2003 GM D eterm in ation of D ead T im e f or a GM Counter,,, 56-1 GM decaying source oscilloscope m ethod. Decaying source 5 6 Mn,. - 3200. Decayin g source paraly zable hybrid fittin g., oscilloscope m ethod discrimination lev el. A b s tra ct T he counting characterisitics of GM counter dependent on count rat e condition w as inv estigated and the dead tim e w as m easured by u sing the decayin g source m ethod an d oscilloscope m ethod. 56 Mn w as chosen as a decayin g source and prepared by irradiatin g m an gan ese by a n eutron beam. T he count rat e w as m easured at fix ed source- t o- detector distan ces for 3200 m initu es. T he dead tim e w as determ ined by fitting th e m easured count rat e w ith th e fittin g form ulas deriv ed from the paraly zable m odel an d th e hybrid m odel. Also, the dead tim e w as determ ined by oscilloscope m ethod w ith picture- reading according t o the ch angin g discrim in ation lev el. 1. GM,,,, surv ey m eter. GM [1].
GM [2,3]. 2..,. GM. thereshold v oltag e, [1].. liv e tim e clock m eth od, pulser m ethod, oscilloscope m eth od [4].,. one param eter non - paraly zable paraly zable, t w o param et er hybrid [2,3]. Non - paraly zable n n [1,5]. Non - paraly zable ob serv ed count rat e tru e count rat e. m = n 1 + n n (1) m ob serv ed count rat e, n tru e count rat e, n non - paraly zable. P araly zable p, p [1,5]. P araly zable ob serv ed count rate true count rat e. m = n e - n p (2) p paraly zable.
Hybrid GM n on - paraly zable, discrim in ation lev el paraly zable [3]. Hybrid ob serv ed count rat e true count rat e. m = n ex p ( - n p ) 1 + n n (3) n n on - paraly zable, p paraly zable. Hybrid (hybrid review ) paraly zable dead period [6,7]. m = n ex p ( - n p ) 1 + n [ n - p ex p ( - n p ) ] (4) 1 initial true count rate 3,000,000 count s/ min 56 Mn paraly zable, n on - paraly zab el, hybrid m odel. hybrid paraly zable non - paraly zable. 3.,. [5-7]. decayin g source m ethod, 2 end w in dow GM tube, scaler, digit al tim er, M CS (m ulti- ch ann el scaler ). GM tube M CS im pedance m at ching preamp. M CS discrimination lev el - 50 m V, 3200 min, 1 m in. Decaying source m ethod.. [2,3]. true count rate
. n ( t) = n 0 e - t + n B G (5) n0 initial true count rat e,, t, nb G. Decayin g source 5 6 Mn. 55 Mn 55 Mn 13.3 barn, 2.578 h ours. ST 1. 1.5 m m, 41.8 m g T eflon sheet 9 10 9 n/ cm 2 s 1. 3 (5). 3 initial tru e count rat e lea st squ are fittin g, initial count rate 3,061,700 9,400 count s/ m in, 19.2 0.4 count s/ m in. fittin g, 4 6. F itting true count rate 4.8 10 4 count s/ m in ob serv ed count rate. P araly zable 580 1 sec 3 10 5 count s/ m in 10%. Hybrid 4.4 10 5 count s/ m in paraly zable 359 1 sec, n onparaly zable 355 2 sec 10%. Hybrid rev iew 6.6 10 5 count s/ m in paraly zable 311 1 sec, n onparaly zable 600 1 sec 10%. 4. O s cill o s c ope m et h o d oscilloscope m eth od oscilloscope [8]... 7 oscilloscope m ethod. non - paraly zable, discrimination lev el paraly zable. Non - paraly zable 500 cps 180 10 sec, 30,000 cps 80 10 sec true count rat e, paraly zable discrim in ation lev el - 30 mv 100 20 sec - 50 m V 500 cps 190 10 sec, 30,000 cps 440 10 sec, - 70 m V 500 cps 330 10 sec, 30,000 cps 900 10 sec.
8 GM true count rate. 5. GM decayin g source m eth od oscilloscope m eth od. ob serv ed count rate, fitting oscilloscope. GM paraly zable hybrid. GM, oscilloscope m eth od.. 1. G.F. Kn oll, Radiation Detection and M easurem ent, pp. 119 128, John W ily & S on s, N ew York, 2000. 2. R.P. Gardn er and L. Liu, Appl. Radiat. Isot. 48 (1997) 1605. 3. R.P. Gardn er and S.H. Lee, A ppl. Radiat. Isot. 53 (2000) 731. 4. A H an dbook of Radioactivity M ea surem ent s Procedures, Nation al Coun cil on Radiation Protection and M easurem ent s, NCRP Report No. 58, 2n d ed., 60, Beth esda, MD, U SA, 1985. 5. R.D. Ev an s, T h e Atom ic Nucleu s, pp.785 818, M cgraw - Hill, N ew York, 1955. 6. J.W. Müller, Nu cl. In str. an d M eth. 112 (1973) 47. 7. J.W. Müller, Nu cl. In str. an d M eth. 117 (1974) 401. 8. H.G. Stev er, P hy s. Rev. 61 (1941) 38.
F ig. 1. T he sim ulation result s of dead tim e m odels at ch angin g dead tim e. F ig. 2. Block diagram of decaying source m ethod by u sin g m ultich ann el scaler. M CS dw ell tim e is 1 m in, discrim in ation lev el - 50 m V. F ig. 3. T he raw dat a of ob serv ed and deriv ed true count rate by decaying source m ethod.
F ig. 4. T he fittin g result and calculat ed count rate det erm in ed by paraly zable m odel. F ig. 5. T he fittin g result and calculat ed count rate det erm in ed by hybrid m odel. F ig. 6. T he fittin g result and calculat ed count rate det erm in ed by hybrid (review ) m odel. F ig. 7. T he m easured dead tim e by oscilloscope m ethod.
1) T rue count rat e : 500 cps 2) T ru e count rat e : 900 cps 3) T rue count rat e : 2700 cps 4) T ru e count rat e : 4500 cp s 5) T rue count rat e : 7800 cps 6) T ru e count rat e : 13300 cp s 7) T rue count rat e : 30000 cps 8) T ru e count rat e : 51100 cp s F ig. 8. T he oscilloscope pattern of th e GM det ect or. Ex posure tim e is 0.25 sec an d trig ger lev el is - 20 m V.