# Exorbitant number of counts per second in the Rutherford experiment

May someone kindly advise on the Rutherford experiment? I have created a Gaussian of $$\mathrm dP_0/\Delta\theta(\theta)$$, where $$P_0$$ is the number of counts of alpha particles through a $$1.05\ \mathrm{mm}\times19.86\ \mathrm{mm}$$ slit divided by the time elapsed and $$\Delta\theta(\theta)$$ is the angular width of the detector. At this stage the Au foil was not used, as this was merely the "background" measurement. I have integrated that Gaussian wrt $$\theta$$ between $$-\infty$$ and $$+\infty$$ to obtain the total $$P_0$$. However, this yielded a total $$P_0$$ equal to $$24\,000\ \mathrm{s^{-1}}$$, which seems exorbitant and wrong. I'd be indebted for any help trying to figure out where the mistake is!

• There isn't really much to go on here. How did you get your result? What is the known strength of the beam? Large numbers are everywhere in physics - what seems so wrong about this one?
– user10851
Jan 9, 2014 at 7:18
• Well, first of all, when used further in the experiment it seems to yield a wrong value for the expected $d\sigma$/d$\Omega$. Moreover, it is unlikely that without the Au leaf and at very small angles the number of counts/second would be that high! Let's examine for instance the readings I obtained for the angle $\theta$=0: number of counts - 1072; time elapsed - 14.1 sec; P0=1072/14.1=76.028 [1/sec]; Δθ = width of slit/distance between source and detector = 1.05mm/44mm = 0.02386; P0/Δθ = 3186.436 [1/sec]. There seems to be something wrong here. Possibly with the evaluation of $\Delta$$\theta$ Jan 9, 2014 at 7:22
• Couple of questions. First, do you know how strong the source is in absolute activity? Remember that $1 \,\mathrm{\mu Cu}$ is still $4 \times 10^4\,\mathrm{Bq}$ and that if you want to measure the rate are high scattering angles you are going to need a considerable rate or a long counting time. Second, how much rate does your data acquisition system register in the absence of the source? That is, are you sure you have your thresholds high enough? Jan 9, 2014 at 15:59
• And as I commented on your previous question about the experiment you are not giving enough detail. Experimental question often turn on the little things. Don't draw a fuzzy word picture, give us a diagram. Jan 9, 2014 at 16:02
• I'm not interested in the energy of the beam, I am interested in the intensity of the beam. You are saying that 24 kCounts/sec is too high, but that is not at all clear to me. So I was looking for a way to estimate the beam intensity from the other direction. If you know the raw activity of the source you can put an upper-limit on the proper measurement for the unscattered beam intensity. Likewise, you can test the DAQ, to see if it registers only the cosmic background in the absence of the beam. Jan 9, 2014 at 16:26