James Smallcombe: Okay, there we go. And I will hand over and say, please ask Syria. Tell us about your work.
Ann-Cecilie Larsen: Well, thank you so much for
Ann-Cecilie Larsen: For the invitation. And yeah, I think this is a great, great initiative really that you organize these kind of meetings and this difficult time.
Ann-Cecilie Larsen: So I will give you a brief introduction, I should say.
Ann-Cecilie Larsen: On how we can constrain neutron capture rates for astrophysical processes where
Ann-Cecilie Larsen: The nuclei, we want to study are difficult to reach experimentally and please go ahead and ask
Questions.
Ann-Cecilie Larsen: During the talk. If you have any
Ann-Cecilie Larsen: Else. Of course it's perfectly fine. Also to ask at the end.
Ann-Cecilie Larsen: So I'd like to start with the thanking all the collaborators.
Ann-Cecilie Larsen: Without them, this would not have been possible last for sure. So a big thanks to to all coworkers.
Ann-Cecilie Larsen: Um, alright, so we happen to live in a universe where there's a whole bunch of different chemical elements.
Ann-Cecilie Larsen: And a big question for for humanity has been where do all these elements come from.
Ann-Cecilie Larsen: And we know today that the lightest elements like hydrogen and helium and maybe some lithium actually originated from the big bang. So from the start of the universe.
Ann-Cecilie Larsen: That everything else somehow must have been made in stars, one way or another.
Ann-Cecilie Larsen: To narrow down a bit further.
Ann-Cecilie Larsen: This question. How were the elements from Ireland to uranium made has been identified as one of the 11 science questions for the new century.
Ann-Cecilie Larsen: And so well. Don't we know anything about this. Well, yes, we do.
Ann-Cecilie Larsen: We know that it's a good idea to look at neutron captures because neutrons. They don't have any charge. And so they don't have any column barrier that they need to overcome.
Ann-Cecilie Larsen: And so it has been identified BACK IN THE 50s, somehow the references fell out here, but that's from Burbage at all and also by Cameron.
Ann-Cecilie Larsen: It was identified that the two
Ann-Cecilie Larsen: Main neutron capture processes responsible for basically making most of the heavy elements that we see today.
Ann-Cecilie Larsen: So one of these processes is the slow nitro capture this accounts for roughly half of the heavy elements. And there's also another process called the rapid neutron capture process which more or less accounts for the other half.
Ann-Cecilie Larsen: And if you look at the figure down here to the left corner there is assume on the heavy element of bounces. So it's the mass on the x axis and abundances relative to silicon on the Y axis.
Ann-Cecilie Larsen: And here you can see that the S process contribution. This is this jagged line.
Ann-Cecilie Larsen: Accounts for quite a bit of the heavy elements and the our process more or less accounts for the rest. And there's also these open squares here which represent nucleotides that are produced in the people says
Ann-Cecilie Larsen: If we now turn to the figure on the right.
Ann-Cecilie Larsen: This is a familiar plot for many of you, I guess the nuclear chart where the dark blue squares represent stable nuclei that we can find on Earth or in our solar system at least today.
Ann-Cecilie Larsen: And the process more or less stays within this valley of stability building up nuclei from, let's say, from Iran and up all the way to basement.
Ann-Cecilie Larsen: While the our process on the other hand,
Ann-Cecilie Larsen: Involves very, very nutrient rich nuclei. So this is out here in the gray area where these nuclei are not even measured in the lab.
Ann-Cecilie Larsen: Well, for the P process, it sort of goes the other way around.
Ann-Cecilie Larsen: We now believe that these nuclei. So this open squares here are made by peeling off neutrons from existing heavy, heavy nuclei that have been made already in the US or the our process, with a few exceptions, but roughly. That's how it goes.
Ann-Cecilie Larsen: Right. So that's all well and good.
Ann-Cecilie Larsen: But do we know where this happens. Well, when it comes to the process, we have a pretty good grip on where this takes place.
Ann-Cecilie Larsen: Because technetium has been observed directly in a red giant star that was Mariel WHO DID THAT THE FIRST TIME BACK IN THE 50s.
Ann-Cecilie Larsen: And since technetium is an unstable elements. So there is no isotope that stable. It means that had to be produced in that star directly. And so that was in fact the first proof that heavy element nuclear synthesis takes place in stores.
Ann-Cecilie Larsen: For the process we have ideas. Definitely. This could take place in supernovae.
Ann-Cecilie Larsen: But when it came to the when it comes to the art process astrophysical sites. This has been
Ann-Cecilie Larsen: heavily debated.
Ann-Cecilie Larsen: Over many years and it's still maybe not completely settled.
Ann-Cecilie Larsen: However,
Ann-Cecilie Larsen: Back in 2017
Ann-Cecilie Larsen: We saw something something fantastic for the first time ever.
Ann-Cecilie Larsen: A neutron star coalition was observed live and that was due to the advances of the Lego and Virgo gravitational wave
Ann-Cecilie Larsen: Detectors that could really pinpoint the place and also time when this collision happened.
Ann-Cecilie Larsen: And then they basically told
Ann-Cecilie Larsen: All people who were involved in and measuring
Ann-Cecilie Larsen: Other types of electromagnetic signatures to pinpoint their to point their telescopes in the direction of this neutron star merger.
Ann-Cecilie Larsen: And what they measured so that indeed this afterglow or this light that they measured following this neutral some merger is fully consistent with the art process taking place in this environment.
Ann-Cecilie Larsen: Alright, so what does that mean
Ann-Cecilie Larsen: Well, from the nuclear physics side, it means it's a huge challenge because this environment means that we have to take many, many nuclei into account when we want to calculate what they produce, which I'll show a little bit more in detail later.
Ann-Cecilie Larsen: But what to keep in mind for us right now is the plot to the right here. And this shows on the on the X axis, this is a logarithmic of the nutrient density in the astrophysical environment where they are process take place and is the temperature in Giga Kelvin. On the y axis.
Ann-Cecilie Larsen: And the question is whether in the stellar environments or this extreme environment with a huge, huge neutron flux. The question is whether
Ann-Cecilie Larsen: This astrophysical site will achieve a so called and gamma, gamma and equilibrium or not.
Ann-Cecilie Larsen: So if there is an n gamma, gamma and like really room. It means that these two reaction rates this auto cancel out.
Ann-Cecilie Larsen: And the only thing that really matters for for the calculation of the yields of what you produce in such an event is basically the masters and the beta decay rates, this sort of governs almost everything.
Ann-Cecilie Larsen: But if there is no such equilibrium reached it means that these rates have to be taken into account explicitly in some in the simulation.
Ann-Cecilie Larsen: And what turns out, is that okay sorry for this. Don't look at the top and the bottom panels, let's focus on the middle panels here. This is a simulation of a neutral surgery event taking
Ann-Cecilie Larsen: Basically bits of the mass that's injected in this event.
Ann-Cecilie Larsen: Calculating the temperature for this kind of box or mass that trajectory and then see how the temperature evolves in this little box as a functional time so it's time here on the x axis temperature on the Y axis.
Ann-Cecilie Larsen: And what you can see here is that some of these trajectories. They are very cold. And so they never reach this and gamma, gamma and the equilibrium
Ann-Cecilie Larsen: And some of them are quite hot so they will probably reach this equilibrium
Ann-Cecilie Larsen: So what does that mean
Ann-Cecilie Larsen: Well, it means for these trajectories that are cold so less than something like one giga Kelvin and also after free south for the hotter trajectories. We need to take into account which on capital rates.
Ann-Cecilie Larsen: Yeah, and I just give a couple of references there that you could look up
Ann-Cecilie Larsen: So that means it's much more work for us, nuclear physicists to provide the reaction rates.
Ann-Cecilie Larsen: Because if you want to do this properly.
Ann-Cecilie Larsen: You need something like 5000 nuclei in your reaction network and you need to take into account, something like 50,000 reaction rates.
Ann-Cecilie Larsen: And this is just a figure I stole from Stefan go really does he showed in the Nordic meeting on nuclear physics, five years ago.
Ann-Cecilie Larsen: So if, if the condition or the astrophysical site was, for example, like a supernova, then one could think that okay you stay. Maybe you're still far out here in the unknown region or the nuclear chart.
Ann-Cecilie Larsen: But you climb up this can be different paths in this kind of chart, depending on the nuclear properties but it's mostly driven by by the masses and the beta decay rates.
Ann-Cecilie Larsen: But for neutral stars. It's a bit different, you sort of start out with an enormous amount of nutrients and you have to somehow end up
Ann-Cecilie Larsen: To the nuclear value of stability in the end, but the way, there is a bit more complicated.
Ann-Cecilie Larsen: So here is a picture showing some snapshots of a reaction network calculation. This is from manpower as well.
Ann-Cecilie Larsen: Taken at different times of, of the, of the merger event.
Ann-Cecilie Larsen: So after hundred milliseconds. There is not much to be seen, because everything is basically neutrals. And then when there is neutral exhausted. So all of the nutrients available has been spent. This is the picture we sit with
Ann-Cecilie Larsen: And then finally, all these super nutrient rich nuclei, they have to decay back to stability.
Ann-Cecilie Larsen: So you see
Ann-Cecilie Larsen: You see the effects of course of the magic numbers have these intensities high intensities around the magic numbers, but you also see that there's a whole bunch of nuclear in between.
Ann-Cecilie Larsen: And somehow we need to take all these guys into account in a proper way.
Ann-Cecilie Larsen: And since there are so neutral rich, it means we don't have any data, maybe we're lucky and have masses for a couple of them.
Ann-Cecilie Larsen: But for all the rest of the properties that we need to take into account, we have to rely on theoretical estimates and so it would be good if these estimates are predicted and robust that we can really trust them.
Ann-Cecilie Larsen: Just briefly, I would also like to mention another process that has sort of popped up.
Ann-Cecilie Larsen: Which is called the intermediate neutron capture process. And if we start out with a figure here to the right, the top panel shows again and network calculation.
Ann-Cecilie Larsen: For us process conditions. So these open boxes. Now they are stable nuclei and the color code shows how much you have of the different replied at a given time.
Ann-Cecilie Larsen: And so again, you can see that this process basically sticks to the value of stability.
Ann-Cecilie Larsen: This intermediate neutron capture process is as the name implies, it's something in between the S process and and the our process.
Ann-Cecilie Larsen: And this is shown here in the in the blocked.
Ann-Cecilie Larsen: Down here to the right and this is taken from Melanie hamels publication.
Ann-Cecilie Larsen: And you can see that somehow the path is shifted towards neutron which nuclei, but not as nutrient rich as for the our process.
Ann-Cecilie Larsen: And then you might ask, okay, why do people talk about this process at all. Well, there are observations astrophysical observations. Whoops, sorry about that.
Ann-Cecilie Larsen: So there are some carbon enhanced metal poor stars in the galactic halo and they are a bit peculiar because they are enriched both with barium, which is a typical s process element and europium, which is the typical our process element.
Ann-Cecilie Larsen: So this is a bit funny how can they both have both barium and Europa in them because the stars are so old that we think they should only have enrichment from one
Ann-Cecilie Larsen: One heavy element production event. So either the process or the process not both at the same time.
Ann-Cecilie Larsen: There's also another star, the post a TV star called suckers object which has a very funny element distribution that doesn't match up with a combination of the are in this process.
Ann-Cecilie Larsen: And so one went back to the suggestion by CO and the roles back in the 70s that maybe they're actually exists an intermediate neutron capture process as well.
Ann-Cecilie Larsen: Which could at least partly explain the features that has been seen in these kind of old stars.
Ann-Cecilie Larsen: And then maybe this is good news for us nuclear physicists, at least, because, okay, these guys are neutral rich, but not as neutral rich as for the our process. So there'll be easily accessible in the lab.
Ann-Cecilie Larsen: Oh, sorry.
Ann-Cecilie Larsen: Coming back to the theoretical estimates for neutron capture rates because this is really what people have to do when we need 50,000 rates.
Ann-Cecilie Larsen: Will actually never measure them all of us, for sure. So what do we do to calculate these guys well we rely on the good old house of Facebook theory.
Ann-Cecilie Larsen: And this is just a rate equation, you don't have to worry about understanding the whole thing. What is important is this thing here, which is the cross section.
Ann-Cecilie Larsen: And this guy needs to be calculated for all the reactions that we need to include in the network.
Ann-Cecilie Larsen: And to do that we use typically theoretical models for the nuclear level density for the gamma decay strength and also for the optical model potential
Ann-Cecilie Larsen: And this is really where
Ann-Cecilie Larsen: Our research comes into play because we would like to try to constrain this cross section from indirect experimental data when it's not possible to measure the cross section directly
Ann-Cecilie Larsen: And so what are these quantities. Well, the level density is simply the number of quantum levels per energy unit as a functional excitation energy and then principles of spin and parity.
Ann-Cecilie Larsen: While the gamma string function or gamma the case strength. This is a measure of the average nuclear electromagnetic response.
Ann-Cecilie Larsen: And per definition, it is the distribution of average reduced partial with as a functional gamma ray energy and this is defined by Bartholomew back in the 1970s.
Ann-Cecilie Larsen: So if you take a look at the figure here to the right, it means that you have. You start out with a set of initial levels.
Ann-Cecilie Larsen: Us over class of levels like a two plus levels, for example, and they all decay down to a set of levels.
Ann-Cecilie Larsen: And the distance here between the levels is very small. And so you can take all these branching ratios. If you want and make an average partial with out of this and this will then represent the average decay strength for this gamma ray and the G to decay down here.
Ann-Cecilie Larsen: And then you might think, okay, don't we know how to calculate these things because this is something that people have been doing for many years now.
Ann-Cecilie Larsen: Well, I'll leave it to you to judge
Ann-Cecilie Larsen: So on the top here, these are level densities on the Y axis as a functional excitation and the G on the x axis here, the black points are data.
Ann-Cecilie Larsen: These other markers and lines. They are calculations.
Ann-Cecilie Larsen: And you can see. So this is our in 56 it's stable.
Ann-Cecilie Larsen: Domain Name 74 it's stable. Note the range. So the y axis here is a logarithmic. So there's quite a discrepancy for quite many of these models.
Ann-Cecilie Larsen: And if you take a look at the lower panels here. This is now the gamma strength on the Y axis and it's gamma ray energy on the x axis again black points data points and then all these lines and other markers, they are different calculations.
Ann-Cecilie Larsen: And again,
Ann-Cecilie Larsen: Even for stable nuclei, like the one to the most left and to the right.
Ann-Cecilie Larsen: Who
Ann-Cecilie Larsen: Is
Ann-Cecilie Larsen: Okay, yeah, I don't know, maybe you're happy with it. I'm not happy with it. I think this is a It's sad. It should have been much better. But that's just how it is. And in the middle panel. This is a nickel 70. So this is a nutrient rich nickel that I'll come back to later.
Ann-Cecilie Larsen: Alright so okay let's take what we have, we have these models.
Ann-Cecilie Larsen: they deviate. As you see,
Ann-Cecilie Larsen: What does that do for the reaction rates. So here I just calculated the reaction rate. This is on the Y axis versus temperature of the stellar environment.
Ann-Cecilie Larsen: And this is for some are in 154 and gamma. So this is the most nutrient rich stable samarium isotopes, but this is now connected with absolutely no constraints on
Ann-Cecilie Larsen: Knowledge from from other experiments. So this is just what the models do if you let them loose, let's say, at each line here is now a specific combination of a level density model and demonstrating functional
Ann-Cecilie Larsen: So even in this case, these uncertainties that we have in the models they translate into almost a factor of talent uncertainty in their reaction, right.
Ann-Cecilie Larsen: And then what happens if you calculate something like scenario 175 so very neutral. Mr. Nicholas know to change to scale here.
Ann-Cecilie Larsen: So then we're talking maybe two orders of magnitude.
Ann-Cecilie Larsen: And there are other cases where there might be three orders of magnitude and certainty for the reaction rates.
Ann-Cecilie Larsen: So, the situation is, I would say not good.
Ann-Cecilie Larsen: So our take on this is to try to constrain these rates by using information on the level density and the demonstration function.
Ann-Cecilie Larsen: There are other approaches as well to constrain these rates, such as the surrogate method for direct capture and give a couple of references.
Ann-Cecilie Larsen: And also the surrogate method for like compound capture and my guess is that you're sure we'll talk about this more in detail next Monday. So I'll come in or say more about this right now.
Ann-Cecilie Larsen: But our approach has been to use what has now for some reason been established, and the Oslo method with variants, like the inverse kinematics or slow method.
Ann-Cecilie Larsen: And the better or slow method and they all attempt to measure the level density and the GM of the case strength so that we can use those quantities to calculate the cross section with better precision.
Ann-Cecilie Larsen: And in this little cartoon. It's just illustrated again why this is important. So there's a neutral, which is captured by some nucleus.
Ann-Cecilie Larsen: And the probability for this nucleus to for the neutron to stick to the nucleus is dependent on the level density and also the gamma ray strength function which is illustrated here.
Ann-Cecilie Larsen: For the compound nucleus. So the received a nucleus after capture
Ann-Cecilie Larsen: So the higher the level density, the more likely for the nucleus to to keep the news trauma, not just immediate again but get rid of the excess and the deal with cameras.
Ann-Cecilie Larsen: And also structures in the gamma strength function, like for example, this is a small pygmy dipole resonance and this the curious up and kind of thing these structures could also increase the probability for the neutron to stick to the nucleus.
Ann-Cecilie Larsen: So what do we do, what is this awesome method that I i just mentioned.
Ann-Cecilie Larsen: Well, it's a method that has been developed over the last 25 years I guess that the also cyclotron laboratory here at the Department of Physics.
Ann-Cecilie Larsen: University of Oslo. And so what we have is a rather small but very cute cyclotron that delivers light iron beams and right now we're running and 16 and maybe proton beam on carbon 12 so this is
Ann-Cecilie Larsen: Exciting.
Ann-Cecilie Larsen: And so the beam is taken out from the cyclotron and guided into the experimental hall where we used to have the cactus array to swim here to the left a sodium iodide array of gamma ray detectors to measure gamma rays, obviously. And then inside that ball of sodium iodide.
Ann-Cecilie Larsen: We had a chamber of where the silicon ring, which you can see down here to the left is placed to measure charged particles emitted in the reactions.
Ann-Cecilie Larsen: So here's just a little cartoon of how that goes. So this is for a helium three beam hitting a target nucleus. And then this is the helium three alpha reaction. So alphas would be measured with the silicon array and then in coincidence. We measure gamma rays.
Ann-Cecilie Larsen: And now we have a new governor detector system that's called Oscar, which causes so 30 laughter No bromide.
Ann-Cecilie Larsen: crystals which are very happy about, because it has excellent timing properties and also an amazing engineer solution.
Ann-Cecilie Larsen: And so I'll just briefly go through the steps of the Oslo method. So this is a crash course.
Ann-Cecilie Larsen: What we need to do is to measure the particles and the gum racing coincidence. So from the charged particle energy. We can transform that into the excitation energy of the residual Lucas. So this is what's on the access on the right upper matrix here.
Ann-Cecilie Larsen: And then we measured the gamma rays. So we plot those together with the they are sort of tagged with a excitation energy we measure them in coincidence.
Ann-Cecilie Larsen: And then we need to correct for the sodium iodide response because no detector is perfect, they will always be Compton scattering single escape doubled escape and so forth that we need to correct for whoops sorry to get the full energy peaks.
Ann-Cecilie Larsen: Having the unfolding matrix, we can extract the distribution of primary gamma rays from that distribution we can get the level density and the gamma strength function.
Ann-Cecilie Larsen: And yeah, if you want to take a look at our data and our publications. They're all put in this webpage
Ann-Cecilie Larsen: And also the the codes and the tools that we use for data analysis are freely available at this GitHub address. Of course, if you find any errors in the codes and some bugs, please let us know.
Ann-Cecilie Larsen: Okay, so I'll not go into all the details of the method, but just sort of outline the principle
Ann-Cecilie Larsen: So this matrix to the right here. Now, these are the primary transitions. So that means when you populate and abuse. I think given excitation energy and it the case but pick out all the first gamma rays that are emitted in the Cascades.
Ann-Cecilie Larsen: And then we say okay this primary matrix this landscape can be described by two vectors.
Ann-Cecilie Larsen: The level density multiplied with the gamma ray transmission coefficient
Ann-Cecilie Larsen: And this cameras transmission coefficient is directly proportional to the gamma ray strength.
Ann-Cecilie Larsen: Okay, that sounds good.
Ann-Cecilie Larsen: But we have to keep in mind that this separation into these two vectors that depends on two criteria being fulfilled. One is that the decay of this level. So the emotional glamorous.
Ann-Cecilie Larsen: Is independent from the way it was formed. So the nucleus doesn't remember how it ended up in that state does this compound decay.
Ann-Cecilie Larsen: And the other criteria is that there should be no strong dependence on excitation and the G spin or parity on the GM of decay strength function. So this is the brain hypothesis if these two are fulfilled, then we can safely do this fit of this landscape with two vectors.
Ann-Cecilie Larsen: And here is an example of an extracted level density for each room at night so level density here on the y axis excitation N G on the x axis and to the right. That is the strength function for the same Lucas.
Ann-Cecilie Larsen: And these error bands. In fact, represent mostly normalization uncertainties in this case.
Ann-Cecilie Larsen: And so that's that's how it looks.
Ann-Cecilie Larsen: And that's all well and good, but we want to do more. So we take our data into reaction calls like talus. And then we can calculate ready to capture cross sections.
Ann-Cecilie Larsen: And so, to the left here is an example for strontium at HP gamma, this is from the data set. I just showed you this zoom at nine data set and we find we take into account all our uncertainties, we can get this prediction for the capture cross section.
Ann-Cecilie Larsen: In the middle is the same thing, but now for neutron capture. So this is dispersed in 161 and gamma and here we have compared to known data as well. This we also did for the P gamma reaction here.
Ann-Cecilie Larsen: And to the very much right this guy here is awesome in 191 and gamma a here. There are actually know experimental data. So this is our prediction.
Ann-Cecilie Larsen: And we have compared with the condolence database for for the process. This is in fact that branch point so called branch point in the process. And so there is no data no direct data.
Ann-Cecilie Larsen: So that's all well and good.
Ann-Cecilie Larsen: Coming back to the gamma strength function. And there's a funny little thing that we saw at low energies.
Ann-Cecilie Larsen: This was a big surprise really when it showed up in the data for the first time, and this was published first in 2004
Ann-Cecilie Larsen: Where it was found more than a factor of 10 enhancement over common theoretical models of the gamma ray strength at blow transitional energies.
Ann-Cecilie Larsen: And this is just a little cartoon or how it looked like.
Ann-Cecilie Larsen: So typical model of the gamma the case strength would look something like this blue line experimentally we saw something like the red line. So it really sort of shoots up here at local government energies.
Ann-Cecilie Larsen: And, well, what do we know about this thing. Well, it has been confirmed by an independent technique independent people
Ann-Cecilie Larsen: As you can see, to the figure to the left here. This is dumb. Here in this case with a DP reaction and gamma. Gamma coincidences. And what we're looking at the I'll found was that the shape of the string function follows very much the same shape as was found in the also experiments.
Ann-Cecilie Larsen: And we also measured the multipolarity of this happened, using cactus and also lengthen and bromide detectors that we borrowed from the Hector plus
Ann-Cecilie Larsen: Array. And we saw that it's dominated by dipole transitions. So this is just an example from this publication.
Ann-Cecilie Larsen: And so far, it's been seen in quite heavy new guy as well. So after samarium
Ann-Cecilie Larsen: And it seems to be maybe some experimental hint that is that it might be a magnetic dipole that really dominates in this in this low energy region.
Ann-Cecilie Larsen: On the theoretical side.
Ann-Cecilie Larsen: There were no predictions of this feature, but lately. There are two different approaches that both sort of gave this low energy enhancement one predicts that this is due to electric type of transitions.
Ann-Cecilie Larsen: And another one predict said, Oh, no, this is probably magnetic dipole transitions. So one is a key RP a calculation and the other one is a shovel the calculation.
Ann-Cecilie Larsen: And well lately. There's been a whole bunch of shell model calculations showing the same trend. But as far as I know, at least not so much on the one side
Ann-Cecilie Larsen: And we don't really know yet. So there's a lot of. Yeah, we need to investigate more on the experimental side to make firm conclusions.
Ann-Cecilie Larsen: But why am I talking about this at all. What does it have to do with the reaction rates.
Ann-Cecilie Larsen: Well, the thing is that if this up and exists, also for neutron which nuclei.
Ann-Cecilie Larsen: Where the neutral separation and he gets really low. It could in fact boost their reaction rates relative to a calculation, where you do not include this no energy enhancement.
Ann-Cecilie Larsen: And this is shown here to the right. So we have calculated the rates, including the event like this one shown here for molybdenum 95
Ann-Cecilie Larsen: And also calculate the rates without the event and just taking the ratio
Ann-Cecilie Larsen: And so as you can see it has a huge impact on the rates for neutron which nuclei so well away from stability.
Ann-Cecilie Larsen: But the question is, of course, is the up and really there for neutral reason. Okay, then we have to measure neutral API.
Ann-Cecilie Larsen: And so we use auto invented this better also method.
Ann-Cecilie Larsen: To investigate nutrient rich and nuclei. And I'll not go into the details because I see. Time flies.
Ann-Cecilie Larsen: But the principle is to take your favorite natural original clues that you can produce in the lab, place it inside a segmented total absorption spectrometer, something like this which is called Sun
Ann-Cecilie Larsen: This is from this is a beautiful detector Ray at the NCL Michigan State University.
Ann-Cecilie Larsen: And then when you have implanted that nucleus measure, the better the electron in coincidence, with all the gamma rays from the dollar.
Ann-Cecilie Larsen: And then if you do that and you some together all these galleries, you get the in this short excitation energy which is shown in the matrix here to the left.
Ann-Cecilie Larsen: And then by taking all the galleries in the individual segments and plotting them as a functional expectation G, you get this Ii Ii gamma spectra, which is the starting point for the also method.
Ann-Cecilie Larsen: So the segments, they give the individual gamma rays and the some of them all gives the excitation energy. That's the principle
Ann-Cecilie Larsen: That ended up in a publication. A couple of years ago, six years ago. Time flies.
Ann-Cecilie Larsen: Where we presented this for the first time.
Ann-Cecilie Larsen: And I think I'll skip this.
Ann-Cecilie Larsen: Right, so just to show you the results. So this was actually for germanium center six which is stable.
Ann-Cecilie Larsen: And that was from better decay or gallium sentence x. So, here to the left is the gamma strength that we measured and to the right is the resulting
Ann-Cecilie Larsen: Constraint neutron capture rate. So what we obtained with our data is in the blue shaded band and the theory predictions for before we had the measurement was a sort of shown in these with these black lines.
Ann-Cecilie Larsen: And so we went back to Michigan State and the measured nickel 70 so this was coble 70 better decaying into nickel 70
Ann-Cecilie Larsen: And this is a picture from the experiment, five years ago and I promise you, there were guys there too. It seems like it's only girls. But yeah, man. There was there, for example, Sean lick
Ann-Cecilie Larsen: It was a fun, fun experiment for sure.
Ann-Cecilie Larsen: Whoops.
Ann-Cecilie Larsen: Let's see. Okay. And that ended up in a publication as well where we sold our results for for this nutrient rich and they call nucleus.
Ann-Cecilie Larsen: This is just I sorry
Ann-Cecilie Larsen: This is how it looks. This is the reaction rate as a function of temperature again with our new data, we could constrain the value to the dark blue band here.
Ann-Cecilie Larsen: And I think I'll skip this.
Ann-Cecilie Larsen: Yeah, I'm just so a little bit more details of the gamma strength function of nickel 70
Ann-Cecilie Larsen: So this is maybe a bit hard to see. It's a bit blurry.
Ann-Cecilie Larsen: But the data points are shown here as the black points again gamma strength as a function of my energy and this blue shaded band is the uncertainty that we obtained
Ann-Cecilie Larsen: So the, the, the black uncertainties our statistical and with the blue band, we include also systematic errors.
Ann-Cecilie Larsen: And we have compared here to theoretical calculations. So this black line here. This own sorry
Ann-Cecilie Larsen: This is he one from accurate calculation and all these different colored lines here. They represent values are calculated with the chairman calculation using different interactions as seen in the legend here.
Ann-Cecilie Larsen: So you can see they all sort of predict this kind of upturn
Ann-Cecilie Larsen: But the absolute magnitude is quite uncertain. It really depends on what kind of interaction and what model space you're using
Ann-Cecilie Larsen: So right now it seems maybe even to be be a bit on the lower side, it's kind of on the lower limit of the experimental uncertainty.
Ann-Cecilie Larsen: So yeah, it could be that maybe the magnetic dipole so or not the full story that we need to include more even strength as well.
Ann-Cecilie Larsen: And just to finish up coming back to the AI process.
Ann-Cecilie Larsen: We have now just been running and experiments here at the cyclotron looking at the team 124 Alfa P reaction to populate antimony 127 and this is if we go back and remember this figure from humble at all for the I process.
Ann-Cecilie Larsen: This is where 127 antimony sets. So we're trying here with the stable beam in a stable target to get out into the unknown for the I process.
Ann-Cecilie Larsen: And yeah, this was run with the 24 MB alpha beam we had Siri and backward angles and Oscar in the close configuration.
Ann-Cecilie Larsen: And this is some preliminary data that we took right now.
Ann-Cecilie Larsen: So here we see alpha. So, this is alpha, alpha prime
Ann-Cecilie Larsen: That media will analyze and then if you see down here. The bottom banana kind of is the protons. Then we have neutrons and triathlons and these we can separate because we plot, then the energy deposited in the back detector versus the energy in the front detector.
Ann-Cecilie Larsen: And by putting a gate around the protons and looking at gamma coming at the same time.
Ann-Cecilie Larsen: We obtain this matrix. So this is now for antimony 127 very preliminary. This is not the full statistics. It's just a small piece of the data.
Ann-Cecilie Larsen: But we see transitions down here that are known for antimony 127 you see the separation and the G and then we see peaks from contaminants. Luckily above the region of interest, which is from the separation N G and down.
Ann-Cecilie Larsen: So I'm finishing now.
Ann-Cecilie Larsen: I'm sure I'm looking
Ann-Cecilie Larsen: For what do they say in the reason we say is lucky the chicken bracing for the church when I say that nuclear physics is awesome. I don't think you need to be convinced about that. I think it's the coolest feel ever
Ann-Cecilie Larsen: And I think also that nowadays it's amazing because there are so many possibilities on the experimental side.
Ann-Cecilie Larsen: And
Ann-Cecilie Larsen: Well, my personal view is that there's also a lot of room for improvement on the theory side, especially when it comes to gamma ray strain functions and the level densities.
Ann-Cecilie Larsen: And I hope I've given you
Ann-Cecilie Larsen: The impression that it is that the Oslo method and the bed also method can provide experimental constraints on nature capture rates for moderately to nutrient rich nuclei.
Ann-Cecilie Larsen: We have a long to do list. We want to extend the bed or slow method to even more neutral nuclei, where we will run into trouble because you open up for example the beta n channel giving an additional background in the spectrum.
Ann-Cecilie Larsen: We've just started using alpha p reactions here at the cyclotron to to start to map out nuclei for the I process.
Ann-Cecilie Larsen: We're continuously working on new and better methods for normalization and investigate systematic errors looking into effects of spin selection, etc, etc. So we're not out of work for a long time to come. I'm sure
Ann-Cecilie Larsen: And with that, I want to say thank you so much for listening.
James Smallcombe: Thank you very much. Answer. So yeah, that was great. I don't know. You'll get any argument from the rest of us. That is very interesting.
James Smallcombe: So yes, any questions as I said back at the start, please do go into that participants menu and use the raise hand button for a question.
James Smallcombe: Even if you don't have a question, get yourself into that participants menu and this you can do a digital clap in there and we can all quietly express our appreciation to talk
James Smallcombe: And again, if you don't have a microphone, please do get down to the chat window and I'll reach out any questions you have.
James Smallcombe: Good. I'm seeing the digital flapping flowing in everyone's having a very good talk a great question. Philip and to be pleased with your head, your question.
Philip Adsley: And can you hear me working great. Thanks.
Philip Adsley: It's hugely impressive and i i do like your method, even though I still think I understand the property.
Philip Adsley: And I have to, especially as such as an office with KG he's tried to explain it to me a few times.
Today,
Philip Adsley: So I have had one very basic question which was just a couple of the end gamma cross section. So you showed a high temperatures and seems to drop quite small. Well, what's that caused by it just is it some other channel coming in or something, a
Philip Adsley: feature which I've never noticed before so
Ann-Cecilie Larsen: Yeah. So let's see, like this one, for example.
Ann-Cecilie Larsen: Oh no, the cross session. Oh, sorry. Sorry, yes. Let me go back. So like this, for example.
Philip Adsley: Yes.
Ann-Cecilie Larsen: Capture. Yeah. So it's basically this a
Ann-Cecilie Larsen: One hour V effect. So the, the slower the neutron, the more likely for it to be captured the faster it goes
Ann-Cecilie Larsen: The chance of it getting scattered or you having end to end reactions. Gross. So, and gamma sort of gets lower and lower, in a way, as the energy goes up as sorry. Yeah. Yes. When the energy goes up.
Ann-Cecilie Larsen: Okay, well, it's the other way around for P gamma, as we can see
Philip Adsley: Yes. Yeah, cuz dependability will
Ann-Cecilie Larsen: Win. Exactly.
Philip Adsley: So the other question I had was
Philip Adsley: So we also method you you kind of reliant on that being a high level that city. Right.
Philip Adsley: Yes. So you were saying about going to bring neutron rich things and the neutron threshold becomes very low.
Philip Adsley: So is there something. Is there a concern that at some point.
Philip Adsley: You have enough metals.
Ann-Cecilie Larsen: Or very much
Ann-Cecilie Larsen: That
Is
Ann-Cecilie Larsen: Also there are. So Tom rusher wrote in one of his papers back in the 90s that the rule of thumb is that you need something like 10 levels PR and then GPM
Ann-Cecilie Larsen: And that is a bit vague may believe, maybe because you could imagine taking a huge energy bin and then you have 10 levels, but that doesn't help you because they're all so why the part that you don't have any statistical behavior and more so, indeed.
Ann-Cecilie Larsen: So what we do is typically that we split our data into slices and we see when things start to change.
Ann-Cecilie Larsen: So like when the especially for the gamma strength. If it starts to change within the errors, then it's typically that we have entered a region where, for example, the sweet transitions can be considerable.
Ann-Cecilie Larsen: And then, then we can't really rely on the method anymore. So this is, is this a very good point. And what's the follow me did back in the days where where I showed this you know where we kept at the rate with and without the up and this is completely ignoring
Ann-Cecilie Larsen: That the level desert. It could be so low that it doesn't actually make any sense to talk about neither a level density or GM or a string, because it's per definition average quantities. So absolutely, you're, you're completely correct
Ann-Cecilie Larsen: So one has to make sure and really study to see whether the nucleus. And the question is, does it really fulfill these criteria or not. Okay. Okay.
Thanks.
You
James Smallcombe: Another question mark. Please go ahead.
Mark-Christoph Spieker: Good morning from Florida. Great talk.
Mark-Christoph Spieker: I hope you're doing well in Oslo.
Ann-Cecilie Larsen: Good.
Mark-Christoph Spieker: That's good. Um, so, since we were already talking about some of the limitations that we might be running into for nutrient rich nuclei, considering the
Mark-Christoph Spieker: Neutron threshold.
Mark-Christoph Spieker: What is your feeling about the influence. I know you talked to a you looked at that a lot. So the brain hypothesis. That's basically where my
Mark-Christoph Spieker: Question would be pointing at, and he showed that it's holding up for the cases where you actually studied it
Mark-Christoph Spieker: But what I'm wondering, especially now that he looked at the show model calculations for the M one part. And if I remember correctly, come, he looked at that.
Mark-Christoph Spieker: At some point, and she showed that these are not part of the ground state gamma strength punch and in the beginning, you mentioned the hot our process and the end gamma, gamma and equilibrium
Mark-Christoph Spieker: How could that be influenced if the gamma ray strength function in the decay is different than the gamma
Mark-Christoph Spieker: Strike function in the incoming chairman. It's also. I mean, it's also an open question for me. Right. And I don't have any feeling at all for that.
Ann-Cecilie Larsen: Oh, that's the that's definitely a very, very good question.
Ann-Cecilie Larsen: I mean,
Ann-Cecilie Larsen: This is, as you said, I mean, many people are studying this just because it's so important. And of course, for all this.
Ann-Cecilie Larsen: Are very many of these theoretical calculations, you have to assume that the brain hypothesis holds and there's also only for the gamma strength but also for the better decay. Right. So you calculate beta decay.
Ann-Cecilie Larsen: Basically Graham said you do accurate PA you hope that this will hold also
Ann-Cecilie Larsen: No high expectation is, basically, and this is the really relevant for beta decay, when you go out there.
Ann-Cecilie Larsen: Far out in the neutral which side where the Q value is very high. So you don't populate just, you know, close to the ground state, and more, but to populate a very broad range of excitation, geez.
Ann-Cecilie Larsen: So on. Yeah, absolutely. For the pygmy that that's really an open question right now and let's say we're working hard on that too.
Ann-Cecilie Larsen: But I think at least it's clear that
Ann-Cecilie Larsen: As Camilla showed. And I think also, there's a paper by peer to mom and Blanca at all where they showed that was for Ireland.
Ann-Cecilie Larsen: Where they calculate the ground state. And once and also the M wants to the two plus. And you see they are really different. So putting these in to the end gamma cross section calculations. They change the cross sections.
Ann-Cecilie Larsen: So,
Ann-Cecilie Larsen: I guess I don't have a short answer for you. I think this is still an open question and
Mark-Christoph Spieker: I'd be looking at it right
Ann-Cecilie Larsen: Yeah, exactly. Yeah, and also really to see where is it fulfilled, where is it not fulfilled. What is the criteria for it to be fulfilled.
Ann-Cecilie Larsen: Because we're having new guy. It's obviously fulfilled like Mark. Now this paper on Neptunian where you have such a high level density that everything looks the same ground so decay.
Ann-Cecilie Larsen: Continuum decay. Everything is the same. But that's not the case for like two nuclei. And especially when you have low level densities.
Thanks.
Ann-Cecilie Larsen: Thank you.
James Smallcombe: Okay. Do we have any other questions, Francis
I don't see anything in the chat.
Mark-Christoph Spieker: I could ask another one.
James Smallcombe: For you're welcome to. If you go
James Smallcombe: Okay.
Mark-Christoph Spieker: So you showed that different method looked at the app and mainly considering reactions that under the given common kinematics could be considered compound nucleus reactions.
Mark-Christoph Spieker: Now for the end gamma reaction and and Phillip brought this up when he asked about the energy dependence of the and gamma cross section and he's still on that slide.
Mark-Christoph Spieker: You have some selectivity to a certain class of nuclei, if you start from the ground state. In the end, gamma reaction, say you do S wave capture you do p with capture or whatever.
Mark-Christoph Spieker: So he might only populate certain class of levels with very specific structures and he touched upon on on that.
Mark-Christoph Spieker: When you discuss that you might run into the problems just having low level densities and then discrete the case governed by structure, rather than statistical properties. I'm wondering
Mark-Christoph Spieker: If you will observe the same thing, not just in the decay, but actually in the population.
Ann-Cecilie Larsen: So I think definitely on the population, but the decay might not be so sensitive
Ann-Cecilie Larsen: So I didn't show it, but I have it.
Mark-Christoph Spieker: Let me see.
Ann-Cecilie Larsen: So for the better, or slow method. This is especially important, right, because better decay can be very or it is selective also in terms of spins and parities
Ann-Cecilie Larsen: So what we did see
Ann-Cecilie Larsen: Some extra stuff, what we did was to measure a light Nicholas where the loudest is low, with both methods. So this is titanium 51 measured at the all CL with a DP reaction. And then at the end, the Cl. We did the scandium 51 better the king into titanium 51
Ann-Cecilie Larsen: And so here, this is just a data. So this is the top panels are from the better decay and the lower panels are from the DP reaction.
Ann-Cecilie Larsen: I mean it's striking. It's so different. Right. If you look at the primaries, for example, to the right here, these matrices. They don't look much the same. Well, you do have the same states in here, but the population intensity is very different.
Ann-Cecilie Larsen: So we're thinking, okay, this is kind of a worst case scenario. So if it works here. Well, then we can't say for sure, but at least it gives us a good indication that it should work also for heavier client lists.
Ann-Cecilie Larsen: And this is what we got. This is a bit difficult to see, maybe this is a level density, but maybe more importantly, the strength
Ann-Cecilie Larsen: We had quite a bit of systematic uncertainties in the normalization. But if you look at the data points. So it's the black squares and the diamonds. They follow each other quite nicely.
Ann-Cecilie Larsen: It's actually, I was shocked.
Ann-Cecilie Larsen: When I saw that it actually worked. So that means ok so the population from the better the k and the DP is very different, very different.
Ann-Cecilie Larsen: But you populate more or less the same states and when they decay. They don't remember how they reformed, they don't know if they came from a better decay or from the DP. And so they just decay by the same way. And so we get the same strength function.
Ann-Cecilie Larsen: So, so that was kind of, for me at least, it was comforting. It's actually published. I forgot to say this is no I should put the reference, sir. So it's in PRC
Ann-Cecilie Larsen: So, so yeah, at least for that case you can see on the level density here is something like hundred per MTV and the beans. Here are
Ann-Cecilie Larsen: Who should remember something like hundred to 200 kV bins, I think. So that's sort of at least for that case it seems to work. Still going lower. We don't know yet. And we have to test even lighten okay but this is the lightest we've tested so far.
Mark-Christoph Spieker: This is a nice result I agree on that.
Mark-Christoph Spieker: Sort of comforting that at least see in the nucleus, which is rather close to stability and has a high level of density, where you populated, you get the two from two very different methods to get the same result. But those two observable.
Yeah.
Nice.
Ann-Cecilie Larsen: Yeah, it's
Ann-Cecilie Larsen: It's a bit surprising. I have to say. But yeah, seems to work.
James Smallcombe: Alright so we got some really good discussions today if there are any last quick questions from people. If you're struggling with the raise hand button. Please do just unmute yourself now.
James Smallcombe: But if I don't see that quickly. Now, I think in that case, we'll, we'll call it to a close there. And so thank you very much and Syria for wonderful talk.
James Smallcombe: Thank you everybody for joining us and please do come back next Monday when, as was mentioned briefly in the door. We've got the
James Smallcombe: New to usher from Livermore, who's going to be talking about nuclear reaction descriptions for Astrophysics and applications that same time 3pm GMT next Monday. And also, don't forget, putting your calendars open meeting the on Tuesdays.
James Smallcombe: Next Tuesday. That's the 15th discussing future
Ann-Cecilie Larsen: community efforts for online seminars, so
James Smallcombe: Thank you very much, all for joining us.
Ann-Cecilie Larsen: And have a good week.
James Smallcombe: Bye for now. Thank you so much.