Journal of Nuclear and Radiochemical Sciences, Vol. 2, No. 1-2, pp. 1-3, 2001

ArticlesDetermination of Degradation Constants of Energetic 7*Li Ion in Liquid Media Usinga Thin Boron Film on Silicon Wafer
Yoichi Sakai,*, a Michael Kenya Kubo,b Chushiro Yonezawa,c Hideaki Matsue,cand Mutsuko Jimbod
aDepartment of Chemistry, Daido Institute of Technology, Takiharu-cho, Minami-ku, Nagoya 457-8530, Japan
School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
cDepartment of Environmental Sciences, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, JapandDepartment of Electrical and Electronics Engineering, Daido Institute of Technology, Takiharu-cho, Minami-ku,Nagoya 457-8530, Japanb

Received: October 23, 2001; In Final Form: November 28, 2001
A novel method to determine degradation constants has been developed for energetic 7*Li ions produced from the10B(n,α)7*Li reaction, moving in liquid media. The energetic 7*Li generated in a thin boron film on silicon wafer
plunged into a liquid sample in which the wafer was immersed. The degradation constants were determined byanalyzing the Doppler-broadened lineshapes of prompt γ-ray at 478 keV emitted from 7*Li. For comparison,degradation constants were also measured for solutions of boron compounds. Values obtained by the two
methods gave fair agreement.

Introduction
Stopping powers of materials for charged particles with subMeV order of kinetic energy have been obtaining growinginterest because of the expanding needs in material science,
radiation therapy, analytical use etc., where the depth profilesof the implanted particles in substrates have critical importance. They have been usually determined by measuring
energy loss of energetic ions through passage of thin foils.
However, it is difficult to adopt such a method for sub-MeV
particles because their short stopping range requires very thinfoils which are available only for limited materials. The
method using stacked thin foils cannot be applicable to gas andliquid materials. Among the methods of measuring stopping
powers of relatively low energy ions, the unique lineshapeanalysis of the 478 keV prompt γ-ray from 7*Li produced via
the 10B(n,α)7*Li reaction has been applied not only to solids
but also to liquids and gases. Usually small amount of boron
mixed or doped in the sample of interest act as a 7*Li source.
An energetic 7* Li with an initial kinetic energy of 840 keV
decays to 7Li with a lifetime of 1.05 × 10−13 s, emitting the γray. Due to coincidental matching of the lifetime and theslowing-down time, the 7 * Li emits the γ-ray in flight inmedium, resulting in Doppler broadening of the spectral lineshape of the γ-ray. The degree of broadening depends onvelocity degradation of 7 * Li in the medium, that is, on theelemental composition and atomic density of the materials.
Previously, we have measured the Doppler broadened lineshapes of the prompt γ-rays of 478 keV in various samples
containing boron using neutron beam guides of JRR-3M at
Japan Atomic Energy Research Institute (JAERI). The“Degradation constant” D was introduced and defined as thereciprocal of the time constant of the degradation in media.1, 2
The stopping power and degradation constant are the values
evaluated for an identical phenomenon from different viewpoints; the former is one from a viewpoint of the materials,
while the latter from the moving charged particles.
Kreutz et al.3 employed the “solution method” and used
*Corresponding author. E-mail: yocsakai@daido-it.ac.jp.
FAX: +81-52-612-5623.

boric acid and its derivatives as boron sources for liquidsamples. Although boric acid is considerably stable, itdissolves to limited range of organic compounds. Boric esters
were also used for organic compounds, but they are not stableto humidity and decompose upon exposure to air. The method
is not applicable to the substances which do not dissolve any
boron compounds with considerable amount. Such limitation
has prevented the measurement of stopping powers of wideclass of compounds by this method.
Our recent works 4 , 5 proved that the lineshapes of theDoppler broadened γ-ray spectra of a thin boron film are asymmetrical. This phenomenon was successfully explained byassuming that energetic 7* Li ions are ejected from the thinboron film. In the present study, we proposed a novel “thinfilm” method using a thin boron film deposited on a siliconwafer as a 7* Li source. By immersing the wafer in sample
liquids we measured the Doppler broadened γ-ray spectra fromthe 7* Li ions ejected from the thin boron layer to the targetliquid.
Experimental
Sample Preparations. Organic solvents were used of a
research grade commercially available from Wako Pure
Chemical Co. For the solution method borane dimethylamine,BH 3NH(CH 3)2, abbreviated as BDA, from Kanto ChemicalCo., Inc. was dissolved in pure liquid media with a weight
concentration of 0.2 %. BDA is chemically stable and sufficiently soluble in various polar and non-polar solvents. About
0.5 ml of the sample solution was placed in a quartz tube (5mmφ × 35 mm height) with a rubber cap. For the thin filmmethod, a mirror-polished silicon wafer of approximately 3 ×5 × 0.05 cm3 was used as a substrate. A thin boron layer ofabout 50 nm was formed on the wafer using the Ar ion sputtering method. The wafer was cut into chips of approximately
1 × 1 cm2.
Prompt -ray Measurements. At the neutron beam guideof JRR-3M at JAERI a solution sample in quartz tube was irradiated with thermal or cold neutrons, the flux of which was 2.4× 107 and 1.1 × 108 cm−2s−1, respectively. A wafer immersed ina liquid sample of about 1 ml in a polyethylene tube was irra-© 2001 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2001

2SakaiJ. Nucl. Radiochem. Sci., Vol. 2, No. 1-2, 2001

10000

aγ-ray detector

liquidthin boron layer

neutronCounts / channel

5000

Figure 1. Conceptual arrangement for the measurement using a B/Siwafer. The thickness of boron layer was 50 nm and the wafer size is1×1 cm2. The distance between the wafer and the detector was 30 cm.
The B/Si wafer was immersed in a liquid sample and the boron layer
was set perpendicular to the measurement axis.

where v0 is the initial velocity of 7*Li , 4.8 × 108 cm s−1. Dvalues are characteristic property of the media in which theenergetic 7*Li ion loses its kinetic energy. The lineshapes of
the Doppler broadened spectra depend greatly on the degradation constant D. Recently we proposed a formula describing
the lineshape g(E) containing D as follows;8[],10000b5000(2)In the above equation, c is the velocity of the light, N 0 thenumber of 7*Li at t = 0, E0 the center energy of the prompt γray emitted from 7*Li (= 478 keV), and λ the decay constant of7*
Li (= 9.49 × 10 1 2 s − 1 ). An actual spectral lineshape ismeasured by convoluting the eq 2 with an instrumental
response function usually assumed to be Gaussian. Therefore,
in order to evaluate a D value, we fitted the combination of theeq 2 and a background function to data points of the prompt γray spectrum.
Results and Discussion
The measured prompt γ-ray spectra at 478 keV of 7*Li for
BF3 gas (900 torr), methanol solution of BDA, and glycerin
solution of BDA are shown in Figure 2. The spectral lineshapes of 478 keV γ-ray for air/B/Si, methanol/B/Si, and glycerin/B/Si are shown in Figure 3. It is expected that a γ-ray
emitted from a 7 * Li ion recoiling forward to media should

4000c

20000–10 010Doppler shift / keVFigure 2. Spectral lineshapes of prompt γ-ray of 478 keV of 7* Li
measured for BF3 (900 torr) (a), methanol solution of BDA with 0.2
wt% (b), and glycerin solution of BDA with 0.2 wt% (c). Theabscissa indicates the Doppler shift from 478 keV.

3000a2000
1000

Counts / channel

diated in the same conditions as described above. The conceptual measurement arrangement is shown in Figure 1, where thethin boron layer was set to be perpendicular to the measurement axis and faced to the detector. The measurements wereperformed using the prompt γ-ray analyzing system installed at
the neutron beam guide by Yonezawa et al.6, 7 The γ-ray spectrometry system consisted of a high purity Ge detector, bismuth germanate shielding detectors, and pulse height analyzers controlled by a personal computer.
Lineshape Analysis. In our previous works, a parameter
called “degradation constant” D was introduced. The velocityof 7*Li at time t, v(t), is expressed in its lifetime in the following;v(t) = vo exp (−Dt),(1)

cN0 λcE−E0 λ−DD−1
2E0ν0 λ−DE0ν0
E0 (1−ν0 /c)≤E ≤E0 (1+ν0 /c).

15000

0silicong(E) =02000

b1000
02000c1000–10 010Doppler shift / keVFigure 3. Spectral lineshapes of prompt γ-ray of 478 keV of 7* Li
measured for air/B/Si (a), methanol/B/Si (b), and glycerin/B/Si (c).
The abscissa indicates the Doppler shift from 478 keV.

Determination of Degradation Constants of Energetic 7*Li Ion

make a positive Doppler energy shift, while that recoilingbackward to silicon substrate should shift its energy to negative. Thus, the measured spectra were analyzed as a superposition of the media and silicon for the positive and negative
Doppler shift, respectively. The estimated Degradation
constants D for air and liquid samples are summarized inTable 1.
The D values for BF3 gas and air/B/Si were estimated fromthe lineshapes to be very small, nearly zero within the experimental error, indicating that no actual velocity degradation
occurred in BF3 gas of 900 torr and air at about 1 atm during
the lifetime of 7*Li. This finding also implies that the boron
layer of the B/Si wafer targets prepared in the present work
was actually thin enough compared with the range of 7*Li that
we can use the B/Si wafers as the source of energetic 7 * Li
implanting to sample media for our purpose. It might beconsidered that 7*Li ions traversing long in boron layer could
affect the lineshape and lead to erroneous D values. But thoseions have originally small velocity component along themeasurement axis and should result in little Doppler shift
whether they experience any degradation or not.
Comparing the D values for methanol, ethyleneglycol, andglycerin by the two methods listed in Table 1, we can conclude
that both the methods gave same values in those organicliquids. This clearly shows that our novel method using a B/Siwafer to determine degradation constants is effectively applicable.
TABLE 1: Degradation Constants (1012s−1). D; Obtained in
the Present Work, DLSS; Calculated According to the LSS
Theory and the Bragg RuleD / 1012s−1

DLSS / 1012s−1“Solution method” “Thin film method”
BF3 gas (900torr)

0.04 (1)

Air (1atm)

0.003
0.02 (15)0.001

Methanol

0.96 (2)1.08 (6)1.13Ethyleneglycol

1.24 (1)1.28 (2)1.46

Glycerin

1.38 (2)1.34 (2)1.63

The measurements for BF3 gas (Thin film method) and air (Solutionmethod) were not carried out since they are impossible to be done inprinciple.
D values contain the (error) in the last one or two decimal place(s)given in brackets. The errors were estimated from fitting results.

J. Nucl. Radiochem. Sci., Vol. 2, No. 1-2, 2001

3

On the other hand, we calculated the degradation constants
using the Lindhard-Scharff-Schiott (LSS) theory 9 and theBragg rule,10 where the simple additivity of stopping power foreach constituent atom was postulated, neglecting any effect ofchemical bonding. The details of calculation procedures arereferred to our previous papers.1, 2 The D values from the LSS
theory (DLSS) are also listed for BF3 (900 torr), air (1 atm),
liquid methanol, liquid ethyleneglycol, and liquid glycerin in
Table 1. It was revealed that the D values experimentallyobtained for methanol, ethyleneglycol, and glycerin are
substantially lower than the respective DLSS values. The difference might be due to some chemical effect on the degradation
processes or stopping powers, which should be very interesting. It is anticipated that there could exist chemical-bondeffects since the velocity range (5 to 2 × 106 m s−1) of 7*Li with
its lifetime overlaps with those of the electrons participating inchemical bonds in medium, leading to the longer interaction
time and consequently the more extent of chemical effect.11
However, we have not yet offered possible concrete explanation on such chemical effects. Further research is necessary totake a clue for making clear the problems.
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