VNU Jo u rn al o f Scicncc, M allicm atics - Physics 24 (2008) 81-87
D eterm ination o f the 15 M eV brem sstrahlung spectrum from thin w target on the microtron M T-17 accelerator
Pham Due K liue\ Bui Van Loat“’*
‘in stitu te o f Physics an d Electronics. Vietnam Academ y o f Science and Technology, 18 H oang Q uoc Viet. Hanoi, Vietnam
^College o f Sciences, VNƯ, 334 Nguyen Trai, Thanh Xuan, Hanoi. Vietnam
Received 23 March 2008; rcccivcd in revised form 28 March 2008
A b stract. Brcnissưaliliuig energy spcctrmn from thin w target produced by 15 McV incident electrons was dctcmiincd by a combination o f measurements aiid llicoretical calculation. The shape o f spcctrum was calculatcd by Moiilc-Carlo mcdiod using úie code EGS4. The photon flux m easiưcmcnts were performed based on tlic activation technique using Ú1C higli pure metallic foils.
The radioactivities o f Ihc iưadiatcd foils were measured by using a gamma spccừomctcr wiUi a high energy resolution HPGe dctcctor. The cxpCTÌmails were carried out at tlie 15 McV electron Microưon MT-17 accelerator located at Institute o f Physics aiid Electronics, Hanoi.
1. I n tr o d u c tio n
E lectro n a c c elera to rs w ith m o d e ra te energy are b ein g u sed tlư o u g h o u t the w o rld for various scien tific an d tech n o lo g ic al fields [1-3], T h e rad iatio n s used at electron accelerators are n o t only the p rim a ry electro n b eam , b u t also th e se c o n d a ry b eam s such as breiTisstrahlung ph oton s an d neutrons, B rem sstrah lu n g p h o to n s are p ro d u c e d fro m direct in teractio n o f fast electrons w ith the nuclei o f the target. N eu tro n s are g e n e ra te d m a in ly fro m p h o to n u clear reactio ns in d u ced b y the b rem ssứ ah lu n g pho to ns. A h ig h in tensity g a m m a so u rc e is a goo d tool for in v estig atin g p h o to n u c lear reactions, rad iatio n affects m ech an ism s a n d p h o to activ atio n analysis [1 -3 '.
In o rd er to an aly ze m o st e x p erim en ts w h en b rem m strah lu n g rad iatio n used, it is necessary to k n o w th e a b so lu te m a g n itu d e o f th e b re m sstra h lu n g specfrim i as a fu n ctio n o f th e photoil energy and o f th e em issio n angle. M an y m e th o d s are av ailable for the in v estig atio n o f b rem ssfrah lu n g specừ a.
The th eo retical p re d ic tio n o f b re m s s tra h k m g sp ectra has been c a ư ie d out u sin g different m eth o d [4;.
A m o n g th em th e s im u la tio n o f electro m ag n etic cascades by m eans o f the M on te-C arlo m ethod lias b een slo w ly g a in in g accqDtance.
D esp ite the rela tiv e ly a d v a n c e d state o f the theoretical calculation, a lot o f accurate, absolute m easurem ents h a v e b een m a d e o f the sp ectru m o f b rem sstrah lu n g p h o to n s [5,6], T here are m any m ethods o f m e a su rin g the b re m s s tra h lu n g su ch as direct m eth o d u sing detectors or th rou gh the use o f com p to n m a g n etic sp e c tro m e te rs, an d in d irect m ethods such as the u se o f p h o to n e u ứ o n tim e o f flight or activ atio n o f sp ecia l m a terials. T h e ad v an tag es and lim itations o f each m e th o d h av e b een discussed elsew here.
• C o rre sp o n d in g a u th o r. E -m ail: lo a tb v @ v iiu .e d u . vn
81
82 P.D. Khue, B. V. Loat / VNU Journal o f Science, Mathematics - Physics 24 (2008) 81-87
T he p u rp o se o f the p re se n t w ork w as to investigate the energy sp ectru m o f b rem sstrah lu n g ph o to n s em itted from the th in
w
targ et b om b ard ed by 15 M eV electro n b e a m fro m the M icro tro n M T -17 accelerato r at the In stitute o f P hysics and Electronics.
In this study, the a ctiv atio n foil techniq ue an d gam m a sp ectru m m e asu re m en t w as used to d eterm in e the p h o to n flux. T h e m ain advantages o f this m eth o d are h ig h sen sitivity, acciư acy and th e ex p erim en tal p ro ced u re is ra th e r sim ple and feasible. B y this way, the p h o to n inten sity can b e d eterm in e d b ased on the activ ity o f the activ ated different foils. F ro m th e ab so lu te p h o to n fluxes w e have co n stru cted the b rem sstrah lim g energy sp ectru m b a s e d on the u n fo ld in g techniq ue m co m b in atio n w ith the sp ecfru m shape w hich w as calculated u sin g th e cod e E G S 4. T he E G S 4 sy stem (EleciTon S how er G am m a 4) is stan d ard for M onte-C arlo calculatio ns o f rad iatio n tran sp o rt [4 7],
2 . E x p e r im e n ta l
T h e M icro tron M T -17 accelerato r can accelerate electron b e a m up to en ergy o f 15 M eV and pro d u ce in tense brem ssfrahluiig an d pho toneu tro ns. The accelera ted electron b e a m hits the W -target to p ro d u c e the b rem ssữ ah lu n g . T h e d im ension o f the W -target is 4 0 m m in diam eter an d thickness o f 1 m m . T he in du ced b rem sstrah lu n g sp ecứ u m covers the energy ran g e fro m zero to 15 M eV .
D u rin g our ex perim ents, the M icro tro n M T -17 accelera to r w as o p erated w ith an elecfron en ergy o f 15 M eV an d 10 b e a m current. T h e iư a d ia tio n tim e w as 137 m in y ie ld in g enough the activities to b e m easured in a g am m a-ray counting sy ste m
In this study, w e u s e d A u an d In foils as the th re sh o ld detectors for the ph oto n flux m easu rem en ts. A ll foils em p lo y ed w ere disk -sh aped w ith diam eter o f 20 m m an d w ith thickness o f 0 .1 m m . F or irradiation , the foils w as p o sitio n ed 4 cm far from the
w
ta rg e t an d at 90 deg ree w ith respect to th e 15 M eV e lecừ o n b e a m d ừ ectio n. The s in p lif ie d exp erim en tal arran g em en t is sh ow n in F ig .I.T h e m ain characteristics o f th e n u clear reaction s in v estig ated an d decay d ata o f the reactio n products are p re se n te d in T able 1 [91.
Fig. 1. Experim ait arrangement for tlie investigation o f Bremssữalilung from ứie w target.
P.D. Khue, B. V. Loat / VNU Journal o f Science. Mathematics - Physics 24 (2008) 81-87
Table 1. N uclcar reactions used for brcinsslralilung spccừum m casurcm m ts
83
Nuclcarrcaclion Threshold energy, E,h(MeV)
Half-life, T ,/2
Main ganuiia - rays Isotopic
abundance % Energy (keV) Intensity, %
8.07 6.183 d 333.03 22.9 100
3 5 5 .6 8 86.9
1091.4 0.15
9.23 49.51 d 190.27 15.4 95.7
588.43 4.39
725.24 4.39
In p ractice, th e m etal foils are activ ated by pho ton s an d rad io iso to p es fo rm ed after the iư ad iatio n s w ere id en tified fro m the p u lse-h eig h t sp ectru m by their gam in a p h o to p e ak energies an d half-lives. T h eir activities w ere d eterm in ed fro m g am m a p h o to p e ak area and d etectio n efficien cies at the p h o to p eak energy. T he av erag e activity o f the activ atio n foils serv ed as p h o to n flux to w h ich the foils w ere exposed. T he re la tio n betw een the average p h o to n flux, (|), and the nu m b er o f d etecte d gam m a rays,
c,
can b e ex p ressed as follow s:r ~ --- z--- ---
ơ A ^ „ £ / ^ F [ l - e x p ( - Ẩ í , . ) ] e x p ( - / l ? J [ l - e x p ( - Ẩ / J ] ^ w here: No is the n u m b e r o f ta rg e t nuclei; s is the p h o to p eak efficiency o f th e detector; is the branching ratio or in ten sity o f the gam m a ray ; \ is the decay constant; F is c o ư e c tio n factor; t, is the irradiation tim e; td is the decay tim e or the tim e betw een end o f irradiatio n an d start o f co un tin g; tc is the m easuring time.
1000
0.1
0.01
...
-1----1__I_I—1. i J --- 1---1__ 1 I I
100
Energy(keV)
1000
100000
10000 o
"1 I 1000
ẫ
r >it= o CD0) 100 j a:
10
Fig. 2. Photopeak efficiency curves o f the gamnia spectrometer wiứi HPGe detector — relative efficiency curve, — absolute efficicncy curve.
In activ atio n m ethod, the actual results o f the m easu rem en ts are the co u n tin g rates o f the irrad iated foils. A fter iư ad iatio n s an d ap p ro priate co oling tim e, th e foils w ere ta k en o ff a n d the in d u c ed gam m a activities w ere m e asu re d b y gam m a spectrom eter. It con sists o f a h ig h p u rity co ax ial germ an iu m H P G e detecto r (C A N B E R R A ), w hich is co u p led to a co m p u ter b a s e d m u ltich an n el an alyzer s y s te m T h e en ergy re so lu tio n o f the sy stem is 1.8 keV at 1.332 o f “ C o stan d ard soiư ce. T he gam rna sp ectra w ere m e asu re d an d an aly zed b y the p ro g ram
s
100 (C anberra).T he p h o to p e a k efficien cy cu rv e o f the gam m a sp ectro m ete r w as calib rated w ith a set o f stan d ard som-ces such as ^ ' A m , ' ” Cs, “ Co, '^^Eu, ' ” B a and T he m ain steps o f the p ro ced u re are ( 1) to d eterm in e the rela tiv e efficien cy curve b a se d on m u lti-en erg y g am m a sources a n d th en (2 ) to ti-ansform th e m e asu re d rela tiv e efficien cy curve to ab so lu te one b a s e d on single energy g am m a sources. T he d etectio n efficiencies w ere fitted b y using the fo llo w in g function:
84 P.D. Khue. B. V. Loat / VNU Journal o f Science, Mathematics - Physics 24 (2008) 81-87
(
2)
w here e is th e d etectio n efficiency, a„ represents the fittin g p aram eters, a n d E is th e energy o f the p h o to peak. T he rela tiv e an d ab so lu te efficiency cu r\’es w ere p resen te d in F ig.2. [5],
3. R e s u lts a n d d isc u ssio n
In this in vestigatio n, the th resh o ld plio ton uclear reactio n s ’” Au(Y,n) A u an d In(Y,n) "In w ere u s e d fo r the p h o to n flu x m easu rem ents. T h e induced g am m a activities w ere m e asu re d b y gam m a spectt-om eter w ith H P G e detector. E ach sam ple w as m e asu re d several tim es in order to follow the decay o f the d ifferen t isotopes. S om e typical gam m a sp e c tra o f the a ctiv ated foils u n d er inv estigation are sh o w n in Fig.3 an d F ig.4, respectively. A fter m ak in g n ece ssary co rrection s for th e u su al exp erim ental errors such as dead tim e, pile-u p, g am m a ray b ran ch in g ratio , self-ab so rp tio n o f gam m a rays an d d etecto r efficiency, th e p h o to n fluxes can b e d eriv ed fro m the m e a siư e d activities b a s e d on eq u atio n (1). T he activ atio n cross sections u sed in our calcu lation s w ere tak en fro m referen ce [9 .
F ro m th e p h o to n fluxes determ in ed b ased on d ifferent tliresh o ld re a c tio n energies, w e calcu la ted th e p h o to n fluxes p e r kW b e a m p o w e r, (Ị) (ph.s-'. s r - '. k w ') , and p re se n te d in T able 2.
Table 2 Integral photon fluxes determined based on different ứircslìold reaction energies.
Nuclear reaction E,„ (MeV) () (ph.s-'.sr-’.k W ') 8.07
9.23
(1.06±0.09)xl0”
(7.44±0.67)xl0'°
Follow ing, the d iffere n tial p h o to n flux in the energy b in AE - E,h(In) - Eih(Au) can b e deriv ed fro m the values o f integ ral p h o to n flux as follows:
A ộ= <ị)(Au) - (|)(In) (3)
F ro m the d ifferen tial p h o to n flu x w e can co n stru cted an ab so lu te b rem sstrah lu n g energy s p e c ứ u m b y a c o ^ i n a t i o n w ith the rela tiv e s p e c ứ u m calcu lated b y u sin g the code EG S4. T he ob ta in ed b rem ssfrah lu n g s p e c ừ u m is p resen te d in Fig.5.
P.D. Khiie, B. V. Loai / VNƯ Journal o f Science, hẩaỉhemalics - Physics 24 (2008) 8Ỉ-87 85
Fig. 5 sh ow that the energ y sp ectru m o f b rem ssừ ah lu n g is co ntinuous the u p p er and th at equals the kinetic energy o f the b o m b a rd in g electron. The slo w in g dow n o f electrons due to io n izatio n losses leads to redu ction o f the high energy p a rt in relatio n to low -energy radiation . T he sh a p e o f the obtained b rem sstrah lu n g sp ectru m is sim ilar to that rqDorted by som e other authors [8,9].
Gainina rny enei gy (ke\")
Fig. 3, Gaimna-ray spcclrum o f Gold foil iưadialed by 15 MeV Brcinssưalilung wiUi ừradiatiou tmie 137 miiij tiic w a itin g lim e 8817 m m , ai)d tJic m easu rin g tim e 30 m in.
Gnimtiii ra y en e rg y (keVỊ
F ig 4. G a m m a-ray sp c c tru m o f Iiidiiun foil iư a d ia te d w idi 15 M eV B rcm sstralilu n g wiOl ừ ra d ia tio n tim e 137 m in , ứ ie w a itin g tim e 3080 m ill, aiid Uie m e a su rm g tim e 30 m ill.
86 P.D. Khue, B. V. Loat / VNU Journal o f Science, Maihematics - Physics 24 (2008) 81-87
P h o to n e n e rg y (M eV )
Fig. 5, Brcmsstrahlung spcctrum from W-target bombarded by 15 MeV electrons from MT-17 accelerator.
T he m a in sources o f th e un certainties for the p re se n t resu lts w ere estim ated due to statistical errors; (0 .5-^ 1% ), the geo m efrical factor for iư a d ia tio n and m e asu re m en t o f the activ atio n foils;
(9.8-^1.5% ), the d etection efficiency: (2-^-3%), n u clear decay ck ta used su ch as h alf-life an d gam m a b ran ch in g ratio: (2-^4%).
In this study, in order to lim it the experim ental e ư o rs, the (y,n) p h o to n u c le a r reactions fo r A u and In w ere u se d as activation detectors, b eca u se o f th eir hig h re a c tio n c ro ss-sec tio n in the energy ran ge o f interest. F u rtherm ore, the interferences caused b y co m p eting reactio n s w ere avoided.
In conclusion, w e can say th at th e obtain ed energy sp ectru m o f b re m sstra lu n g p h o to n s are useful n o t only for n u c le a r d ata m easurem ents, b u t also help in u n d erstan d in g th e n u clear in teraction processes in v o lv ed in th e p ro d u ctio n o f brem sstrahlung. F or p ra c tic a l ap plication s, the o b tain ed data
are useful in m ak in g d etailed sh ield in g calculations and p h oto activ atio n analysis.
A c k n o w le d g e m e n ts. T h e autho rs are gratefu l to Prof. N g u y en V a n D o fo r his continuous in terest in this w ork. W e also w o uld lik e to th a n k the colleagues in the C en ter o f N u c le a r P hysics, Institute o f Physics an d E lectronics for their help diưiĩig the experim ent. T his w o rk is fin an cially su p p o rted by Q G -07-06 project.
R e fe re n c e s
[1] Y. M.Tsipennyuk, The Microiron development and application^ Taylor & Francis, 2002.
[2] V.L. Ausleuder et al., Bremsstrahlung converters for powerful industrial electron accelerators, Radial Fhys. Chem. 7Ỉ (2004) 295.
[3] P.Liihorte et al., Applied radiation research around a 15 McV higli average power linac. Racliat. Phys. Chem. 55 (1999)761.
P,D. Khue, B. V. Locit / VNƯ Journal o f Science, Mathematics - Physics 24 (2008) 8 Ỉ-8 7 87
[4] K-Van Ijxcro, w . Mondelacrs, Full Montercarlo simulation imd optimization o f a higli power brenx^stralilung converter, Radiat. Phvs. Chem. 49 (1997) 207.
[5] Nguyen Van Do, Pham Due KJiue, Angular characterization o f 15 McV and 65 MeV brciTisstrahlung photons from w*
target, Communications in Physics Vol. 15 N o.l (2005) 1.
[6] D. J.s. Findlav, Analytic representation of bremsstrahung spectra from thick radiators as function o f photon energy and angle, MW. ĩnstr. a n d M e tk A276 (1989) 589.
[7] Richard B. Fieslone, Table o f Isotopes, Wiley-Iiitcrscience, 1996.
[8] A. Calziido, E. Vano, V. Degado, L. Gonzalez, 42 MeV bremsstralilung spectrum iuialysis by photoactivivation method, N ucl Instr.cmd. Meth. 225 (1984) 232.
[9] http://depni.sinp.msu.ru/cgi-bin/exfV3.cgi.
