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A moving target for accelerated charged particle induced X-ray measurement
Nuclear Instruments and Methods 171 {198(I) 207 214 ' N,,~rth-llolland Publishin.. ('cunpany
A MOVING TARGET FOR ACCELERATED CttARGED PARTICLE INDUCED X-RAY MEASUREMENT L.S. ( ' t l U A N ( ; *. K. SIIIMA. tl. I{BltlARA, R. SI!KI and T. MIKUMO landem ,l¢'cch'rator ("enter. l'hc ~nirersity o~ ls,kuha. Ibaraki. 300-31 Japan Received I Scplenlbcr 1978 and in revised t'c,rm 19 Scl',tember 1979
In all attcn lpt to atIain good reproducibililx as x~cll as to e[lable all absohltc dctcrlllination ill the lllt'asurclllenl of X-ray t]u,,~resccnces, resuhing from b o m b a r d m e n l of a heterogeneous sample by accelerated charged particles, a moving-target mechanism incorporating an electronic remolc control system has been devised. The system is designed to scan the whole sample area with a
chosell constanl linear speed, by a fixed particle beanl with a cross-sectional area a small fraction of thai of tl'tc sample. Using 16 MeV protons and 40 McV oxygen-ion beams, test runs of this syslem showed that the altempled objectives are attainable wilh good accuracies: reproducibility of the data for a given target is better fllan 3f;. the Imearity of the calibration curve is m good agreemcnl, within the weighing errors of the standard elements and the uncertainty due to beam current l'luctt,ation. v.ith the expected values, and the results of absolute determinalions using both metal foils and hetero.t~enec,us f,owder san> pies arc in ,.,ood agreement v,ith acccI',ted results using dit'tercnt methods. I)cta,iled accounts of the w~oving-targel system, and the test for rcpruducibility and lincarity are presented. An absolute dclerlllirlatJon oI" tile quantities relaled to accelerated charged-particle induced X-ray fluorescence usillg tile moving target is presented for samples in different forms.
i. Introduction
wouht be required. In practice, tile sample area must be as large as necess:ny it:, al],,+w for good cotnltillg statistics and. a htmlogeneous sample caniio| easily he prepared. Ito,,vever. it is not easy to produce a h o m o gencous tlean3 to cover the whole heterc~geneous sample area l i t ) l I.O iDove a h t ) n l t ) g e n e o t l s bca131 spot t o
Particle induced X-ray fluorescence emission (PIXIe) has been shown Io be a pronlising analytical tool for the elemental analysis o f a varielv o f substances, ltowever, diffict,lties involved in the preparation o f Ihe samples for an absolute determination o f the elemental contents have so far limited the scope of tiffs analytical m e t h o d in actt, al applications. Conventional methods used in qttatltitative analysis by means ol" P I X I o f the elements in chemical. biological and ecological samples arc o f a "'comparafive" nature: establishment o f a calibration curve or doping the sample with reference standards I I I. For the use of a calibration curve, the conditions of lhe standards and tile samples must be identical which presents great difficulty in the usual cases o f sanlple preparation. On the other hand. doping o f reference standards in a sample requires that the sample should be dissolved m solution, otherwise :, hornogeneous particle beam covering the whole target sample area or a h o m o g e n e o u s sample coupled with a small beam spot o f cross section smaller than t i l e sample area
scan, with a c o n s t a n t
linear speed, throughout the
heteroger]eous sarnple area. In the present e x p e r i m e n t , a "'moving-target'" m e t h o d has been developed with the aim o f o v e r c o m ing the difficulties mentioned above and. further, for an absolute determination in PIXI'. In this m e t h o d . the particle beam cross-section is so squeezed as to be considered as a h o m o g e n e o u s beam, covering only a small portion o f the heterogeneous sample area. The sample is rotated to scan through the fixed particle beam, t)l" constant intensity, with the same linear speed for every portion o f the target area. As the fluctt, alion o f the beam intensity is small, it can be assulned to remain at a constant value during the irradiation period even if the scanning process is repeated many times. In one c o m p l e t e scanning o f the sample area, A cm 2, o f the heterogeneous sample through the fixed beam, o f cross-section a c n l 2 and COtlManl intensity 1 particles per second, the average n u m b e r o f reactions
* Visiting research scientist from The ('hinese University of llong Kong, Shatin, NI, llong Kong. 207
I..S. (Tmang t>t al. / A m o v i n g target
208
where o in cm 2 is the cross section for tile reaction of interest, n t is the average number of target atoms per cm 2 of the heterogeneous sample, with n and t denoting target atoms per cm 3 and thickness, in cm, respectively, w i is the target mass for ith portion which has thickness I i cm, N A is Avogadro's number, M is the atomic mass. a in cm z is the fixed beam cross-section, aild N s is the total number of portions for the whole area of the target, of total nlass W, scanned by the fixed particle beam. It is seen from eq. (1) that if the total target mass, W, and the total target area, A. of the sample is fixed, the average nuinber of reactions produced per second, Ya, is independent of how the mass W is distributed on the total area A. Therefore, by keeping the beam current at a fixed value, a good reproduction o f the measured activity and good linearity for a calibration curve can be obtained in a "comparison" method of quantitative analysis by means o f PIXE. Furthermore, if due corrections for tile detector efficiency, geometry, and attenuation of the X-rays arc made, an absohite analysis by means of PIXI { is possible. It should be emphasized that eq. (1) is tile same as that for a fixed target of a heterogeneous sample o f total mass W and total area A being irradiated by a honlogeneous beam of intensity I with a cross-section large, than the sample area.
driven by a d c motor is used for rotation of the main circular disk in order to change the sample position and to set the target sample in the irradiation position. Another dc motor drives another gear system to spin tile target sample about its own central axis along with a transverse motion. Microswitches are actuated by the cams for switching of the electric power applied to tile dc motors o n / o f f and, further, to change tile terminal voltage of the dc motor for varying the velocity of spinning of the target as a flmction of the distance between the center of the target and the point of the irradiation, so that the linear speed with which tile sample scans through the fixed beam can bc kept at the same vah,e for every portion of the target area scanned. A gear system interconnecting tile main sample holder disk and the target sample holder moves the central shaft, through an eccentric radial wheel, in the transverse direction. I mm per complete spin of the target holder. In tile present systeni, one con> plete scanning of the sample area of 12 toni diaineter iS made when the target completes its sixth spin. Oil another six complete spins o f the target, it will have returned It) its initial position, i.e., tree ccniipletc scans of the sample area of 12 mm diameter has been completed, and an electric signal will be sent out to the control panel for a visible indication. The spin velocity of the target can be changed within the tolerable applied voltage of tile dc motor. However, an optinmm voltage range must be used for meeting the requirement set by an equal linear scanning speed of the target throughout its whole area. This mechanism has been designed to change the speed of the spin at the end of each two-thirds of a colnpletc spin. As a rest, It, tile constant linear speed with which tile whole sample area is being scanned through tile fixed beam is approximately 0.8 mm s - ' . The beam track on the target is a spiral one (see fig. 2A).
2. Moving-target system
3. Target samples
A photograph o f the moving-target assembly installed in tile sactiering chamber is shown in fig. 1. It is composed of three main units, namely, a driving naechanisnL driving motors, and an electronic remote control, in the driving nlechanism, a large circular disk o f 20 cm diameter whose outermost circumference contains ten circular holes of 40 nnn diameter each for the accommodation of the samples, is attached to tile central shaft. A gear system which is
In order to verify the versatile capabilities of tile moving-target device applied in the measurenlents of PIXE as implied in eq. (1), various types of target sanlples of known elemental contents were used. These target samples in different forms include: (A) reagent grade pure metals (zinc and lead), carbonates (potassium and barium) and nitrate (strontium) which are dissolved in nitric acid arid then dropped on a Nuclepore membrane 121 of 5 ~ m
produced per second. Ya, will be *~"S ( w i i V A / M a t i ) ti i-: 1 )'a = loiTt
= Io
............
/WS
.'Ys
:\" A ~ Wi i- 1 = Io . . . . . . . .
,Vsa,~!
x~rA i l Io
•
-
1 (1)
AM
L.S. Chuang et al. / / I moving tar eet
2(19
Fig. 1. A photo of" the moving-target assembly installed in the sc;lllcring chamber.
thickness and dried in an electric-oven. Various combinations o f the five elements in known quantities were made into four samples, i.e., sample AI to A4 in table I A. (B) Reagent grade pure metal (zinc) and sulphide (BaSO4) in powder form were dropped onto a sticky region, 10 mm diameter, of a Nuclepore membrane of 5/am thickness and the sticker was dried in a vacuum-electric oven at 120°C. The sampies containing different amounts o f known elements, sample BI to B4. are tabulated in table lB. Thc samples which were prepared by this method are extremely heterogeneous. Therefore, they are thought to be able to serve the present purpose perfectly. The degree of hcterogeneity for these samples is illustrated with a sketch shown in fig. 2A. (C) A Chinese medicine, Corydalis ambigua Chain. et Schlechl. (Fumariaceae) in powder form, whose elemental contents have been partially analyzed using 14 MeV neutrons 13]. The powder sample was suspended in liquid paraffin using a supersonic shaker
and then precipitated on a Nuclepore nlembrane o f 5 /am thickness by means of a centrifugal separator. It o was then dried in a vacuum-oven at 1,,0 C, 14]. Its potassium and phosphorus contents determined by means o f 14 MeV neutron activatiot~ analysis are tabulated in table IC. A microscopic photograph (×100) o f its formation is shown in fig. 2B. (D)Thin metal foils, i.e., Cu and Cr which are supplied commercially are used as self-supporting target samples. The thicknesses o f the foils are given in table 1D.
4. Experimental procedure The experiment was carried out at the 12 UD Pelletron Tandem Accelerator o f tile University o f Tsukuba. In order to compare the present results with the previous results using the fixed-target technique in this laboratory, 16 MeV proton and 40 MeV oxygen-ion beams of currents ranging from 0.5 I 0 nA
210
L.S. Chuang et al, /,4 morhUt target
A 12
B
ram0
1pro SAMPLE
BEAM TRACK : CONSTANT LINEAR SPE E D 03 m m / s e c I:ig. 2. A sketch (A) and a microscopic photo (B) showing the degrees of heterogeneities of the target samples. A. Materials in powder form were dropped at random on Nucleporc membrane. B. Materials in powder form were suspended in liquid paraffin and then precipitated on a Nuclepore membrane by means of centrifugal force.
were used. The b e a m was s q u e e z e d to less than 2 m m
5 / a m thick Nuclepore m e m b r a n e which allows the
d i a m e t e r at the target w h i c h is placed at 45 ° to the
p r o t o n beana to go through the target and be col-
i n c i d e n t b e a m d i r e c t i o n . The sample is m o u n t e d on a
lected in a Faraday cup placed b e h i n d the target for
Table 1 I.;lemental contents for various types of the target samples. A. Materials were dissolved in solution and then dropped onto Nuclepore membrane to dry. B. Materials in powder form were dropped at random on Nuclepore membrane. C. Materials in powder form were suspended in liquid paraffin and then precipitated on a Nuclepore membrane by means of centrifugal separator. 1). Thin metal foil. Element
1A
llenlcnt
1B
Sample (tag)
Satnplc (+ug) .
K Zn Sr Ba Pb
K P
AI
A2
A3
A4
8 8 8 8 8
4 4 4 4 8
0.8 0.8 0.8 0.8 8
0.4 0.4 0.4 0.4 8
.
.
.
.
.
.
.
.
.
.
.
.
BI S Zn Ba
1 I)
C (Chinese Medicine: Fumariaccae) (f/,)
(ug cm -2)
Cu Cr
.
.
.
.
.
.
.
.
.
B2
B3
B4
213
76 1320
213 1130
912
324
912
1390
1C
2.84 0.40
.
D1
D2
307
614
D3
243
L.S. (7tuang et al. / A
integration. For the oxygen-ion beam, two Faraday cups located on both sides o f the target sample were operated m order It) determine the change o f the charge states o f the ()s÷ beam, it) passing through the target material. Duc corrections were applied for the increase in the beam current reading resulting from the change of the charge state o f the oxygen-ion beam. X-rays emitted at 90 ° pass through the vacuum chamber window of 7.6 ~n| thick Be-foil, an air path of about 2 cm, and then a Si(Li) detector window of 25 /.tin thick Be-foil to reach the Si(Li) detector, which has a resolution or about 200 eV at 6.4 keV and an effective area o f 12.5 I l l l l l 2. It t o o k about 23 s to change the target sample and 8.3 rain to finish one complete scan of the target. As the main sample holder disk is made to rotate in both the clockwise and counter-clockwise directions, it was easy to bring the ZnS screen which was set in one of the ten sample holders, into the target position for checking the beam condition. Furthermore, as the time of each revolution o f the spinning target is known, measurement of the X-rays emitted from any target region of interest could easily be selected according to the timing using a stop-watch. In lhe present experiment, however, X-rays emitted from the whole constant area o f the target of 12 mm diameter were counted, as required by tile condition provided in eq. (I}, although the sample materials were confined in tile central region of about 5 10 mm diameter, except the metal-foil san)pies which were self-supporting, the sample mate,ial being extended throughout the whole beam area, 12 mm diameter. It was thought that in this way the performance of tile moving-target could be rigorm,sly tested. Each one of the standard target samples was irradiated separately up to a few ,uC. Two to four runs were made for each standard sample in the moving-target mode operation. A fixed-target mode operation for four o f the standard samples was made in order to compare the differences between the two modes of operation: the proton beam was adjusted to hit three different positions o f the target samples in the three separate runs.
5. R e s u l t s a n d d i s c u s s i o n
A typical X-ray spectrum is shown in fig. 3. For analysis of the reproducibility and linearity of the
mo
211
'it g tarxet
~-
t3 --
Ep
16 MeV
O
3 !6 ~C
-i I I
5 ffm Nuclepcr¢'
Backing ~3 & N
_J UJ Z Z
n,hi n O3 t-Z
'g3
''.l : .I: ~;
3;
,:,,
"i i;*
10-
I.~_
"'.1
i
,,
I
i,
, ~
;. t
" L i:
i
!" :E
,'~ fill ii
102_ i.¸
t
< r~
i x
•
I. . . . .
o
26o
6-o
CHANNEL
NUNBER
.[
_. :.-.-.,.,,,k. _J
600
Fig. 3. Typical X-ray spectra of the target samples. Spcctrt, m of a sample containing K, Zn, Sr, Ba and Pb irradiated with a 16 McV proton beam.
data taken in the moving-target mode operation, and to compare them with those taken with the fixedtarget mode operation, as well as for an absolute determination of X-ray production cross-section and the elemental contents of the samples, SOlne representative peaks of the X-rays were used. Net peak counts per unit current integrator reading of the selected X-rays were tabulated for various checks. .5.1. R e p r o d u c i b i l i t ) ,
lit the case of the moving-larget mode operation, using tile proton and oxygen-ion bean)s, tile fluctuation o f the values o f the net peak counts per unit current integrator reading of the X-rays, for two to four repeated runs, were of the order o f a few per cent about the mean value, whenever the current integrator readings for the same period o f irradiation o f the target sample in the repeated runs were consistent within the order of 20~. The corresponding results for fixed-target mode operation with a small beam showed a few tens o f per cent fluctuation (fig. 4), Two repeated runs o f the moving-target mode operation o f the Cr-foil with the current integrator readings for the same period o f irradiation differing by a factor o f two, resulted in a reproducibility of about 45~. This degree of reproducibility [or a thin
212
I,.S. Ctntattg ct al. ./ ,4 morit g target t
(A r e-K
-
-K\
Zn-K ~MOVING '~ Ba-L ] TARGET • Sr-K; ',)/ FIXED TARGET EXPECTABLE
q
. . . . . . . . . n Zn-K (Powc~rk, I x S -K ,powder) tMOVlNG o Ba-L ,jgowder)) TARGET :,af C u - K (¢ci/', ' : FIXED TARGET 'l. OI - EXPECTABLE ,~f /,~
i =
20
"\
/
~" I0"
,',,.-tf '
(~)f I /
I "4t .<
¢~
/
@"
/
7"
/
I . ~ O.5{u~, • ~ I
[
="
,,/ 4
o J
0
',B,
r
. . . . . 10 20 WA-x/WA-4
i
/ /
; /
0 [/-0
. . . . . . . . . 0.5 --1)0
WB-x~WB-1
or WB-2
Iig. 4. Net peak counts per unit current integrator reading versus tile corresponding mass of an element for moving-target and lqxcd-larget operarion modes results using a 16 McV proton beam. A. Net peak counts per unit current integrator reading for a given X-ra3 of any one element, (CAX), normalized to that for sample A4,(,('..\4) versus the corresponding quantitiy for the masses, (h'Ax/lt~A4): (x denotes 1, 2 or 3 for samples A1 to A3). B. Net peak counts per unit current integrator reading for a given X-ray of any one clement, (('Bx). normalized to thai for sample B1 in case of Zn, (CBI), and that for sample B2 in cases of S and Ba, {('B2). versus the corresponding quantity for the masses, (IVl~x/IVl~I ) or (ll'Bx/h'B2): (for samples B1 to P,4). In addition, the data for ('u tsamplcs I)1 and 1)2) are plotted accordingly,
foil is o f the same order o f magnitude as for the heterogeneous target samples presently used a further confirmation of the ability of the movingtarget to change a heterogeneous target sample into a nearly homogeneous one in effect. The degree of reproducibility was fotmd to be solely dependent on tile degree of the consistency of the beam intensity for the repeated runs as predicted by e q . ( 1 ) . If the number of spins of the target sample could be increased suflicicntly to average-out the fluctuating beam intensity to a constant value for the repeated runs, a better reproducibility should be obtainable. In the present experiment, two complete scans of the target samples were made in most of the experimental runs.
of the X-ray f r o m a certain peak X to that fronl the Pb-L~ peak in the same sample, (CJC,b). and the content in ,ug, ICx, o f the element x for which the X-ray peak is being considered are represented by four straight lines. These results were obtained both
!
5[
*K-Kd
' 41
/, ..
/
(32 5.2.1. Linearity in terms of comparison with an hlternal reference Using the same amounts o f Pb contents in each of the four standard target samples (i.e., sample A1 to A4 in table 1A), normalization of the other elements in any one of the target samples was made against the content of Pb in the same sample. The values thus obtained werc plotted as shown in fig. 5 for the 16 MeV proton beam irradiation. The relationships between the values of the ratio of tile net peak counts per/,1("
e
Zn-Kd o Bo-L~ • Sr-Ka o
/
,
~ /
,
J
i L
/
,
,,
"
i .4
,
-
id
00
/
................ '
. .2. . . .
;. . . . 6. .
8"
Wx [jag] I'ig. 5. The ratio of the net peak counts per pC of tile X-ray from a certain peak of an element x to that from the P1)-I,a peak in the same sample, ( C . x . / ( ' l , h ) v e r s u s nlass of the element x, (W,.) results using a 16 MeV proton beam,
L.S. (Ttuang ct al. /..I morin.e target
l\)r t]xed-target and moving-target modes of operalion, suggesting that the samples used, which were prepared by dissolving the sample in solution and then dropping it to dr',' on the target backing, have distributions t)f the elements in eqt, al r;.llit:,s, but not necessarily htmlogeneouslv, throtJghout the sample area. As shown above, in using an internal rel~'rence (e.g.. Pb in the present cz,se). only when the iIlaSs of the internal rel'erence element. I~'s, is known, the lllaSS oJ" an Ullknt~wn I.l:.,~ Call h e calculated from the relationship: (7'.,,.e s Os ,.11.v IK,. --- r~'~ .......
(2 }
where C denotes the nel peak counts per nnit current integrator reading corrected for the absorF, ticm in the target, tile windows and air-path: c. ~; and M are the efficiency of the detector for a given X.ray energy, X-ray production cross-section a n d a t o m i c n l a s s , respectively, and with the subscripts x and s denoting the unknowll :.llld the standard element respectively. It should be noted that this method is the commonly practiced means of quantitative analysis of elements in soluble samples in PIXE analysis. A sample which cannot be prepared by means of dissolving it in solution and dropping it to dry is difficult to analyze by the fixed-target mode. In that case the moving-target tnode operation is a good alternative. 5.2.2. l, im'aritv in terms of absoh, te comparison: c o n s t n w t k m o.f a calibration cun'e
Comparisons of the net peak counts per t, nit current integrator reading for any one type of X-ray with the corresponding elemental mass should result in linear proportionality: in fig. 4A are shown the results of comparisons using a 16 MeV proton beam for the target samples AI, 2. 3 and 4 (table IA); the results of comparisons, also using a 16 MeV proton beam, for the powder samples B1, 2, 3 and 4 (table I B) which were prepared by dropping the powder samples randomly onto the Nuclepore membranes and for the metal foil samples DI and 2 (table I D) are shown in fig. 4B. A least squares lit for the experimental results shown in tig. 4A and B, resulting in straight lines superposed, within the experimental error, on the straight lines that would be expected. In fig. 4A and 4B, the results of the same comparison for a few cases using fixed-target mode operation are plotted with the symbol ( )f marking the experimental points. In both cases of the fixed-target mode
213
operation, random scalterings of the experimental points from these expected points z,re well demonstrafed. Thus. il is inferred that the moving-target is capable o f performing an absolute determination of PIXI! for ally salllple forlll as long as the reqttirements for an absolute determination imposed by Nuclear Physics are satisfactorily ft,llillet]. 5.3. A hsohtle determhtations
Using the moving-targe! mode operation, X-ray production cross-sections of thin foils have been measured. For 16 MeV protons, for instance, Ct,-K X-ray production cross-section was 4.1 X I0 22 cm2 ?_ 20Vr, which agrees with the prediction froth the binary encounter approximation (BIiA} [5] within 185;. Fo, t\fils with known average thickness but with tmconfirtned utuformity, the use of the moving-target is expected to provide :, better rest, It. With the use of BI-A cross sections, the absolute contents of potassium and phosphorus in a Chinese medicinal sample, Furmariaceae. have been determined, using a 16 MeV proton beam, to be (2.7± 0.3)!; and (0.33 + 0.2)~:; respectively. The comparison of the present results with those of 14 MeV net,tron activation analy'sis (see table IC) [31 shows a good ~,greement between them. I:urthermore. very small fractions of other elements (iron and manganese) have been found by the present PIXE. It should be added before concluding, that in addi0on [o the merits presented heretofore the movinglarger should be very t, seful in medical biological applications of PIXE. If hundreds of thousands of living cells of the same category are aligned on the Nuclepore membrane for irradiation with an adequate particle beam intensity for keeping the cells alive, a quantitative result with good statislics could possibly be obtained.
This work is partially supported by the Nuclear anti Solid State Research Project, The University of Tst,kuba. The authors wish to express their thanks to Professor J. Sanada for his continuous interest and helpful suggestions. The kind arrangements and help of the accelerator group and the enthusiastic assistance during the machine time of Drs. S. Seki, T. lshihara, T. Aoki and K. Kobayashi and Mr. T. Kawano are gratefully acknowledged.
214
L.S. Chuang et al. //1 mot,ing target
References [l I K. Ishii, S. Morita, tl. Tawata, T.C. ('hu, I1. Kaji and 1. Shiokawa, Nucl. Instr. and Meth. 126 (1975) 75; N.I.. Mangalson, K.K. Nielson, M.W. ttill, I).J. t-atottght and L.I). llansen, Proc. Third Conf. on Application of small accelerators, Vol. 1 (1974) 163. [2] A product of Nuclepore Corp., Pleasanton, (!a USA.
[3t L.S. Chuang, K.W. Yue, P.K. ('han and W.Y. Chiu,('omparative Medical - East and West, Vol. VI, No. 1 (1978) 37. 14] I. Sugai, Inst. Nucl. Study, Univ. of Tokyo, Report INS-PL-134, (Dec. 1977) unpublished. IS] J.I). (;arcia, R.J. Fortner and T.M. Kavanagh, Rev. Mod. Phys. 45 (1973) I 11.
A MOVING TARGET FOR ACCELERATED CttARGED PARTICLE INDUCED X-RAY MEASUREMENT L.S. ( ' t l U A N ( ; *. K. SIIIMA. tl. I{BltlARA, R. SI!KI and T. MIKUMO landem ,l¢'cch'rator ("enter. l'hc ~nirersity o~ ls,kuha. Ibaraki. 300-31 Japan Received I Scplenlbcr 1978 and in revised t'c,rm 19 Scl',tember 1979
In all attcn lpt to atIain good reproducibililx as x~cll as to e[lable all absohltc dctcrlllination ill the lllt'asurclllenl of X-ray t]u,,~resccnces, resuhing from b o m b a r d m e n l of a heterogeneous sample by accelerated charged particles, a moving-target mechanism incorporating an electronic remolc control system has been devised. The system is designed to scan the whole sample area with a
chosell constanl linear speed, by a fixed particle beanl with a cross-sectional area a small fraction of thai of tl'tc sample. Using 16 MeV protons and 40 McV oxygen-ion beams, test runs of this syslem showed that the altempled objectives are attainable wilh good accuracies: reproducibility of the data for a given target is better fllan 3f;. the Imearity of the calibration curve is m good agreemcnl, within the weighing errors of the standard elements and the uncertainty due to beam current l'luctt,ation. v.ith the expected values, and the results of absolute determinalions using both metal foils and hetero.t~enec,us f,owder san> pies arc in ,.,ood agreement v,ith acccI',ted results using dit'tercnt methods. I)cta,iled accounts of the w~oving-targel system, and the test for rcpruducibility and lincarity are presented. An absolute dclerlllirlatJon oI" tile quantities relaled to accelerated charged-particle induced X-ray fluorescence usillg tile moving target is presented for samples in different forms.
i. Introduction
wouht be required. In practice, tile sample area must be as large as necess:ny it:, al],,+w for good cotnltillg statistics and. a htmlogeneous sample caniio| easily he prepared. Ito,,vever. it is not easy to produce a h o m o gencous tlean3 to cover the whole heterc~geneous sample area l i t ) l I.O iDove a h t ) n l t ) g e n e o t l s bca131 spot t o
Particle induced X-ray fluorescence emission (PIXIe) has been shown Io be a pronlising analytical tool for the elemental analysis o f a varielv o f substances, ltowever, diffict,lties involved in the preparation o f Ihe samples for an absolute determination o f the elemental contents have so far limited the scope of tiffs analytical m e t h o d in actt, al applications. Conventional methods used in qttatltitative analysis by means ol" P I X I o f the elements in chemical. biological and ecological samples arc o f a "'comparafive" nature: establishment o f a calibration curve or doping the sample with reference standards I I I. For the use of a calibration curve, the conditions of lhe standards and tile samples must be identical which presents great difficulty in the usual cases o f sanlple preparation. On the other hand. doping o f reference standards in a sample requires that the sample should be dissolved m solution, otherwise :, hornogeneous particle beam covering the whole target sample area or a h o m o g e n e o u s sample coupled with a small beam spot o f cross section smaller than t i l e sample area
scan, with a c o n s t a n t
linear speed, throughout the
heteroger]eous sarnple area. In the present e x p e r i m e n t , a "'moving-target'" m e t h o d has been developed with the aim o f o v e r c o m ing the difficulties mentioned above and. further, for an absolute determination in PIXI'. In this m e t h o d . the particle beam cross-section is so squeezed as to be considered as a h o m o g e n e o u s beam, covering only a small portion o f the heterogeneous sample area. The sample is rotated to scan through the fixed particle beam, t)l" constant intensity, with the same linear speed for every portion o f the target area. As the fluctt, alion o f the beam intensity is small, it can be assulned to remain at a constant value during the irradiation period even if the scanning process is repeated many times. In one c o m p l e t e scanning o f the sample area, A cm 2, o f the heterogeneous sample through the fixed beam, o f cross-section a c n l 2 and COtlManl intensity 1 particles per second, the average n u m b e r o f reactions
* Visiting research scientist from The ('hinese University of llong Kong, Shatin, NI, llong Kong. 207
I..S. (Tmang t>t al. / A m o v i n g target
208
where o in cm 2 is the cross section for tile reaction of interest, n t is the average number of target atoms per cm 2 of the heterogeneous sample, with n and t denoting target atoms per cm 3 and thickness, in cm, respectively, w i is the target mass for ith portion which has thickness I i cm, N A is Avogadro's number, M is the atomic mass. a in cm z is the fixed beam cross-section, aild N s is the total number of portions for the whole area of the target, of total nlass W, scanned by the fixed particle beam. It is seen from eq. (1) that if the total target mass, W, and the total target area, A. of the sample is fixed, the average nuinber of reactions produced per second, Ya, is independent of how the mass W is distributed on the total area A. Therefore, by keeping the beam current at a fixed value, a good reproduction o f the measured activity and good linearity for a calibration curve can be obtained in a "comparison" method of quantitative analysis by means o f PIXE. Furthermore, if due corrections for tile detector efficiency, geometry, and attenuation of the X-rays arc made, an absohite analysis by means of PIXI { is possible. It should be emphasized that eq. (1) is tile same as that for a fixed target of a heterogeneous sample o f total mass W and total area A being irradiated by a honlogeneous beam of intensity I with a cross-section large, than the sample area.
driven by a d c motor is used for rotation of the main circular disk in order to change the sample position and to set the target sample in the irradiation position. Another dc motor drives another gear system to spin tile target sample about its own central axis along with a transverse motion. Microswitches are actuated by the cams for switching of the electric power applied to tile dc motors o n / o f f and, further, to change tile terminal voltage of the dc motor for varying the velocity of spinning of the target as a flmction of the distance between the center of the target and the point of the irradiation, so that the linear speed with which tile sample scans through the fixed beam can bc kept at the same vah,e for every portion of the target area scanned. A gear system interconnecting tile main sample holder disk and the target sample holder moves the central shaft, through an eccentric radial wheel, in the transverse direction. I mm per complete spin of the target holder. In tile present systeni, one con> plete scanning of the sample area of 12 toni diaineter iS made when the target completes its sixth spin. Oil another six complete spins o f the target, it will have returned It) its initial position, i.e., tree ccniipletc scans of the sample area of 12 mm diameter has been completed, and an electric signal will be sent out to the control panel for a visible indication. The spin velocity of the target can be changed within the tolerable applied voltage of tile dc motor. However, an optinmm voltage range must be used for meeting the requirement set by an equal linear scanning speed of the target throughout its whole area. This mechanism has been designed to change the speed of the spin at the end of each two-thirds of a colnpletc spin. As a rest, It, tile constant linear speed with which tile whole sample area is being scanned through tile fixed beam is approximately 0.8 mm s - ' . The beam track on the target is a spiral one (see fig. 2A).
2. Moving-target system
3. Target samples
A photograph o f the moving-target assembly installed in tile sactiering chamber is shown in fig. 1. It is composed of three main units, namely, a driving naechanisnL driving motors, and an electronic remote control, in the driving nlechanism, a large circular disk o f 20 cm diameter whose outermost circumference contains ten circular holes of 40 nnn diameter each for the accommodation of the samples, is attached to tile central shaft. A gear system which is
In order to verify the versatile capabilities of tile moving-target device applied in the measurenlents of PIXE as implied in eq. (1), various types of target sanlples of known elemental contents were used. These target samples in different forms include: (A) reagent grade pure metals (zinc and lead), carbonates (potassium and barium) and nitrate (strontium) which are dissolved in nitric acid arid then dropped on a Nuclepore membrane 121 of 5 ~ m
produced per second. Ya, will be *~"S ( w i i V A / M a t i ) ti i-: 1 )'a = loiTt
= Io
............
/WS
.'Ys
:\" A ~ Wi i- 1 = Io . . . . . . . .
,Vsa,~!
x~rA i l Io
•
-
1 (1)
AM
L.S. Chuang et al. / / I moving tar eet
2(19
Fig. 1. A photo of" the moving-target assembly installed in the sc;lllcring chamber.
thickness and dried in an electric-oven. Various combinations o f the five elements in known quantities were made into four samples, i.e., sample AI to A4 in table I A. (B) Reagent grade pure metal (zinc) and sulphide (BaSO4) in powder form were dropped onto a sticky region, 10 mm diameter, of a Nuclepore membrane of 5/am thickness and the sticker was dried in a vacuum-electric oven at 120°C. The sampies containing different amounts o f known elements, sample BI to B4. are tabulated in table lB. Thc samples which were prepared by this method are extremely heterogeneous. Therefore, they are thought to be able to serve the present purpose perfectly. The degree of hcterogeneity for these samples is illustrated with a sketch shown in fig. 2A. (C) A Chinese medicine, Corydalis ambigua Chain. et Schlechl. (Fumariaceae) in powder form, whose elemental contents have been partially analyzed using 14 MeV neutrons 13]. The powder sample was suspended in liquid paraffin using a supersonic shaker
and then precipitated on a Nuclepore nlembrane o f 5 /am thickness by means of a centrifugal separator. It o was then dried in a vacuum-oven at 1,,0 C, 14]. Its potassium and phosphorus contents determined by means o f 14 MeV neutron activatiot~ analysis are tabulated in table IC. A microscopic photograph (×100) o f its formation is shown in fig. 2B. (D)Thin metal foils, i.e., Cu and Cr which are supplied commercially are used as self-supporting target samples. The thicknesses o f the foils are given in table 1D.
4. Experimental procedure The experiment was carried out at the 12 UD Pelletron Tandem Accelerator o f tile University o f Tsukuba. In order to compare the present results with the previous results using the fixed-target technique in this laboratory, 16 MeV proton and 40 MeV oxygen-ion beams of currents ranging from 0.5 I 0 nA
210
L.S. Chuang et al, /,4 morhUt target
A 12
B
ram0
1pro SAMPLE
BEAM TRACK : CONSTANT LINEAR SPE E D 03 m m / s e c I:ig. 2. A sketch (A) and a microscopic photo (B) showing the degrees of heterogeneities of the target samples. A. Materials in powder form were dropped at random on Nucleporc membrane. B. Materials in powder form were suspended in liquid paraffin and then precipitated on a Nuclepore membrane by means of centrifugal force.
were used. The b e a m was s q u e e z e d to less than 2 m m
5 / a m thick Nuclepore m e m b r a n e which allows the
d i a m e t e r at the target w h i c h is placed at 45 ° to the
p r o t o n beana to go through the target and be col-
i n c i d e n t b e a m d i r e c t i o n . The sample is m o u n t e d on a
lected in a Faraday cup placed b e h i n d the target for
Table 1 I.;lemental contents for various types of the target samples. A. Materials were dissolved in solution and then dropped onto Nuclepore membrane to dry. B. Materials in powder form were dropped at random on Nuclepore membrane. C. Materials in powder form were suspended in liquid paraffin and then precipitated on a Nuclepore membrane by means of centrifugal separator. 1). Thin metal foil. Element
1A
llenlcnt
1B
Sample (tag)
Satnplc (+ug) .
K Zn Sr Ba Pb
K P
AI
A2
A3
A4
8 8 8 8 8
4 4 4 4 8
0.8 0.8 0.8 0.8 8
0.4 0.4 0.4 0.4 8
.
.
.
.
.
.
.
.
.
.
.
.
BI S Zn Ba
1 I)
C (Chinese Medicine: Fumariaccae) (f/,)
(ug cm -2)
Cu Cr
.
.
.
.
.
.
.
.
.
B2
B3
B4
213
76 1320
213 1130
912
324
912
1390
1C
2.84 0.40
.
D1
D2
307
614
D3
243
L.S. (7tuang et al. / A
integration. For the oxygen-ion beam, two Faraday cups located on both sides o f the target sample were operated m order It) determine the change o f the charge states o f the ()s÷ beam, it) passing through the target material. Duc corrections were applied for the increase in the beam current reading resulting from the change of the charge state o f the oxygen-ion beam. X-rays emitted at 90 ° pass through the vacuum chamber window of 7.6 ~n| thick Be-foil, an air path of about 2 cm, and then a Si(Li) detector window of 25 /.tin thick Be-foil to reach the Si(Li) detector, which has a resolution or about 200 eV at 6.4 keV and an effective area o f 12.5 I l l l l l 2. It t o o k about 23 s to change the target sample and 8.3 rain to finish one complete scan of the target. As the main sample holder disk is made to rotate in both the clockwise and counter-clockwise directions, it was easy to bring the ZnS screen which was set in one of the ten sample holders, into the target position for checking the beam condition. Furthermore, as the time of each revolution o f the spinning target is known, measurement of the X-rays emitted from any target region of interest could easily be selected according to the timing using a stop-watch. In lhe present experiment, however, X-rays emitted from the whole constant area o f the target of 12 mm diameter were counted, as required by tile condition provided in eq. (I}, although the sample materials were confined in tile central region of about 5 10 mm diameter, except the metal-foil san)pies which were self-supporting, the sample mate,ial being extended throughout the whole beam area, 12 mm diameter. It was thought that in this way the performance of tile moving-target could be rigorm,sly tested. Each one of the standard target samples was irradiated separately up to a few ,uC. Two to four runs were made for each standard sample in the moving-target mode operation. A fixed-target mode operation for four o f the standard samples was made in order to compare the differences between the two modes of operation: the proton beam was adjusted to hit three different positions o f the target samples in the three separate runs.
5. R e s u l t s a n d d i s c u s s i o n
A typical X-ray spectrum is shown in fig. 3. For analysis of the reproducibility and linearity of the
mo
211
'it g tarxet
~-
t3 --
Ep
16 MeV
O
3 !6 ~C
-i I I
5 ffm Nuclepcr¢'
Backing ~3 & N
_J UJ Z Z
n,hi n O3 t-Z
'g3
''.l : .I: ~;
3;
,:,,
"i i;*
10-
I.~_
"'.1
i
,,
I
i,
, ~
;. t
" L i:
i
!" :E
,'~ fill ii
102_ i.¸
t
< r~
i x
•
I. . . . .
o
26o
6-o
CHANNEL
NUNBER
.[
_. :.-.-.,.,,,k. _J
600
Fig. 3. Typical X-ray spectra of the target samples. Spcctrt, m of a sample containing K, Zn, Sr, Ba and Pb irradiated with a 16 McV proton beam.
data taken in the moving-target mode operation, and to compare them with those taken with the fixedtarget mode operation, as well as for an absolute determination of X-ray production cross-section and the elemental contents of the samples, SOlne representative peaks of the X-rays were used. Net peak counts per unit current integrator reading of the selected X-rays were tabulated for various checks. .5.1. R e p r o d u c i b i l i t ) ,
lit the case of the moving-larget mode operation, using tile proton and oxygen-ion bean)s, tile fluctuation o f the values o f the net peak counts per unit current integrator reading of the X-rays, for two to four repeated runs, were of the order o f a few per cent about the mean value, whenever the current integrator readings for the same period o f irradiation o f the target sample in the repeated runs were consistent within the order of 20~. The corresponding results for fixed-target mode operation with a small beam showed a few tens o f per cent fluctuation (fig. 4), Two repeated runs o f the moving-target mode operation o f the Cr-foil with the current integrator readings for the same period o f irradiation differing by a factor o f two, resulted in a reproducibility of about 45~. This degree of reproducibility [or a thin
212
I,.S. Ctntattg ct al. ./ ,4 morit g target t
(A r e-K
-
-K\
Zn-K ~MOVING '~ Ba-L ] TARGET • Sr-K; ',)/ FIXED TARGET EXPECTABLE
q
. . . . . . . . . n Zn-K (Powc~rk, I x S -K ,powder) tMOVlNG o Ba-L ,jgowder)) TARGET :,af C u - K (¢ci/', ' : FIXED TARGET 'l. OI - EXPECTABLE ,~f /,~
i =
20
"\
/
~" I0"
,',,.-tf '
(~)f I /
I "4t .<
¢~
/
@"
/
7"
/
I . ~ O.5{u~, • ~ I
[
="
,,/ 4
o J
0
',B,
r
. . . . . 10 20 WA-x/WA-4
i
/ /
; /
0 [/-0
. . . . . . . . . 0.5 --1)0
WB-x~WB-1
or WB-2
Iig. 4. Net peak counts per unit current integrator reading versus tile corresponding mass of an element for moving-target and lqxcd-larget operarion modes results using a 16 McV proton beam. A. Net peak counts per unit current integrator reading for a given X-ra3 of any one element, (CAX), normalized to that for sample A4,(,('..\4) versus the corresponding quantitiy for the masses, (h'Ax/lt~A4): (x denotes 1, 2 or 3 for samples A1 to A3). B. Net peak counts per unit current integrator reading for a given X-ray of any one clement, (('Bx). normalized to thai for sample B1 in case of Zn, (CBI), and that for sample B2 in cases of S and Ba, {('B2). versus the corresponding quantity for the masses, (IVl~x/IVl~I ) or (ll'Bx/h'B2): (for samples B1 to P,4). In addition, the data for ('u tsamplcs I)1 and 1)2) are plotted accordingly,
foil is o f the same order o f magnitude as for the heterogeneous target samples presently used a further confirmation of the ability of the movingtarget to change a heterogeneous target sample into a nearly homogeneous one in effect. The degree of reproducibility was fotmd to be solely dependent on tile degree of the consistency of the beam intensity for the repeated runs as predicted by e q . ( 1 ) . If the number of spins of the target sample could be increased suflicicntly to average-out the fluctuating beam intensity to a constant value for the repeated runs, a better reproducibility should be obtainable. In the present experiment, two complete scans of the target samples were made in most of the experimental runs.
of the X-ray f r o m a certain peak X to that fronl the Pb-L~ peak in the same sample, (CJC,b). and the content in ,ug, ICx, o f the element x for which the X-ray peak is being considered are represented by four straight lines. These results were obtained both
!
5[
*K-Kd
' 41
/, ..
/
(32 5.2.1. Linearity in terms of comparison with an hlternal reference Using the same amounts o f Pb contents in each of the four standard target samples (i.e., sample A1 to A4 in table 1A), normalization of the other elements in any one of the target samples was made against the content of Pb in the same sample. The values thus obtained werc plotted as shown in fig. 5 for the 16 MeV proton beam irradiation. The relationships between the values of the ratio of tile net peak counts per/,1("
e
Zn-Kd o Bo-L~ • Sr-Ka o
/
,
~ /
,
J
i L
/
,
,,
"
i .4
,
-
id
00
/
................ '
. .2. . . .
;. . . . 6. .
8"
Wx [jag] I'ig. 5. The ratio of the net peak counts per pC of tile X-ray from a certain peak of an element x to that from the P1)-I,a peak in the same sample, ( C . x . / ( ' l , h ) v e r s u s nlass of the element x, (W,.) results using a 16 MeV proton beam,
L.S. (Ttuang ct al. /..I morin.e target
l\)r t]xed-target and moving-target modes of operalion, suggesting that the samples used, which were prepared by dissolving the sample in solution and then dropping it to dr',' on the target backing, have distributions t)f the elements in eqt, al r;.llit:,s, but not necessarily htmlogeneouslv, throtJghout the sample area. As shown above, in using an internal rel~'rence (e.g.. Pb in the present cz,se). only when the iIlaSs of the internal rel'erence element. I~'s, is known, the lllaSS oJ" an Ullknt~wn I.l:.,~ Call h e calculated from the relationship: (7'.,,.e s Os ,.11.v IK,. --- r~'~ .......
(2 }
where C denotes the nel peak counts per nnit current integrator reading corrected for the absorF, ticm in the target, tile windows and air-path: c. ~; and M are the efficiency of the detector for a given X.ray energy, X-ray production cross-section a n d a t o m i c n l a s s , respectively, and with the subscripts x and s denoting the unknowll :.llld the standard element respectively. It should be noted that this method is the commonly practiced means of quantitative analysis of elements in soluble samples in PIXE analysis. A sample which cannot be prepared by means of dissolving it in solution and dropping it to dry is difficult to analyze by the fixed-target mode. In that case the moving-target tnode operation is a good alternative. 5.2.2. l, im'aritv in terms of absoh, te comparison: c o n s t n w t k m o.f a calibration cun'e
Comparisons of the net peak counts per t, nit current integrator reading for any one type of X-ray with the corresponding elemental mass should result in linear proportionality: in fig. 4A are shown the results of comparisons using a 16 MeV proton beam for the target samples AI, 2. 3 and 4 (table IA); the results of comparisons, also using a 16 MeV proton beam, for the powder samples B1, 2, 3 and 4 (table I B) which were prepared by dropping the powder samples randomly onto the Nuclepore membranes and for the metal foil samples DI and 2 (table I D) are shown in fig. 4B. A least squares lit for the experimental results shown in tig. 4A and B, resulting in straight lines superposed, within the experimental error, on the straight lines that would be expected. In fig. 4A and 4B, the results of the same comparison for a few cases using fixed-target mode operation are plotted with the symbol ( )f marking the experimental points. In both cases of the fixed-target mode
213
operation, random scalterings of the experimental points from these expected points z,re well demonstrafed. Thus. il is inferred that the moving-target is capable o f performing an absolute determination of PIXI! for ally salllple forlll as long as the reqttirements for an absolute determination imposed by Nuclear Physics are satisfactorily ft,llillet]. 5.3. A hsohtle determhtations
Using the moving-targe! mode operation, X-ray production cross-sections of thin foils have been measured. For 16 MeV protons, for instance, Ct,-K X-ray production cross-section was 4.1 X I0 22 cm2 ?_ 20Vr, which agrees with the prediction froth the binary encounter approximation (BIiA} [5] within 185;. Fo, t\fils with known average thickness but with tmconfirtned utuformity, the use of the moving-target is expected to provide :, better rest, It. With the use of BI-A cross sections, the absolute contents of potassium and phosphorus in a Chinese medicinal sample, Furmariaceae. have been determined, using a 16 MeV proton beam, to be (2.7± 0.3)!; and (0.33 + 0.2)~:; respectively. The comparison of the present results with those of 14 MeV net,tron activation analy'sis (see table IC) [31 shows a good ~,greement between them. I:urthermore. very small fractions of other elements (iron and manganese) have been found by the present PIXE. It should be added before concluding, that in addi0on [o the merits presented heretofore the movinglarger should be very t, seful in medical biological applications of PIXE. If hundreds of thousands of living cells of the same category are aligned on the Nuclepore membrane for irradiation with an adequate particle beam intensity for keeping the cells alive, a quantitative result with good statislics could possibly be obtained.
This work is partially supported by the Nuclear anti Solid State Research Project, The University of Tst,kuba. The authors wish to express their thanks to Professor J. Sanada for his continuous interest and helpful suggestions. The kind arrangements and help of the accelerator group and the enthusiastic assistance during the machine time of Drs. S. Seki, T. lshihara, T. Aoki and K. Kobayashi and Mr. T. Kawano are gratefully acknowledged.
214
L.S. Chuang et al. //1 mot,ing target
References [l I K. Ishii, S. Morita, tl. Tawata, T.C. ('hu, I1. Kaji and 1. Shiokawa, Nucl. Instr. and Meth. 126 (1975) 75; N.I.. Mangalson, K.K. Nielson, M.W. ttill, I).J. t-atottght and L.I). llansen, Proc. Third Conf. on Application of small accelerators, Vol. 1 (1974) 163. [2] A product of Nuclepore Corp., Pleasanton, (!a USA.
[3t L.S. Chuang, K.W. Yue, P.K. ('han and W.Y. Chiu,('omparative Medical - East and West, Vol. VI, No. 1 (1978) 37. 14] I. Sugai, Inst. Nucl. Study, Univ. of Tokyo, Report INS-PL-134, (Dec. 1977) unpublished. IS] J.I). (;arcia, R.J. Fortner and T.M. Kavanagh, Rev. Mod. Phys. 45 (1973) I 11.