationchamber. The details of the sampling conditions of the air aredescribed in References 9–12.
Possible species of 11C in the irradiated air would be 11C,
11
CO2 , 11C-hydrocarbons, and H11CN. A measurement of theconcentration of these species was carried out by a sampling

Radioactive Airborne Species Formed in the Air in High Energy

method using alkaline traps and a cold trap. After removing
the radioactive aerosols with the membrane filter at a flow rateof 0.3 L/min, 11CO2 was collected in the first KOH trap and11
CO was collected in the second KOH trap after being oxidized to 11CO2 by the hopcalite. The radioactivity in each trap
was measured by γ -ray spectrometry. The total 11C activity
was obtained by a system consisting of a membrane filter, a Pd
catalyst and a KOH trap. The Pd catalyst maintained at 550 ˚Coxidized both 11CO and 11C-hydrocarbons to 11CO2. The collection of H11CN was carried out by the cold trap kept at -25 ˚C, inwhich H11CN could be collected. The sampled air was also
analysed with a radio-gaschromatograph.
A new filter technique was developed for the analysis ofindividual chemical species of 13N. The filter packs for collecting 13NO2 and 13NO consisted of Na 2 CO3 -,NiO-, and Co2O3 impregnated filter arranged in series.10 After removing
radioactive aerosols by passing through a Teflon filter, H13NO2
and H13NO3 were collected by a Na 2 CO3 -impregnated filter.
13
NO2 and 13NO were separately collected on the NiO-, andCo2O3-impregnated filters, respectively. Further, selective collection systems for H13NO2 and H13NO3 consist of two sets ofassemblies. One was for the total H13NO3 and H13NO2, and theother was for H13NO2 alone. H13NO3 and H13NO2 were selectively stripped from the sample air into a scrubbing solution(0.05M NaOH) as a nitrite and nitrate, respectively. The nitrate
was reduced with hydrazine sulfate in the presence of Cu(ll) tonitrite, which was then reduced to 13NO with ascorbic acidsolution. H13NO3 was determined by the difference between
the two measurements.

J. Nucl. Radiochem. Sci., Vol. 7, No. 1, 2006 R33

Figure 3. Typical γ-ray spectrum of the radioactivity collected on
the filter.

3. Results and Discussion(a) Gaseous and aerosol fractions of airborne nuclides. The
typical γ-ray spectrum of radioactivities collected on the filter
is shown in Figure 3. The number of radioactive nuclides foundin this experiment was relatively small compared to the numberof possible radioactive nuclides expected to be produced. This
could be explained as follows; (a) their concentrations were too
low to detect by γ-ray spectroscopy. (b) Some of the nuclides
were pure-β emitters. The airborne radioactivity concentrations for individual nuclides changed from one measurement toanother depending on the conditions of machine operation,such as the beam intensity and the degree of beam loss. The
concentrations for the principal nuclides were ~7×10 -1 Bq/cm3
for 11C, ~1.5 Bq/cm3 for 15O,~ 5.9×10 -5 Bq/cm3 for 3H, ~1.7×10 -4
Bq/cm 3 for 7 Be, ~ 9.6×10 -5 Bq / cm 3 for 24Na, and ~5.6×10 -5
Bq/cm3 for 38S.
The aerosol fractions were estimated to be ~1.9% for 11C and13
N, 0.5% for 15O, and ~0.05% for 18F. For 38S, 38Cl, and 39Cl,
they were found to have both aerosol and gaseous components
from measurements using a NaOH filter paper. The aerosol
fractions were 50% for 38S, 78% for 38Cl, and 72% for 39Cl. In
the case of 38Cl, the genetic relation between 38S and 38Cl was
taken into account. The radioactive nuclides other than those
nuclides were present as 100% aerosols. A summary of thesemeasurements is given in Table 1. So far, little is known about
the fractions of gaseous and aerosol components for the
nuclides produced in the air of accelerator tunnels. These
results give useful information for estimating internal radiation
doses.
(b) Particle size distribution of radioactive aerosols andmechanisms of radioactive aerosol formation. Figure 4
shows the relations of A/A0 vs Q obtained by using the PPDB
for 7 Be-, 24 Na-, and 38 S-aerosols. Measurements for otherradioactive nuclides, except those three nuclides, were not conducted because of their low radioactivities. The experimental
errors in the figure are the standard deviations for the counting
statistics. The solid lines indicate the best results of the leastsquares fitting. Statistical analyses gave the parameters rg

Figure 4. The relationship of A/A0 vs flow-rate for 7Be-, 24Na-, and38
S-aerosols. : experimental, — : calculated using SALS code.

•

R34 J. Nucl. Radiochem. Sci., Vol. 7, No. 1, 2006

Kondo

(geometric mean radii) and σg (standard geometric deviation).
They were determined to be rg = 0.027 ± 0.005 µm and log σg= 0.36 ± 0.02 for 7Be, rg = 0.027 ± 0.01 µm and logσg = 0.44 ±0.04 for 38S, and rg =0.028 ± 0.009 µm and logσg = 0.33 ± 0.02
for 24Na-aerosols. The obtained log-normal distributions areshown in Figure 5, together with that of the non-radioactive
aerosols observed during the machine operation.
As Figure 5 shows, the particle size distributions for 7Be-,
24
Na- and 38S-aerosols are very similar to each other, although
the standard geometric deviations are slightly different. It isvery interesting that these aerosols are of about the same radii
regardless of the differences of the physical and chemical properties among them. This fact suggests that the aerosol formation of these nuclides proceeds via a simple physical adsorption
to ambient non–radioactive aerosols.13 The aerosol size distributions for other isotopes, such as iron, scandium, and manganese isotopes, were not determined because of their low
radioactivities, though their size distributions are considered tobe similar to those for 7Be, 24Na, and 38S.
Naturally occurring radioactive aerosols of radon and/or
thoron daughters also have a similar size distribution, but theirgeometric mean radii are considerably greater than the present
values. It is also reported that the radioactive aerosols of fission products from atomic bomb tests have different size distributions in the stratosphere and troposphere, depending on thecharacteristics of the ambient non-radioactive aerosols.17
The size distribution of non-radioactive aerosols obtained inthis experiment was of a log-normal shape with a geometricmean radius of 0.01 µm and logσg = 0.3, as shown in Figure 5,
and the aerosol concentrations were (~1.5–4.3)×10 5 /cm 3 ,depending on the beam intensity. It is very interesting that the
non-radioactive aerosols formed during machine operation
were of sizes much smaller than those of the non-radioactive
aerosols present in the natural environment. Most atmosphericaerosols in the submicron region are generally considered tocontain sulfate formed through the photochemical oxidation of
SO218 of which the concentration in the atmospheric air is 5–1.5µg/m 3.19 In this experiment, the air is exposed to very highradiation of about 104 R/h at the sampling inlet. Under these
conditions, the non-radioactive submicron aerosols in the air of
EP-2 tunnel are considered to be mostly sulfate aerosols formed
through radiolytic oxidation reactions of SO2. Indeed, it was
confirmed by ion-chromatographic analysis that the non-radioactive aerosols in the size range below 0.45 µm in diameter,

contained sulfate compounds in significant quantity.
Therefore, it seems likely that the 7Be-, 24Na-, and 38S-aerosols are formed by the attachment of these nuclides to the nonradioactive aerosols containing SO 42-, which are formed byradiation-induced reactions. These radioactive aerosol size distributions (F(r)) can be determined by multiplying the nonradioactive aerosol size distribution (N(r)) by the attachment
coefficient β(r). β(r) was calculated using the following equation, assuming that these radioactive atoms are neutral and theattachment proceeds via simple physical adsorption:2–β(r) = π r–ν ,(5)
1+ ν rFigure 5. The log-normal distributions of 7Be-, 24Na-, and 38S-aerosols. 0 : non-radioactive aerosl, 1: 24Na, 2: 7Be, 3: 38S.

Figure 6. Comparison between the log-normal aerosol size distributions obtained experimentally and those calculated.
—— : experimental, - - - - - - : calculated; 1: 24Na, 2: 7Be, 3: 38S

20( 4D )

where r is an aerosol radius, ν– the average velocity and D the
diffusion coefficient. D and ν– are defined by the following
equations:D=3
L π k T (m+M) ,
8
mM

(6)

ν– =

8kT
πM ,

(7)

where m and M are the mass of nuclide and air molecule,respectively, T the temperature (296 K), k Boltzmann’s constant,and L the mean-free path. By using the thus-calculated β(r) for
each nuclide, the radioactive size distribution (F (r)) was
obtained. As depicted in Figure 6, the thus-calculated distributions were almost the same as those for 7 Be-, 24Na-, and 38Saerosols. This is indicative that these aerosols are formed by asimple physical attachment to the ambient sulfate aerosols4.
In addition, the simple adsorption of the nuclide to the ambient aerosols was confirmed by the experiments using Ar gas
containing DOP aerosols of a given poly-disperse size distribution,21,22 and the sample gas was irradiated by well-collimatedmonoenergetic neutrons of 65 MeV, where no significant radiations without neutrons were concomitant. It was confirmedthat 38Cl- and 39Cl-aerosols were formed only for the argon gas
containing the DOP aerosols and were not observed without
the DOP aerosols. Figure 7 shows the DOP size distribution
and the activity-weighted size distribution. Here, the activityweighted size distribution was obtained from the total activityof 38Cl and 39Cl to the end of sampling. The distribution curves
were normalized to the respective maximum concentrations.

Radioactive Airborne Species Formed in the Air in High Energy

J. Nucl. Radiochem. Sci., Vol. 7, No. 1, 2006 R35
Table 1: (a) Fractions of aerosol and gseous componentsand (b) chemical species of gaseous components(a) Fractions of aerosol and gaseous components
Nuclide
11

C
N
15
O
18
F
38
Cl***
39
Cl***
38
S***
13

Component
Aerosol Component
Gaseous Component
1.9 %98.1 %
1.9 %*
98.1 %
0.5 %**
99.5 %
0.05 %**
99.95 %
73 %
27 %
72 %
28 %
50 %
50 %

* : ±0.4%, ** : ±(0.02~0.04)%, *** : A NaOH loaded filter wasused to collect gaseous components.

(b) Chemical species
Figure 7. Size distribution of 38Cl and 39Cl attached DOP aerosols
formed by the irradiation of 45 MeV neutrons○:measured number size distribution, ●:measured activity size distribution

11

C species11
CO : ~80%,

11

CO2 : ~20%

13

N species
13
N2 : ~55%,

The activity-weighted size distribution curve slightly shifted
toward larger particle diameter, compared to the DOP size distribution, and the computed distribution was well fitted to theone obtained experimentally. This result is similar to those for7Be-, 24Na-, and 38S-aerosols, and supports the formation mechanism of aerosol formation mentioned above.
The radionuclides produced via nuclear reactions would havelarge kinetic energies and/or charges at their birth and interact
with the surrounding as hot atoms. After completion of these
hot atom reactions, most of them are likely to attach to the surrounding non-radioactive aerosols. The present results support
that the radioactive aerosol formation proceeds by a simple
physical attachment to the ambient non-radioactive aerosols
with a certain distribution.
(c) Chemical forms of gaseous 11C and 13 N and their formation mechanisms. Table 1 summarizes the chemical compositions of gaseous 11C and 13N. 11C and 13N atoms behave as
hot atoms and react with the ambient molecules to lead to theformation of 11C- and 13N-labeled species. Wolf et al.23, 24 extensively studied the reaction products of energetic 11C atoms produced in N2-O2 gases of various compositions using 3-GeV and
10-MeV protons. They found that the primary products are11
CO and 11CO2 , and they explained the formation of theseproducts by the following hot 11C* atom reactions, which leads
11
CO and 11CN:
11

C* + O2 → 11CO + O,
C* + N2 → 11CN + N.

11

(1)
(2)

The reaction (1) is exothermic by -128 kcal/mol, while thereaction (2) is endothermic by 50 kcal/mol. Thus formed 11CN
is instantaneously oxidized to 11CO2.
11

CN + O2 → 11CON + O,
CON + O2 → 11CO2 + NO.

11

(3)
(4)

They also estimated that the reactivity of energetic 11C atoms
toward O2 is nine-times larger than that toward to N2 . From
the reactivities of 11C with O2 and N2, and their contents in theair, the percentages of 11CO and 11CO2 as primary products
were evaluated to be about 70% and 30%, respectively. The
composition is close to those, as shown in Table 1, and the present result supports the idea that the 11C species are formed by
the above mentioned hot atom reactions.
On the other hand, the major species for 13N were 13N2 and
13
NO2 , and although smaller quantities of 13NO and H13NO2
were also observed, significant quantities of H13NO3 were notdetected. The recoil 13N atoms would attack nitrogen molecules by hot-atom reactions, leading directly the 13N2 formation.

13

NO2 : ~45%, others : less than 1%

It is also considered that a large fraction of the kinetically thermalized recoil 13N atoms would be electronically excited at thetime of the reaction. The excited states are considered to beN(2D) and N(2P), having excitation energies of 2.38 and 3.57
eV, respectively. Hence, the excited 13N* atoms could supply
the activation energy necessary for an exchange with nitrogenmolecules 25:
13

N* + N2 → 13NN* + N,
N*(2D,2P) + N2 → 13NN* + N,
13
NN* + M → 13NN + M*.
13

(5)
(6)
(7)

The kinetic data indicate that 13 NO 2 could be produced
through a subsequent oxidation of 13NO with radiolytic products of O3. 13NO is a primary product from hot and thermal
reactions of recoil 13N atom with oxygen molecules;
13

NO + O3 → 13NO2 + O2.
13

(8)

13

The relative yields of N2 and NO2 suggest that the recoil
N atoms are more likely to react faster with oxygen molecules
by a factor of about 2.5 as compared with the reaction withnitrogen molecules.
The G-value for ozone formation in air was also calculated
to be 6.4 (molecules/100 eV) at a dose rate of 6×1016 eVg-1s-1,
which was slightly smaller than the value of 7.4 obtained byWillis et al.26 at a high dose rate of 1023 eVg-1s-1.
13

4. Summary
The formation of radioactive aerosols was explained by asimple physical attachment of radioactive nuclides to ambient
aerosols, and the size distributions of radioactive aerosols
depended on the size distributions of ambient aerosols. It isconsidered that the non-radioactive aerosols of a submicron
range contain sulfate formed by radiolytic reactions in the airof accelerator tunnels, where the air is exposed to very highradiations.
Most of 11C and 13N produced in the air are present in gaseous species, and are formed through hot-atom reactions andfollowing thermal reactions. The chemical compositions of 11C
and 13N were extensively studied. The results can be used forquantitative evaluations of the internal doses.

R36 J. Nucl. Radiochem. Sci., Vol. 7, No. 1, 2006
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